Existing Annotations Review

GO Term	Evidence	Action	Reason
GO:0001671 ATPase activator activity	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for ATPase activator activity based on phylogenetic inference (PANTHER). This is the core molecular function of AHSA1. Aha1 was identified as the activator of Hsp90 ATPase in yeast and human, stimulating ATPase activity 5-fold in vitro (PMID:12604615) and confirmed across multiple studies (PMID:12504007, PMID:29127155, PMID:27353360). The IBA annotation is well supported by conserved function across yeast (S. cerevisiae Aha1, Hch1) and S. pombe orthologs included in the PANTHER family. Reason: ATPase activator activity is the primary, evolutionarily conserved molecular function of AHSA1. Multiple independent experimental studies confirm this function (PMID:12504007, PMID:12604615, PMID:27353360, PMID:29127155), and the IBA phylogenetic inference is sound, supported by orthologs in yeast. Supporting Evidence: PMID:12504007 A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for the in vivo Hsp90-dependent activation of clients such as v-Src, implicating them as cochaperones of the Hsp90 system. PMID:12604615 Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90 5-fold
GO:0005829 cytosol	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for cytosol localization based on phylogenetic inference. AHSA1 is predominantly cytosolic, consistent with its role as an HSP90 co-chaperone. UniProt records cytoplasm/cytosol as the primary subcellular location (PMID:11554768), and HPA immunofluorescence data supports cytosol localization (GO_REF:0000052). Reason: Cytosol is the well-established primary localization of AHSA1, consistent with its function as an HSP90 co-chaperone in the cytoplasm. Supported by IDA from HPA and UniProt subcellular location annotation.
GO:0006457 protein folding	IBA GO_REF:0000033	ACCEPT	Summary: IBA annotation for protein folding based on phylogenetic inference. AHSA1 participates in protein folding indirectly by stimulating the HSP90 ATPase cycle, which is required for HSP90-dependent client protein maturation. Yeast Aha1 deletion impairs activation of the Hsp90 client v-Src (PMID:12504007, PMID:12604615), and human AHSA1 competes with inhibitory co-chaperones to regulate HSP90 client chaperoning (PMID:27353360, PMID:29127155). The term is at an appropriate level of generality for the biological process. Reason: Protein folding is an appropriate biological process for AHSA1, which participates in HSP90-mediated client protein folding/maturation by stimulating the HSP90 chaperone cycle. The IBA inference is supported by experimental evidence in yeast and human showing that Aha1 is required for efficient activation of Hsp90 clients. Supporting Evidence: PMID:12604615 Aha1 and Hch1 contributed to efficient activation of the heterologous Hsp90 client protein v-Src PMID:29127155 Tsc1 is a new co-chaperone for Hsp90 that inhibits its ATPase activity ... prevents the activating co-chaperone Aha1 from binding the middle domain of Hsp90
GO:0001671 ATPase activator activity	IEA GO_REF:0000002	ACCEPT	Summary: IEA annotation for ATPase activator activity inferred from InterPro domain IPR015310 (AHSA1-like_N). This is consistent with the core function of AHSA1 and is well supported by experimental evidence across multiple publications. Reason: The InterPro-to-GO mapping is correct. The AHSA1 N-terminal domain (Aha1_N, Pfam PF09229) is the domain responsible for binding the HSP90 middle domain and stimulating ATPase activity (PMID:12604615). This IEA annotation is redundant with the IBA and IDA annotations but correctly captures the function.
GO:0005783 endoplasmic reticulum	IEA GO_REF:0000044	KEEP AS NON CORE	Summary: IEA annotation for ER localization mapped from UniProt subcellular location. UniProt notes that AHSA1 "may transiently interact with the endoplasmic reticulum" based on PMID:11554768 (Sevier and Machamer 2001), which identified AHSA1 (then called p38) as interacting with VSV G glycoprotein. The ER localization is likely a minor or transient association rather than a primary localization. Reason: The ER association is based on early work showing AHSA1 interacts with VSV G glycoprotein and may transiently associate with the ER (PMID:11554768). This is not a core localization for AHSA1, whose primary function occurs in the cytosol. The UniProt annotation itself qualifies this as transient.
GO:0005829 cytosol	IEA GO_REF:0000044	ACCEPT	Summary: IEA annotation for cytosol localization mapped from UniProt subcellular location vocabulary. Consistent with the primary localization of AHSA1 and supported by IDA (HPA) and IBA evidence. Reason: Cytosol is the well-established primary localization of AHSA1. This IEA annotation is consistent with the IDA and IBA annotations for the same term and is correctly mapped from UniProt subcellular location.
GO:0051087 protein-folding chaperone binding	IEA GO_REF:0000002	ACCEPT	Summary: IEA annotation for protein-folding chaperone binding inferred from InterPro domain IPR015310. AHSA1 directly binds HSP90, which is a protein-folding chaperone, so this annotation is accurate. The binding is well characterized: the N-terminal domain of AHSA1 binds the middle domain of HSP90 (PMID:12604615), and the C-terminal domain stabilizes the dimerized N-terminal domains of HSP90 (PMID:33808352). Reason: AHSA1 is a well-established HSP90-binding co-chaperone. The InterPro-to-GO mapping correctly captures the chaperone binding function. This is supported by multiple experimental studies (PMID:12504007, PMID:12604615, PMID:27353360, PMID:29127155).
GO:0005515 protein binding	IPI PMID:12604615 Aha1 binds to the middle domain of Hsp90, contributes to cli...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Lotz et al. 2003, which demonstrated that Aha1 binds to the middle domain of Hsp90 using biochemical approaches including co-immunoprecipitation and direct binding assays. The WITH column indicates interaction with HSP90AA1 (P07900). While the interaction is real and well-characterized, "protein binding" is uninformative since the specific interaction is better captured by GO:0051879 (Hsp90 protein binding). Reason: The underlying interaction with HSP90 is the core function of AHSA1 and is well-documented (PMID:12604615). However, GO:0005515 (protein binding) is too vague. The interaction is more precisely captured by GO:0051879 (Hsp90 protein binding), which is already annotated from other evidence. Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:12604615 We have identified Aha1 (activator of Hsp90 ATPase) and its relative Hch1 (high copy Hsp90 suppressor) as binding partners of Hsp90 in Saccharomyces cerevisiae. By using genetic and biochemical approaches, the middle domain of Hsp90 (amino acids 272-617) was found to mediate the interaction with Aha1 and Hch1.
GO:0005515 protein binding	IPI PMID:16696853 A yeast 2-hybrid analysis of human GTP cyclohydrolase I prot...	KEEP AS NON CORE	Summary: IPI protein binding annotation from IntAct, based on Swick and Kapatos 2006, which identified AHSA1 as interacting with GCH1 (GTP cyclohydrolase I, P30793) using yeast two-hybrid screen and validated by GST pull-down assay. The authors note that "the physiological relevance of the Aha1-GCH1 interaction requires further study" and speculate it may recruit GCH1 into the eNOS/Hsp90 complex. This is a secondary, possibly indirect interaction. Reason: The interaction with GCH1 was detected by yeast two-hybrid and validated by GST pull-down (PMID:16696853), but its physiological relevance is uncertain. The authors themselves state the relevance "requires further study." This may reflect indirect bridging through HSP90 rather than a direct functional interaction. Keeping as non-core since the interaction was validated but may not represent a core AHSA1 function. Supporting Evidence: PMID:16696853 The interaction of one of these clones, Activator of Heat Shock 90 kDa Protein (Aha1), with GCH1 was validated by glutathione-s-transferase (GST) pull-down assay. Although the physiological relevance of the Aha1-GCH1 interaction requires further study
GO:0005515 protein binding	IPI PMID:19875381 A proteomic investigation of ligand-dependent HSP90 complexe...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Gano and Simon 2010, which identified AHSA1 as a component of HSP90 complexes by tandem affinity purification and LC-MS/MS. The WITH column indicates interaction with HSP90AA1 (P07900). This is a high-throughput proteomics study that characterized the nucleotide-dependent HSP90 interactome. The AHSA1-HSP90 interaction is well established. Reason: The interaction with HSP90 is the core function of AHSA1. GO:0005515 (protein binding) is uninformative; the specific interaction is better captured by GO:0051879 (Hsp90 protein binding). Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:19875381 We identified 52 known and novel components of HSP90 complexes that are regulated by these ligands, including several co-chaperones.
GO:0005515 protein binding	IPI PMID:20618441 CHIP participates in protein triage decisions by preferentia...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Stankiewicz et al. 2010. The WITH column indicates interaction with HSP90AB1 (P08238). This study investigated CHIP-mediated ubiquitination and tested the influence of Aha1 on HSP90 ATPase activity in the context of CHIP. AHSA1 is mentioned as an HSP90 co-chaperone tested for its effect on CHIP function. The interaction with HSP90 is incidental to the main focus of the paper. Reason: The interaction detected is between AHSA1 and HSP90AB1, which is the core binding partner. GO:0005515 is too vague; GO:0051879 (Hsp90 protein binding) is the appropriate specific term. Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:20618441 CHIP did not influence the ATPase cycle of Hsp90 in the absence of co-chaperones or in the presence of the Hsp90 cochaperones Aha1 or p23.
GO:0005515 protein binding	IPI PMID:25036637 A quantitative chaperone interaction network reveals the arc...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Taipale et al. 2014, a large-scale quantitative chaperone interaction network study. The WITH column indicates interaction with HSP90AB1 (P08238). This is a systematic proteomics study mapping chaperone-client interactions. AHSA1 was identified as part of the HSP90 interaction network. Reason: The interaction detected is with HSP90AB1. GO:0005515 is too vague for AHSA1, whose binding to HSP90 is its core function. GO:0051879 (Hsp90 protein binding) is the appropriate specific term. Proposed replacements: Hsp90 protein binding
GO:0005515 protein binding	IPI PMID:30382094 Structure and pro-toxic mechanism of the human Hsp90/PPIase/...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Oroz et al. 2018, which determined the solution structure of the human Hsp90/FKBP51/Tau complex. The WITH column indicates interaction with HSP90AB1 (P08238). AHSA1 is mentioned as restoring HSP90 ATPase activity that was decreased by FKBP51 binding. This is a functional assay demonstrating AHSA1's role as an HSP90 ATPase activator. Reason: The interaction is with HSP90AB1 and reflects AHSA1's core function as an HSP90 co-chaperone. GO:0005515 is uninformative; GO:0051879 (Hsp90 protein binding) properly captures this interaction. Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:30382094 Hsp90 ATPase activity was restored by the addition of Aha1, a strong enhancer of Hsp90 ATPase activity through compaction of the Hsp90 conformation
GO:0005515 protein binding	IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce...	MODIFY	Summary: IPI protein binding annotation from IntAct, based on Cho et al. 2022 (OpenCell project), which used endogenous tagging and mass spectrometry to map protein-protein interactions across the human proteome. The WITH column indicates interactions with HSP90AA1 (P07900) and HSP90AB1 (P08238). This is a large-scale systematic study confirming the known AHSA1-HSP90 interaction. Reason: The interactions detected are with both HSP90 isoforms (HSP90AA1 and HSP90AB1), consistent with AHSA1's core function. GO:0005515 is too vague; GO:0051879 (Hsp90 protein binding) properly captures this. Proposed replacements: Hsp90 protein binding
GO:0051879 Hsp90 protein binding	IEA GO_REF:0000120	ACCEPT	Summary: IEA annotation for Hsp90 protein binding from combined automated methods (ARBA, mouse ortholog, Ensembl). This is well-supported by extensive experimental evidence: AHSA1 directly binds HSP90AA1 and HSP90AB1 (PMID:12504007, PMID:12604615, PMID:25486457, PMID:27353360, PMID:29127155). HSP90 binding is the core molecular interaction of AHSA1. Reason: Hsp90 protein binding is the core molecular interaction of AHSA1. The automated annotation is correct and supported by multiple independent experimental studies. AHSA1 binds both HSP90 isoforms through its N-terminal and C-terminal domains.
GO:0005829 cytosol	IDA GO_REF:0000052	ACCEPT	Summary: IDA annotation for cytosol localization from Human Protein Atlas (HPA) based on curation of immunofluorescence data. This is consistent with AHSA1's role as a cytosolic HSP90 co-chaperone and is supported by UniProt subcellular location annotation (PMID:11554768). Reason: Cytosol is the primary localization of AHSA1, directly demonstrated by immunofluorescence (HPA) and consistent with its function as an HSP90 co-chaperone in the cytoplasm.
GO:0036506 maintenance of unfolded protein	EXP PMID:37486705 Human Aha1's N-terminal extension confers it holdase activit...	MARK AS OVER ANNOTATED	Summary: EXP annotation for maintenance of unfolded protein from DisProt, based on Tang et al. 2023, which demonstrated that the N-terminal extension (M1-R16) of human Aha1 confers holdase activity in vitro. The holdase activity prevents aggregation of heat-denatured MBP but is abolished by high NaCl concentration. This activity is mediated by the N-terminal extension unique to higher eukaryote Aha1 proteins and is absent from the conserved core domains. Reason: The holdase activity demonstrated in vitro maps to the N-terminal extension (M1-R16) unique to higher eukaryotes, not the conserved Aha1 core domains. The activity is abolished by high NaCl concentration, suggesting electrostatic-driven non-specific interactions (PMID:37486705). No in vivo evidence supports this as a physiological function. AHSA1's core function is HSP90 ATPase activation, not autonomous chaperone activity. Supporting Evidence: PMID:37486705 the highly conserved N-terminal extension spanning M1 to R16 in Aha1 from higher eukaryotes is responsible for the holdase activity of the protein PMID:37486705 since the high concentration of NaCl could abolish the holdase activity of Aha1, the electrostatic interactions mediated by those charged residues in Aha1's N-terminal extension are thus indicated to play a crucial role in the substrate recognition
GO:0036506 maintenance of unfolded protein	IDA PMID:37486705 Human Aha1's N-terminal extension confers it holdase activit...	MARK AS OVER ANNOTATED	Summary: IDA annotation for maintenance of unfolded protein from DisProt, based on the same study as above (Tang et al. 2023). This is a duplicate annotation with different evidence code (IDA vs EXP) from the same reference, and the same concerns apply. Reason: Same rationale as the EXP annotation from the same reference. The holdase activity is an in vitro observation dependent on the non-conserved N-terminal extension (M1-R16) and abolished by high salt, suggesting non-specific electrostatic interactions. Not established as a physiological function of AHSA1. Supporting Evidence: PMID:37486705 the highly conserved N-terminal extension spanning M1 to R16 in Aha1 from higher eukaryotes is responsible for the holdase activity of the protein
GO:0044183 protein folding chaperone	EXP PMID:33808352 Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Li...	MARK AS OVER ANNOTATED	Summary: EXP annotation for protein folding chaperone from DisProt, based on Hu et al. 2021, which demonstrated using NMR HSQC titrations and ThT assays that full-length human Aha1 interacts with alpha-synuclein and inhibits its aggregation in vitro. The chaperone-like activity requires the N-terminal extension (M1-W27) and/or C-terminal RLF motif, which are peripheral to the conserved core domains (PMID:33808352). The core construct Aha128-335 showed no significant interaction with alpha-synuclein. Reason: The protein folding chaperone annotation for AHSA1 represents an over-annotation. The chaperone-like activity is an in vitro observation dependent on peripheral regions (N-terminal extension and C-terminal RLF motif) not present in the conserved Aha1 core. The core construct Aha128-335 showed no significant interaction with alpha-synuclein (PMID:33808352). No in vivo evidence supports autonomous chaperone function. AHSA1's established role is as an HSP90 ATPase activator co-chaperone, not as an independent protein folding chaperone. Supporting Evidence: PMID:33808352 Without the presence of M1-W27 fragment and RLF (R336L337F338) motif, no significant chemical shift perturbations were observed for the NMR resonances of the residues in Aha128-335 upon the addition of alpha-synuclein PMID:33808352 extensive in vivo studies need to be conducted to precisely decipher the functional roles of Aha1 under different physiological and pathological conditions
GO:0044183 protein folding chaperone	IDA PMID:33808352 Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Li...	MARK AS OVER ANNOTATED	Summary: IDA annotation for protein folding chaperone from DisProt, based on the same study (Hu et al. 2021). This is a duplicate annotation with different evidence code (IDA vs EXP) from the same reference, and the same concerns apply. Reason: Same rationale as the EXP annotation from the same reference. The chaperone-like activity depends on peripheral regions absent from the conserved Aha1 core, has no in vivo validation, and does not represent a core function of AHSA1. Supporting Evidence: PMID:33808352 Without the presence of M1-W27 fragment and RLF (R336L337F338) motif, no significant chemical shift perturbations were observed for the NMR resonances of the residues in Aha128-335 upon the addition of alpha-synuclein
GO:0051082 unfolded protein binding	EXP PMID:33808352 Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Li...	MARK AS OVER ANNOTATED	Summary: GO:0051082 (unfolded protein binding) is being obsoleted (go-ontology#30962). The annotation from DisProt is based on PMID:33808352 (Hu et al. 2021), which used NMR HSQC titrations and ThT assays to show that full-length human Aha1 interacts with intrinsically disordered alpha-synuclein and inhibits its aggregation in vitro. However, the core construct Aha128-335 (lacking the N-terminal M1-W27 extension and C-terminal RLF motif) showed no significant interaction with alpha-synuclein, indicating this is not a property of the conserved Aha1 core domains. The authors themselves describe this as "chaperone-like activity" and note that in vivo significance remains unestablished. AHSA1's primary, evolutionarily conserved function is as an HSP90 ATPase activator, not as an independent chaperone. The unfolded protein binding annotation is an over-annotation that conflates an in vitro observation with a core molecular function. Furthermore, GO:0051082 is scheduled for obsoletion. The holdase/chaperone-like activity demonstrated in vitro is better captured by the existing GO:0044183 (protein folding chaperone) and GO:0036506 (maintenance of unfolded protein) annotations already present on this gene. Reason: AHSA1 is primarily an HSP90 co-chaperone whose core function is stimulating HSP90 ATPase activity (PMID:12504007, PMID:29127155). The unfolded protein binding annotation is based on in vitro NMR titration experiments showing that full-length Aha1 interacts with alpha-synuclein (PMID:33808352), but this interaction depends on the N-terminal extension (M1-W27) and C-terminal RLF motif, which are peripheral to the conserved Aha1 domains and absent in lower eukaryotes. The authors state that "extensive in vivo studies need to be conducted to precisely decipher the functional roles of Aha1 under different physiological and pathological conditions" (PMID:33808352). A follow-up study (PMID:37486705) confirmed the holdase activity maps to the N-terminal extension (M1-R16) and is driven by electrostatic interactions that can be abolished by high NaCl concentration, suggesting non-specific binding. Additionally, GO:0051082 is being obsoleted. The chaperone-like activity is already captured by GO:0044183 and GO:0036506 annotations on this gene from the same research group. Supporting Evidence: PMID:33808352 Without the presence of M1-W27 fragment and RLF (R336L337F338) motif, no significant chemical shift perturbations were observed for the NMR resonances of the residues in Aha128−335 upon the addition of α-synuclein PMID:33808352 In particular, since Aha1 has been reported to drive the production of pathological tau aggregates by acting as Hsp90's co-chaperone [59], which is in opposite to the inhibition effect of Aha1 on α-synuclein's aggregation observed by us, extensive in vivo studies need to be conducted to precisely decipher the functional roles of Aha1 under different physiological and pathological conditions. PMID:37486705 the highly conserved N-terminal extension spanning M1 to R16 in Aha1 from higher eukaryotes is responsible for the holdase activity of the protein PMID:37486705 since the high concentration of NaCl could abolish the holdase activity of Aha1, the electrostatic interactions mediated by those charged residues in Aha1's N-terminal extension are thus indicated to play a crucial role in the substrate recognition
GO:0001671 ATPase activator activity	IDA PMID:29127155 Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facil...	ACCEPT	Summary: IDA annotation for ATPase activator activity based on Woodford et al. 2017, which showed that AHSA1 activates HSP90 ATPase activity and competes with the inhibitory co-chaperone TSC1 for binding to the HSP90 middle domain. Phosphorylation of Aha1-Y223 increases its affinity for HSP90 and displaces TSC1, providing a regulatory switch for the chaperone cycle. This is direct experimental evidence for the core function of AHSA1. Reason: This is strong direct assay evidence for the core molecular function of AHSA1. The study demonstrates ATPase activation of HSP90 by AHSA1 and elucidates the regulatory mechanism involving phosphorylation of Y223 (PMID:29127155). Supporting Evidence: PMID:29127155 phosphorylation of Aha1-Y223 increases its affinity for Hsp90 and displaces Tsc1, thereby providing a mechanism for equilibrium between binding of these two co-chaperones to Hsp90
GO:0032781 positive regulation of ATP-dependent activity	IDA PMID:29127155 Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facil...	ACCEPT	Summary: IDA annotation for positive regulation of ATP-dependent activity based on Woodford et al. 2017. AHSA1 stimulates the ATPase activity of HSP90, which is an ATP-dependent molecular chaperone. This annotation captures the regulatory biological process aspect of AHSA1's core function. The term is appropriate as AHSA1 positively regulates HSP90's ATP-dependent chaperone cycle. Reason: This annotation accurately captures the biological process corresponding to AHSA1's core molecular function. AHSA1 positively regulates HSP90's ATP-dependent chaperone cycle by stimulating its ATPase activity (PMID:29127155, PMID:12504007, PMID:27353360). Supporting Evidence: PMID:29127155 Tsc1 is a new co-chaperone for Hsp90 that inhibits its ATPase activity ... prevents the activating co-chaperone Aha1 from binding the middle domain of Hsp90. Conversely, phosphorylation of Aha1-Y223 increases its affinity for Hsp90 and displaces Tsc1
GO:0045296 cadherin binding	HDA PMID:25468996 E-cadherin interactome complexity and robustness resolved by...	REMOVE	Summary: HDA annotation for cadherin binding from BHF-UCL, based on Guo et al. 2014, a large-scale quantitative proteomics study of the E-cadherin interactome using proximity biotinylation. AHSA1 was identified among 561 proteins in the vicinity of the cytoplasmic tail of E-cadherin. This is a high-throughput proximity labeling approach that detects proteins near E-cadherin, not necessarily direct binding partners. AHSA1 has no known functional relationship to cadherins, and its presence may reflect its abundance as a cytoplasmic chaperone co-factor. Reason: Cadherin binding is not a plausible molecular function for AHSA1, an HSP90 co-chaperone. The HDA evidence comes from proximity biotinylation proteomics (PMID:25468996), which identifies proteins in the general vicinity of E-cadherin, not direct binding partners. AHSA1 is an abundant cytosolic protein and likely a background hit. There is no functional relationship between AHSA1 and cadherin biology.
GO:0051879 Hsp90 protein binding	IPI PMID:29127155 Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facil...	ACCEPT	Summary: IPI annotation for Hsp90 protein binding based on Woodford et al. 2017, with interaction partner HSP90AA1 (P07900). This study demonstrated that AHSA1 binds to the middle domain of HSP90, and this interaction is enhanced by phosphorylation of Aha1-Y223. AHSA1 competes with TSC1 for HSP90 binding. This is the core molecular interaction of AHSA1. Reason: Hsp90 protein binding is the core molecular interaction of AHSA1, directly demonstrated by biochemical assays including binding competition with TSC1 (PMID:29127155). The interaction is mediated by the N-terminal domain of AHSA1 binding the HSP90 middle domain, and the C-terminal domain stabilizing the HSP90 N-terminal domain dimer. Supporting Evidence: PMID:29127155 The C-terminal domain of Tsc1 (998-1,164 aa) forms a homodimer and binds to both protomers of the Hsp90 middle domain. This ensures inhibition of both subunits of the Hsp90 dimer and prevents the activating co-chaperone Aha1 from binding the middle domain of Hsp90.
GO:0005515 protein binding	IPI PMID:25486457 Middle domain of human Hsp90 isoforms differentially binds A...	MODIFY	Summary: IPI protein binding annotation from UniProt, based on Synoradzki and Bieganowski 2015, which demonstrated that Aha1 interacts preferentially with HSP90alpha (HSP90AA1) over HSP90beta (HSP90AB1), with the distinction depending on the middle domain of HSP90. The WITH column indicates interactions with both HSP90AA1 (P07900) and HSP90AB1 (P08238). This study provides insight into isoform-specific binding of AHSA1 to HSP90. Reason: The interactions detected are with both HSP90 isoforms, which is the core binding function of AHSA1. GO:0005515 is too vague; GO:0051879 (Hsp90 protein binding) is the appropriate specific term. Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:25486457 the Hsp90 co-chaperone Aha1 interacts preferentially with Hsp90alpha. The distinction depends on the middle domain of Hsp90.
GO:0001671 ATPase activator activity	IDA PMID:27353360 The FNIP co-chaperones decelerate the Hsp90 chaperone cycle ...	ACCEPT	Summary: IDA annotation for ATPase activator activity based on Woodford et al. 2016, which showed that AHSA1 competes with the inhibitory FNIP co-chaperones for HSP90 binding, providing a reciprocal regulatory mechanism. The paper demonstrates AHSA1's role as an ATPase activator in the context of competition with FNIP1/FNIP2. Reason: Direct experimental evidence for AHSA1's core function as an HSP90 ATPase activator. The study demonstrates that FNIPs compete with the activating co-chaperone Aha1 for binding to HSP90 (PMID:27353360), confirming AHSA1's role in activating the HSP90 ATPase cycle. Supporting Evidence: PMID:27353360 FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins
GO:0005515 protein binding	IPI PMID:27353360 The FNIP co-chaperones decelerate the Hsp90 chaperone cycle ...	MODIFY	Summary: IPI protein binding annotation from UniProt, based on Woodford et al. 2016. The WITH column indicates interactions with HSP90AA1 (P07900) and FNIP1 (Q8NFG4). The interaction with HSP90AA1 is AHSA1's core function. The interaction with FNIP1 reflects the competitive binding relationship at the HSP90 middle domain rather than a direct AHSA1-FNIP1 interaction. Reason: The primary interaction detected is with HSP90AA1, which is AHSA1's core binding partner. GO:0005515 is too vague. The HSP90 binding is captured by GO:0051879. The interaction with FNIP1 (Q8NFG4) is indirect, reflecting competitive binding to HSP90 rather than direct AHSA1-FNIP1 binding. Proposed replacements: Hsp90 protein binding Supporting Evidence: PMID:27353360 FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins
GO:0051087 protein-folding chaperone binding	IDA PMID:27353360 The FNIP co-chaperones decelerate the Hsp90 chaperone cycle ...	ACCEPT	Summary: IDA annotation for protein-folding chaperone binding based on Woodford et al. 2016, which demonstrated AHSA1 binding to HSP90. Since HSP90 is a protein-folding chaperone, this annotation is accurate and represents AHSA1's core interaction with HSP90. Reason: AHSA1 directly binds HSP90, which is a protein-folding chaperone. This annotation correctly captures AHSA1's core interaction and is supported by direct biochemical evidence (PMID:27353360). Supporting Evidence: PMID:27353360 FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins
GO:0070062 extracellular exosome	HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos...	KEEP AS NON CORE	Summary: HDA annotation for extracellular exosome localization based on Gonzales et al. 2009, a large-scale proteomics study of urinary exosomes that identified 1132 proteins. AHSA1 was detected among these proteins. This is a high-throughput proteomics study, and the presence of AHSA1 in exosomes likely reflects its high abundance as a cytoplasmic protein rather than a specific exosome localization. Reason: AHSA1 was detected in urinary exosomes by mass spectrometry (PMID:19056867). While the detection is real, exosome localization is not a core function or localization of AHSA1. Many abundant cytoplasmic proteins are detected in exosome proteomics studies. This is a minor, non-functional localization.
GO:0001671 ATPase activator activity	IDA PMID:12504007 Activation of the ATPase activity of hsp90 by the stress-reg...	ACCEPT	Summary: IDA annotation for ATPase activator activity based on Panaretou et al. 2002, the foundational study that identified Aha1 as an activator of Hsp90 ATPase. This study demonstrated that Aha1 binds directly to Hsp90 and stimulates its ATPase activity in vitro, and is required for in vivo Hsp90-dependent activation of clients such as v-Src. This is the primary publication establishing AHSA1's core function. Reason: This is the foundational study that defined AHSA1's core function as an HSP90 ATPase activator (PMID:12504007). Direct experimental evidence including in vitro ATPase assays and in vivo client activation assays. Supporting Evidence: PMID:12504007 A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for the in vivo Hsp90-dependent activation of clients such as v-Src, implicating them as cochaperones of the Hsp90 system. In vitro, Aha1 and its shorter homolog, Hch1, stimulate the inherent ATPase activity of yeast and human Hsp90.
GO:0051087 protein-folding chaperone binding	IDA PMID:12504007 Activation of the ATPase activity of hsp90 by the stress-reg...	ACCEPT	Summary: IDA annotation for protein-folding chaperone binding based on Panaretou et al. 2002, which demonstrated that Aha1 binds directly to Hsp90. Since HSP90 is a protein-folding chaperone, this annotation correctly captures the core binding interaction of AHSA1. Reason: AHSA1 directly binds HSP90, a protein-folding chaperone, as demonstrated by the foundational study (PMID:12504007). This is a core interaction of AHSA1. Supporting Evidence: PMID:12504007 A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90

Core Functions

AHSA1 (AHA1) is the most potent known stimulator of HSP90 ATP hydrolysis, functioning as an HSP90 co-chaperone that accelerates the HSP90 conformational/ATPase cycle via conserved NxNNWHW and RKxK motifs in its N-terminal domain, which engage the HSP90 middle domain and catalytic loop (residues 370-390). The C-terminal domain stabilizes the dimerized HSP90 N-terminal domains. AHSA1 can bind asymmetrically (one molecule sufficient for stimulation) or as two molecules per HSP90 dimer with distinct functional outcomes (DOI:10.1016/j.bpj.2023.07.020). A metazoan-specific ICD (aa ~1-20) confers HSP90-independent holdase activity and negatively regulates ATPase stimulation (DOI:10.1038/s44319-024-00193-8). AHSA1 competes with inhibitory co-chaperones TSC1 and FNIP1/FNIP2 for HSP90 binding, and phosphorylation of Aha1-Y223 modulates the competition by increasing affinity for HSP90. AHSA1 influences maturation of diverse HSP90 clients including kinases, steroid receptors, and Dicer1 (affecting microRNA biogenesis) (DOI:10.1093/nar/gkac528).

Molecular Function:

ATPase activator activity

Directly Involved In:

protein folding positive regulation of ATP-dependent activity

Cellular Locations:

cytosol

Supporting Evidence:

PMID:12504007
A ubiquitous family of stress-regulated proteins have been identified (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for the in vivo Hsp90-dependent activation of clients such as v-Src.
PMID:12604615
Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90 5-fold.
PMID:29127155
phosphorylation of Aha1-Y223 increases its affinity for Hsp90 and displaces Tsc1, thereby providing a mechanism for equilibrium between binding of these two co-chaperones to Hsp90.
PMID:27353360
FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90, thereby providing a reciprocal regulatory mechanism for chaperoning of client proteins.

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000044

Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

GO_REF:0000120

Combined Automated Annotation using Multiple IEA Methods

PMID:12504007

Activation of the ATPase activity of hsp90 by the stress-regulated cochaperone aha1.

PMID:12604615

Aha1 binds to the middle domain of Hsp90, contributes to client protein activation, and stimulates the ATPase activity of the molecular chaperone.

PMID:16696853

A yeast 2-hybrid analysis of human GTP cyclohydrolase I protein interactions.

PMID:19056867

Large-scale proteomics and phosphoproteomics of urinary exosomes.

PMID:19875381

A proteomic investigation of ligand-dependent HSP90 complexes reveals CHORDC1 as a novel ADP-dependent HSP90-interacting protein.

PMID:20618441

CHIP participates in protein triage decisions by preferentially ubiquitinating Hsp70-bound substrates.

PMID:25036637

A quantitative chaperone interaction network reveals the architecture of cellular protein homeostasis pathways.

PMID:25468996

E-cadherin interactome complexity and robustness resolved by quantitative proteomics.

PMID:25486457

Middle domain of human Hsp90 isoforms differentially binds Aha1 in human cells and alters Hsp90 activity in yeast.

PMID:27353360

The FNIP co-chaperones decelerate the Hsp90 chaperone cycle and enhance drug binding.

PMID:29127155

Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facilitates folding of kinase and non-kinase clients.

PMID:30382094

Structure and pro-toxic mechanism of the human Hsp90/PPIase/Tau complex.

PMID:33808352

Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Like Activity.

PMID:35271311

OpenCell: Endogenous tagging for the cartography of human cellular organization.

PMID:37486705

Human Aha1's N-terminal extension confers it holdase activity in vitro.

DOI:10.1038/s44319-024-00193-8

Recruitment of Ahsa1 to Hsp90 is regulated by a conserved peptide that inhibits ATPase stimulation.

AHSA1 contains a metazoan-specific intrinsic chaperone domain (ICD, aa ~1-20) that dampens ATPase stimulation by interfering with NxNNWHW function and controls regulated recruitment to HSP90
NxNNWHW and RKxK are conserved motifs central to AHSA1 ATPase stimulatory function; RKxK stabilizes the HSP90 catalytic loop to facilitate ATP hydrolysis

DOI:10.1016/j.bpj.2023.07.020

Aha1 regulates Hsp90's conformation and function in a stoichiometry-dependent way.

One or two Aha1 molecules can bind per HSP90 dimer, with binding stoichiometry differentially regulating HSP90 conformational properties and ATPase activity

DOI:10.1093/nar/gkac528

HSP90 and Aha1 modulate microRNA maturation through promoting the folding of Dicer1.

HSP90 and AHA1 modulate microRNA maturation by promoting folding and stability of Dicer1; AHA1 depletion reduces Dicer1 protein and mature miRNA levels

DOI:10.1186/s13046-021-02220-1

AHSA1 is a promising therapeutic target for cellular proliferation and proteasome inhibitor resistance in multiple myeloma.

AHSA1 binds bufalin at K137; the inhibitor KU-177 targets the same site, reduces AHSA1-HSP90A interaction, and reverses proteasome inhibitor resistance in multiple myeloma

DOI:10.3389/fnmol.2024.1509280

The role of Aha1 in cancer and neurodegeneration.

Review framing Aha1 dysregulation as a driver of disease phenotypes in cancer, cystic fibrosis, and neurodegeneration via HSP90 proteostasis network imbalance
HSP90/Aha1 protein-protein interaction interface proposed as a therapeutic target with potential to spare essential baseline HSP90 functions

DOI:10.1038/s41580-023-00640-9

Structural and functional complexity of HSP90 in cellular homeostasis and disease.

Authoritative review noting co-chaperone regulation by AHA1 contributes to HSP90 functional complexity in disease and discusses chemical inhibition of the Aha1-HSP90 complex

📚 Additional Documentation

Deep Research Falcon

(AHSA1-deep-research-falcon.md)

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organism_full: Homo sapiens (Human).
protein_family: Belongs to the AHA1 family. .
protein_domains: Aha1. (IPR036338); AHSA1-like_N. (IPR015310); ASHA1/2-like_C. (IPR013538);
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: O95433
Protein Description: RecName: Full=Activator of 90 kDa heat shock protein ATPase homolog 1; Short=AHA1; AltName: Full=p38;
Gene Information: Name=AHSA1; Synonyms=C14orf3; ORFNames=HSPC322;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the AHA1 family. .
Key Domains: Aha1. (IPR036338); AHSA1-like_N. (IPR015310); ASHA1/2-like_C. (IPR013538); START-like_dom_sf. (IPR023393); Aha1_N (PF09229)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "AHSA1" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'AHSA1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene AHSA1 (gene ID: AHSA1, UniProt: O95433) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

UniProt Accession: O95433
Protein Description: RecName: Full=Activator of 90 kDa heat shock protein ATPase homolog 1; Short=AHA1; AltName: Full=p38;
Gene Information: Name=AHSA1; Synonyms=C14orf3; ORFNames=HSPC322;
Organism (full): Homo sapiens (Human).
Protein Family: Belongs to the AHA1 family. .
Key Domains: Aha1. (IPR036338); AHSA1-like_N. (IPR015310); ASHA1/2-like_C. (IPR013538); START-like_dom_sf. (IPR023393); Aha1_N (PF09229)

MANDATORY VERIFICATION STEPS:

Check if the gene symbol "AHSA1" matches the protein description above
Verify the organism is correct: Homo sapiens (Human).
Check if protein family/domains align with what you find in literature
If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Please provide a comprehensive research report on the gene AHSA1 (gene ID: AHSA1, UniProt: O95433) in human.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Title: Functional Annotation Research Report — Human AHSA1 (AHA1/p38), UniProt O95433

Publication window emphasized: 2023–2024 where available.

Executive summary

AHSA1 (also called AHA1; historically described as the “Activator/Accelerator of Hsp90 ATPase”) encodes a conserved HSP90 co‑chaperone that is the most potent known stimulator of HSP90 ATP hydrolysis. Its primary molecular function is to bind HSP90 and accelerate progression of the HSP90 conformational/ATPase cycle, thereby modulating maturation of HSP90 client proteins (including kinases and steroid receptors) and affecting proteostasis and disease phenotypes in cancer and neurodegeneration. Multiple recent studies show that human AHSA1 contains an N‑terminal metazoan-specific regulatory element (intrinsic chaperone domain; ICD) that both confers HSP90-independent “holdase” activity and tunes recruitment to HSP90 by dampening the activity of the adjacent ATPase-stimulatory NxNNWHW motif. (hussein2024recruitmentofahsa1 pages 1-2)

Gene/protein identity verification (required disambiguation)

Target identity: AHSA1 (Homo sapiens), UniProt O95433.

2.1 Naming and synonyms

AHSA1 is referred to as Ahsa1 or Aha1 in the chaperone literature and is explicitly described as “Activator/Accelerator of Hsp90 ATPase,” a co‑chaperone that potently stimulates HSP90 ATPase activity. (hussein2024recruitmentofahsa1 pages 1-2)

A 2024 review likewise defines Aha1 as “Activator of Hsp90 ATPase activity homolog 1” and describes it as the most potent accelerator of HSP90 ATPase. (blagg2024theroleof pages 1-2)

2.2 Family/domain architecture consistent with UniProt O95433

Human AHSA1 is described as a two-domain protein (N‑terminal domain and C‑terminal domain) connected by a flexible linker; both domains are required for full stimulation of HSP90 ATPase. (hussein2025decipheringahsa1a pages 37-41)

Key conserved motifs in the N‑terminal domain are repeatedly identified across Aha-type co‑chaperones, including (i) NxNNWHW (central for ATPase stimulation) and (ii) RKxK (important for catalytic-loop regulation). (hussein2024recruitmentofahsa1 pages 1-2, hussein2025decipheringahsa1a pages 37-41)

A mechanistic synthesis explicitly connects the AlphaFold model identifier AF-O95433-F1 to human AHSA1, supporting that the literature entity is the UniProt O95433 protein. (hussein2025decipheringahsa1a pages 37-41)

A supporting schematic/sequence alignment from a 2024 primary paper visually shows AHSA1 domain architecture and the conserved ICD (aa 1–20), NxNNWHW, and RKxK motifs. (hussein2024recruitmentofahsa1 media 467b9fc0)

Key concepts and definitions (current understanding)

3.1 HSP90 chaperone cycle and co-chaperone control

HSP90 is an abundant chaperone whose ATP-binding and ATP-hydrolysis-driven conformational changes support folding/maturation of a large client set (>400 clients). (blagg2024theroleof pages 1-2)

Co‑chaperones regulate the timing and direction of HSP90 conformational transitions; AHA1/AHSA1 is distinctive in that it markedly accelerates HSP90 ATP hydrolysis. (mondol2023aha1regulateshsp90’s pages 1-2)

3.2 Primary molecular function of AHSA1: stimulation of HSP90 ATPase

Definition: AHSA1/AHA1 is the HSP90 co‑chaperone that “potently stimulates” HSP90 ATPase activity by promoting transition to the closed, N‑terminally dimerized state and by engaging catalytic elements of HSP90. (liu2022hsp90andaha1 pages 1-2, hussein2024recruitmentofahsa1 pages 1-2)

Mechanistic model: AHA1 binds HSP90’s middle domain and accelerates ATP hydrolysis, likely via interaction with the HSP90 catalytic loop (residues 370–390). (blagg2024theroleof pages 1-2)

3.3 Functional sequence features (motifs) and how they map to mechanism

NxNNWHW motif: A conserved motif in the AHSA1 N‑domain that drives ATPase stimulation; deletion of N‑terminal residues encompassing this motif reduces ATPase stimulation and alters ATP-binding kinetics (lower apparent Km for ATP in the cited work). (hussein2025decipheringahsa1aa pages 41-45)

RKxK motif: Proposed to stabilize the HSP90 catalytic loop and position a critical arginine to facilitate ATP binding/hydrolysis; mutations in RKxK drastically impair AHSA1’s ability to enhance HSP90 ATPase activity. (hussein2025decipheringahsa1a pages 41-45, hussein2025decipheringahsa1aa pages 41-45)

Asymmetry: AHSA1 binds HSP90 asymmetrically in structural/biochemical studies; a single AHSA1 molecule can be sufficient to stimulate ATPase activity, consistent with ordered hydrolysis across the HSP90 dimer. (hussein2025decipheringahsa1aa pages 41-45)

3.4 Autonomous (HSP90-independent) chaperone/holdase activity

Recent work supports that AHSA1 has HSP90-independent holdase activity requiring its extreme N‑terminus (first ~16–27 residues depending on assay), where it can prevent aggregation of model substrates (e.g., luciferase/rhodanese) and bind aggregation-prone/disordered proteins such as α‑synuclein. (hussein2025decipheringahsa1aa pages 41-45)

In cells, an HSP90-binding-defective AHSA1 mutant (E67K) can still rescue levels of certain client proteins, supporting an HSP90-independent component to AHSA1 function. (hussein2025decipheringahsa1aa pages 41-45)

Subcellular localization and where AHSA1 acts

The retrieved primary and review excerpts largely discuss AHSA1 as part of the cytosolic HSP90 machinery (consistent with HSP90’s essential cytosolic role); explicit compartmental localization statements for AHSA1 itself were not present in the evidence snippets captured here. Therefore, localization here is inferred from its demonstrated biochemical and cellular interactions with HSP90 and cytosolic client maturation assays rather than from direct localization experiments. (liu2022hsp90andaha1 pages 1-2, hussein2024recruitmentofahsa1 pages 1-2)

Pathways and biological processes involving AHSA1 (mechanistic framing)

5.1 Proteostasis and client maturation

AHSA1 regulates late-stage folding/activation steps by accelerating HSP90 ATPase cycling, thereby influencing client maturation/release; this mechanism underpins downstream effects across diverse client classes (kinases, steroid receptors) and disease-associated assemblies (“heteroprotein complexes” centered on HSP90). (blagg2024theroleof pages 1-2, mondol2023aha1regulateshsp90’s pages 1-2)

5.2 microRNA biogenesis via Dicer1 folding (example of a defined client consequence)

A primary study (Nucleic Acids Research, 2022) demonstrates that HSP90 and AHA1 modulate microRNA maturation by promoting folding/stability of Dicer1, with genetic depletion of Aha1 reducing Dicer1 protein and mature miRNAs, and rescue depending on AHA1 functional regions. (liu2022hsp90andaha1 pages 1-2)

Recent developments and latest research (prioritizing 2023–2024)

6.1 2024: Intrinsic chaperone domain (ICD) as a metazoan-specific regulator of recruitment and ATPase stimulation

A 2024 EMBO Reports study identifies an additional conserved ~20-aa peptide in metazoan AHSA1 immediately preceding NxNNWHW, termed the intrinsic chaperone domain (ICD). The ICD diminishes HSP90 ATPase stimulation by interfering with NxNNWHW function and also controls regulated recruitment/interaction with HSP90 in cells; its deletion can cause loss of interaction with HSP90 and the glucocorticoid receptor. (hussein2024recruitmentofahsa1 pages 1-2)

These domain/motif relationships are supported visually by the paper’s schematic and sequence alignment showing ICD (aa 1–20), NxNNWHW, and RKxK. (hussein2024recruitmentofahsa1 media 467b9fc0)

6.2 2023: Stoichiometry-dependent regulation of HSP90 by Aha1

A 2023 Biophysical Journal study reports that one or two Aha1 molecules can bind per HSP90 dimer and that this binding stoichiometry differentially regulates HSP90 properties; in their system, two Aha1 molecules coordinate to strongly stimulate ATPase activity and promote unfolding of the HSP90 middle domain. (mondol2023aha1regulateshsp90’s pages 1-2)

6.3 2023–2024: Therapeutic repositioning toward co-chaperone interfaces

A 2024 review argues that historical N-terminal HSP90 ATP-pocket inhibitors were limited by broad client degradation and dose-limiting toxicities, motivating interest in targeting HSP90–co‑chaperone protein–protein interfaces such as HSP90/Aha1. (blagg2024theroleof pages 1-2)

A 2023 Nature Reviews Molecular Cell Biology article likewise notes the structural/functional complexity of HSP90 in disease and discusses small-molecule approaches aimed at modulating HSP90-dependent folding by targeting Aha1, including compounds reported to inhibit the Aha1–HSP90 complex. (chiosis2023structuralandfunctional pages 9-10)

Current applications and real-world implementations

7.1 Drug discovery: direct AHSA1 targeting and inhibitors

Multiple myeloma: A 2022 translational study identifies AHSA1 as a unique target of the natural product bufalin using a proteome microarray; it maps the binding site to AHSA1 lysine 137 (K137). Mutation of K137 reduced AHSA1–HSP90A interaction and suppressed downstream effects. The same study identifies KU-177 as an AHSA1-selective inhibitor that targets the same site and reports that bufalin and KU-177 reduce proliferation of MRD-positive cells from primary and recurrent MM patient samples; KU-177 also reverses AHSA1-driven proteasome inhibitor resistance. (gu2022ahsa1isa pages 1-2)

7.2 Drug discovery strategy: disrupting HSP90/AHA1 protein–protein interaction

A 2024 review summarizes accumulating evidence that disease states can feature HSP90-centered heteroprotein complexes containing Aha1, and proposes that small-molecule disruptors of the HSP90/Aha1 complex can be effective in models of cancer and neurodegeneration, potentially sparing essential baseline HSP90 functions compared with pan-HSP90 inhibitors. (blagg2024theroleof pages 1-2)

7.3 Cystic fibrosis (CFTR proteostasis) as a mechanistic application area

AHSA1 silencing enhances CFTR folding and can restore function of the disease-associated ΔF508 CFTR variant, positioning AHSA1 as a potential proteostasis-modulating target in cystic fibrosis. (hussein2024recruitmentofahsa1 pages 1-2)

7.4 Neurodegeneration/tauopathy mechanism-based application area

Overexpression of AHSA1 in mouse brains has been reported to increase Tau aggregation and neuronal cell death, and reviews link Aha1-dependent client maturation to tau phosphorylation pathways. These data underpin the rationale for targeting the HSP90/Aha1 axis in tauopathies. (hussein2024recruitmentofahsa1 pages 1-2, blagg2024theroleof pages 1-2)

7.5 Biomarker applications in cancer prognosis and therapy response

Multiple myeloma: elevated AHSA1 expression is associated with relapse and poor outcomes (supporting potential biomarker utility) and is mechanistically tied to activation of CDK6 and PSMD2 via HSP90A co-chaperoning. (gu2022ahsa1isa pages 1-2)

Lung adenocarcinoma: a 2024 prognostic modeling paper reports AHSA1 as part of an immune-escape-related prognostic signature and provides hazard ratios/AUCs (see statistics section). (jia2024theprognosticvalue pages 11-11, jia2024theprognosticvalue pages 8-9)

Expert opinions and authoritative analyses (selected)

8.1 AHSA1 as a disease driver via proteostasis network imbalance

A 2024 expert review frames Aha1 dysregulation as a driver that can push HSP90 complexes toward disease phenotypes in cystic fibrosis, cancer, and neurodegeneration, and identifies the HSP90/Aha1 interface as a promising therapeutic focus. (blagg2024theroleof pages 1-2)

8.2 AHSA1 as an actionable node within the broader HSP90 disease network

A high-impact 2023 Nature Reviews Molecular Cell Biology review highlights that co-chaperone regulation (exemplified by AHA1) contributes to HSP90 functional complexity in disease and notes efforts to chemically inhibit the Aha1–HSP90 complex, consistent with a growing consensus that co-chaperone- and state-selective strategies may improve therapeutic windows. (chiosis2023structuralandfunctional pages 9-10)

Relevant statistics and quantitative data (recent and/or high-authority)

9.1 HSP90 system-level quantities

Client breadth: HSP90 is described as stabilizing/maturing >400 client proteins. (blagg2024theroleof pages 1-2)

Abundance: HSP90 is estimated at ~1–2% of total cellular protein in healthy cells and ~4–6% in stressed/transformed cells. (blagg2024theroleof pages 1-2)

Kinome dependence: ~60% of the human kinome is described as dependent on HSP90/Cdc37 complexes for maturation and stability (contextualizing why HSP90-cycle modulation by Aha1 can have strong signaling consequences). (blagg2024theroleof pages 1-2)

9.2 AHSA1 mechanistic quantities

Cellular abundance ratio: AHSA1/Aha1 is reported ~30-fold less abundant than HSP90. (hussein2025decipheringahsa1aa pages 41-45)

Stoichiometry/asymmetry: a single AHSA1 molecule can be sufficient to stimulate HSP90 ATPase activity; structural studies show asymmetric binding with one AHSA1 NTD resolved in a semi-hydrolyzed state. (hussein2025decipheringahsa1aa pages 41-45)

Binding stoichiometry: 2023 work reports one or two Aha1 molecules can bind per HSP90 dimer and that stoichiometry changes functional outcomes. (mondol2023aha1regulateshsp90’s pages 1-2)

HSP90 catalytic loop residues implicated in Aha1 effect: residues 370–390 are identified as the catalytic-loop region believed to interact functionally with Aha1 to accelerate ATP hydrolysis. (blagg2024theroleof pages 1-2)

Mapped inhibitor binding residue: AHSA1-K137 is identified as a specific binding site for bufalin and KU-177 in a multiple myeloma study. (gu2022ahsa1isa pages 1-2)

9.3 Cancer prognosis metrics (2024)

In a 2024 LUAD prognostic study, AHSA1-associated statistics shown include highly significant p-values (e.g., P=9.4e−25 and P=6.9e−4) and hazard ratios displayed in the excerpt (HR=0.46, 95% CI 0.31–0.69; and HR=5.98, 95% CI 1.85–19.38), alongside time-dependent ROC AUCs for survival prediction at 365/1095/1825 days including 0.69 (0.62–0.76), 0.70 (0.61–0.78), and 0.70 (0.49–0.90) in one panel. (jia2024theprognosticvalue pages 11-11)

Separately in the same 2024 paper, time-point ROC AUCs reported for a prognostic model include 0.70 (0.62–0.77) at 365 days, 0.63 (0.57–0.70) at 1,095 days, and 0.66 (0.58–0.74) at 1,825 days. (jia2024theprognosticvalue pages 8-9)

Limitations and open questions (based on retrieved evidence)

10.1 Context dependence of client outcomes

Even while AHSA1’s core biochemical activity (HSP90 ATPase acceleration) is robust, downstream effects on specific client proteins can be inconsistent across studies and model systems, implying that functional annotation should distinguish (i) the conserved mechanistic role in the HSP90 cycle from (ii) context-dependent phenotypes. (hussein2025decipheringahsa1a pages 41-45)

10.2 Localization evidence gap in the retrieved excerpts

Direct subcellular localization evidence for AHSA1 (e.g., microscopy-based localization under basal vs stress conditions) was not present in the excerpts retrieved here, and should be addressed with dedicated localization studies or curated databases if needed. (liu2022hsp90andaha1 pages 1-2, hussein2024recruitmentofahsa1 pages 1-2)

Selected key references (with URLs and publication dates)

• Hussein SK et al. “Recruitment of Ahsa1 to Hsp90 is regulated by a conserved peptide that inhibits ATPase stimulation.” EMBO Reports. Published Jun 2024. https://doi.org/10.1038/s44319-024-00193-8 (hussein2024recruitmentofahsa1 pages 1-2)

• Blagg BSJ, Catalfano KC. “The role of Aha1 in cancer and neurodegeneration.” Frontiers in Molecular Neuroscience. Published Dec 2024. https://doi.org/10.3389/fnmol.2024.1509280 (blagg2024theroleof pages 1-2)

• Chiosis G et al. “Structural and functional complexity of HSP90 in cellular homeostasis and disease.” Nature Reviews Molecular Cell Biology. Published Jul 2023. https://doi.org/10.1038/s41580-023-00640-9 (chiosis2023structuralandfunctional pages 9-10)

• Mondol T et al. “Aha1 regulates Hsp90’s conformation and function in a stoichiometry-dependent way.” Biophysical Journal. Published Sep 2023. https://doi.org/10.1016/j.bpj.2023.07.020 (mondol2023aha1regulateshsp90’s pages 1-2)

• Liu X et al. “HSP90 and Aha1 modulate microRNA maturation through promoting the folding of Dicer1.” Nucleic Acids Research. Published Jun 2022. https://doi.org/10.1093/nar/gkac528 (liu2022hsp90andaha1 pages 1-2)

• Gu C et al. “AHSA1 is a promising therapeutic target for cellular proliferation and proteasome inhibitor resistance in multiple myeloma.” J Exp Clin Cancer Res. Published Jan 2022. https://doi.org/10.1186/s13046-021-02220-1 (gu2022ahsa1isa pages 1-2)

• Jia HR et al. “The prognostic value of immune escape-related genes in lung adenocarcinoma.” Translational Cancer Research. Published Jun 2024. https://doi.org/10.21037/tcr-23-2295 (jia2024theprognosticvalue pages 11-11)

Recommended functional annotation (concise)

Molecular function: AHSA1 (AHA1) is an HSP90 co‑chaperone that accelerates HSP90 ATPase activity and conformational cycling, likely via contacts to HSP90’s middle domain and catalytic loop; AHSA1 contains conserved NxNNWHW and RKxK motifs required for maximal ATPase stimulation and can act asymmetrically with substoichiometric abundance relative to HSP90. (hussein2024recruitmentofahsa1 pages 1-2, hussein2025decipheringahsa1aa pages 41-45, blagg2024theroleof pages 1-2)

Additional function: AHSA1 harbors an N‑terminal metazoan ICD (aa ~1–20) that confers HSP90-independent holdase activity and negatively regulates ATPase stimulation by interfering with NxNNWHW; this region also controls regulated recruitment to HSP90 in cells. (hussein2024recruitmentofahsa1 pages 1-2)

Disease relevance and applications: AHSA1 is implicated in CFTR proteostasis, tauopathy mechanisms, and cancer progression/therapy response, and is being explored both as a drug target (direct inhibitors such as KU‑177/bufalin-binding site K137 in MM; and PPI disruptors of HSP90/Aha1) and as a prognostic/therapeutic biomarker in cancer cohorts. (gu2022ahsa1isa pages 1-2, blagg2024theroleof pages 1-2, jia2024theprognosticvalue pages 11-11)

References

(hussein2024recruitmentofahsa1 pages 1-2): Solomon K Hussein, Rakesh Bhat, Michael Overduin, and Paul LaPointe. Recruitment of ahsa1 to hsp90 is regulated by a conserved peptide that inhibits atpase stimulation. EMBO Reports, 25:3532-3546, Jun 2024. URL: https://doi.org/10.1038/s44319-024-00193-8, doi:10.1038/s44319-024-00193-8. This article has 7 citations and is from a highest quality peer-reviewed journal.
(blagg2024theroleof pages 1-2): Brian S. J. Blagg and Kevin C. Catalfano. The role of aha1 in cancer and neurodegeneration. Frontiers in Molecular Neuroscience, Dec 2024. URL: https://doi.org/10.3389/fnmol.2024.1509280, doi:10.3389/fnmol.2024.1509280. This article has 1 citations.
(hussein2025decipheringahsa1a pages 37-41): SK Hussein. Deciphering ahsa1: a biochemical and proteome-wide analysis of its role in chaperone regulation. Unknown journal, 2025.
(hussein2024recruitmentofahsa1 media 467b9fc0): Solomon K Hussein, Rakesh Bhat, Michael Overduin, and Paul LaPointe. Recruitment of ahsa1 to hsp90 is regulated by a conserved peptide that inhibits atpase stimulation. EMBO Reports, 25:3532-3546, Jun 2024. URL: https://doi.org/10.1038/s44319-024-00193-8, doi:10.1038/s44319-024-00193-8. This article has 7 citations and is from a highest quality peer-reviewed journal.
(mondol2023aha1regulateshsp90’s pages 1-2): Tanumoy Mondol, Laura-Marie Silbermann, Julia Schimpf, Leonie Vollmar, Bianca Hermann, Katarzyna (Kasia) Tych, and Thorsten Hugel. Aha1 regulates hsp90’s conformation and function in a stoichiometry-dependent way. Biophysical Journal, 122:3458-3468, Sep 2023. URL: https://doi.org/10.1016/j.bpj.2023.07.020, doi:10.1016/j.bpj.2023.07.020. This article has 20 citations and is from a domain leading peer-reviewed journal.
(liu2022hsp90andaha1 pages 1-2): Xiaochuan Liu, Yen-Yu Yang, and Yinsheng Wang. Hsp90 and aha1 modulate microrna maturation through promoting the folding of dicer1. Nucleic Acids Research, 50:6990-7001, Jun 2022. URL: https://doi.org/10.1093/nar/gkac528, doi:10.1093/nar/gkac528. This article has 11 citations and is from a highest quality peer-reviewed journal.
(hussein2025decipheringahsa1aa pages 41-45): SK Hussein. Deciphering ahsa1: a biochemical and proteome-wide analysis of its role in chaperone regulation. Unknown journal, 2025.
(hussein2025decipheringahsa1a pages 41-45): SK Hussein. Deciphering ahsa1: a biochemical and proteome-wide analysis of its role in chaperone regulation. Unknown journal, 2025.
(chiosis2023structuralandfunctional pages 9-10): Gabriela Chiosis, Chander S. Digwal, Jane B. Trepel, and Len Neckers. Structural and functional complexity of hsp90 in cellular homeostasis and disease. Nature Reviews Molecular Cell Biology, 24:797-815, Jul 2023. URL: https://doi.org/10.1038/s41580-023-00640-9, doi:10.1038/s41580-023-00640-9. This article has 171 citations and is from a domain leading peer-reviewed journal.
(gu2022ahsa1isa pages 1-2): Chunyan Gu, Yajun Wang, Lulin Zhang, Li Qiao, Shanliang Sun, Miaomiao Shao, Xiaozhu Tang, Pinggang Ding, Chao Tang, Yuhao Cao, Yanyan Zhou, Mengjie Guo, Rongfang Wei, Nianguang Li, Yibei Xiao, Jinao Duan, and Ye Yang. Ahsa1 is a promising therapeutic target for cellular proliferation and proteasome inhibitor resistance in multiple myeloma. Journal of Experimental & Clinical Cancer Research : CR, Jan 2022. URL: https://doi.org/10.1186/s13046-021-02220-1, doi:10.1186/s13046-021-02220-1. This article has 36 citations.
(jia2024theprognosticvalue pages 11-11): Hao Ran Jia, Wen Chao Li, and Lin Wu. The prognostic value of immune escape-related genes in lung adenocarcinoma. Translational Cancer Research, 13:2647-2661, Jun 2024. URL: https://doi.org/10.21037/tcr-23-2295, doi:10.21037/tcr-23-2295. This article has 9 citations.
(jia2024theprognosticvalue pages 8-9): Hao Ran Jia, Wen Chao Li, and Lin Wu. The prognostic value of immune escape-related genes in lung adenocarcinoma. Translational Cancer Research, 13:2647-2661, Jun 2024. URL: https://doi.org/10.21037/tcr-23-2295, doi:10.21037/tcr-23-2295. This article has 9 citations.

Citations

blagg2024theroleof pages 1-2
chiosis2023structuralandfunctional pages 9-10
jia2024theprognosticvalue pages 11-11
jia2024theprognosticvalue pages 8-9
https://doi.org/10.1038/s44319-024-00193-8
https://doi.org/10.3389/fnmol.2024.1509280
https://doi.org/10.1038/s41580-023-00640-9
https://doi.org/10.1016/j.bpj.2023.07.020
https://doi.org/10.1093/nar/gkac528
https://doi.org/10.1186/s13046-021-02220-1
https://doi.org/10.21037/tcr-23-2295
https://doi.org/10.1038/s44319-024-00193-8,
https://doi.org/10.3389/fnmol.2024.1509280,
https://doi.org/10.1016/j.bpj.2023.07.020,
https://doi.org/10.1093/nar/gkac528,
https://doi.org/10.1038/s41580-023-00640-9,
https://doi.org/10.1186/s13046-021-02220-1,
https://doi.org/10.21037/tcr-23-2295,

📄 View Raw YAML

id: O95433
gene_symbol: AHSA1
product_type: PROTEIN
status: IN_PROGRESS
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  Activator of 90 kDa heat shock protein ATPase homolog 1 (AHA1/AHSA1) is the
  most potent known stimulator of HSP90 ATP hydrolysis, functioning as an HSP90
  co-chaperone that accelerates the HSP90 conformational/ATPase cycle. AHSA1 is a
  two-domain protein (N-terminal and C-terminal) connected by a flexible linker;
  the N-terminal domain binds the HSP90 middle domain via conserved NxNNWHW and
  RKxK motifs (required for maximal ATPase stimulation and catalytic-loop
  positioning), while the C-terminal domain stabilizes the dimerized HSP90
  N-terminal domains (DOI:10.1038/s44319-024-00193-8). AHSA1 is ~30-fold less
  abundant than HSP90 and can act asymmetrically, with a single AHSA1 molecule
  sufficient to stimulate HSP90 ATPase activity. One or two AHSA1 molecules can
  bind per HSP90 dimer, with stoichiometry differentially regulating HSP90
  properties (DOI:10.1016/j.bpj.2023.07.020). AHSA1 competes with inhibitory
  co-chaperones FNIP1 and TSC1 for HSP90 binding, providing reciprocal regulation
  of client protein chaperoning. A metazoan-specific N-terminal intrinsic
  chaperone domain (ICD, aa ~1-20) both confers HSP90-independent holdase activity
  (preventing aggregation of model substrates) and dampens ATPase stimulation by
  interfering with NxNNWHW function, also controlling regulated recruitment to
  HSP90 in cells (DOI:10.1038/s44319-024-00193-8). AHSA1 modulates maturation of
  HSP90 clients including kinases, steroid receptors, and Dicer1 (affecting
  microRNA biogenesis) (DOI:10.1093/nar/gkac528). AHSA1 is being explored as a
  therapeutic target in multiple myeloma (bufalin/KU-177 binding at K137), cystic
  fibrosis (CFTR proteostasis), and neurodegeneration (tauopathies)
  (DOI:10.1186/s13046-021-02220-1, DOI:10.3389/fnmol.2024.1509280).
alternative_products:
- name: '1'
  id: O95433-1
- name: '2'
  id: O95433-2
  sequence_note: VSP_055797
existing_annotations:
- term:
    id: GO:0001671
    label: ATPase activator activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      IBA annotation for ATPase activator activity based on phylogenetic inference
      (PANTHER). This is the core molecular function of AHSA1. Aha1 was identified
      as the activator of Hsp90 ATPase in yeast and human, stimulating ATPase
      activity 5-fold in vitro (PMID:12604615) and confirmed across multiple studies
      (PMID:12504007, PMID:29127155, PMID:27353360). The IBA annotation is well
      supported by conserved function across yeast (S. cerevisiae Aha1, Hch1) and
      S. pombe orthologs included in the PANTHER family.
    action: ACCEPT
    reason: >-
      ATPase activator activity is the primary, evolutionarily conserved molecular
      function of AHSA1. Multiple independent experimental studies confirm this
      function (PMID:12504007, PMID:12604615, PMID:27353360, PMID:29127155), and
      the IBA phylogenetic inference is sound, supported by orthologs in yeast.
    supported_by:
      - reference_id: PMID:12504007
        supporting_text: >-
          A ubiquitous family of stress-regulated proteins have been identified
          (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are
          required for the in vivo Hsp90-dependent activation of clients such as
          v-Src, implicating them as cochaperones of the Hsp90 system.
      - reference_id: PMID:12604615
        supporting_text: >-
          Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90
          5-fold
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      IBA annotation for cytosol localization based on phylogenetic inference.
      AHSA1 is predominantly cytosolic, consistent with its role as an HSP90
      co-chaperone. UniProt records cytoplasm/cytosol as the primary subcellular
      location (PMID:11554768), and HPA immunofluorescence data supports
      cytosol localization (GO_REF:0000052).
    action: ACCEPT
    reason: >-
      Cytosol is the well-established primary localization of AHSA1, consistent
      with its function as an HSP90 co-chaperone in the cytoplasm. Supported by
      IDA from HPA and UniProt subcellular location annotation.
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      IBA annotation for protein folding based on phylogenetic inference. AHSA1
      participates in protein folding indirectly by stimulating the HSP90 ATPase
      cycle, which is required for HSP90-dependent client protein maturation. Yeast
      Aha1 deletion impairs activation of the Hsp90 client v-Src (PMID:12504007,
      PMID:12604615), and human AHSA1 competes with inhibitory co-chaperones to
      regulate HSP90 client chaperoning (PMID:27353360, PMID:29127155). The term
      is at an appropriate level of generality for the biological process.
    action: ACCEPT
    reason: >-
      Protein folding is an appropriate biological process for AHSA1, which
      participates in HSP90-mediated client protein folding/maturation by
      stimulating the HSP90 chaperone cycle. The IBA inference is supported by
      experimental evidence in yeast and human showing that Aha1 is required for
      efficient activation of Hsp90 clients.
    supported_by:
      - reference_id: PMID:12604615
        supporting_text: >-
          Aha1 and Hch1 contributed to efficient activation of the heterologous
          Hsp90 client protein v-Src
      - reference_id: PMID:29127155
        supporting_text: >-
          Tsc1 is a new co-chaperone for Hsp90 that inhibits its ATPase activity
          ... prevents the activating co-chaperone Aha1 from binding the middle
          domain of Hsp90
- term:
    id: GO:0001671
    label: ATPase activator activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation for ATPase activator activity inferred from InterPro domain
      IPR015310 (AHSA1-like_N). This is consistent with the core function of
      AHSA1 and is well supported by experimental evidence across multiple
      publications.
    action: ACCEPT
    reason: >-
      The InterPro-to-GO mapping is correct. The AHSA1 N-terminal domain
      (Aha1_N, Pfam PF09229) is the domain responsible for binding the HSP90
      middle domain and stimulating ATPase activity (PMID:12604615). This IEA
      annotation is redundant with the IBA and IDA annotations but correctly
      captures the function.
- term:
    id: GO:0005783
    label: endoplasmic reticulum
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: >-
      IEA annotation for ER localization mapped from UniProt subcellular location.
      UniProt notes that AHSA1 "may transiently interact with the endoplasmic
      reticulum" based on PMID:11554768 (Sevier and Machamer 2001), which
      identified AHSA1 (then called p38) as interacting with VSV G glycoprotein.
      The ER localization is likely a minor or transient association rather than
      a primary localization.
    action: KEEP_AS_NON_CORE
    reason: >-
      The ER association is based on early work showing AHSA1 interacts with VSV
      G glycoprotein and may transiently associate with the ER (PMID:11554768).
      This is not a core localization for AHSA1, whose primary function occurs in
      the cytosol. The UniProt annotation itself qualifies this as transient.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: >-
      IEA annotation for cytosol localization mapped from UniProt subcellular
      location vocabulary. Consistent with the primary localization of AHSA1
      and supported by IDA (HPA) and IBA evidence.
    action: ACCEPT
    reason: >-
      Cytosol is the well-established primary localization of AHSA1. This IEA
      annotation is consistent with the IDA and IBA annotations for the same
      term and is correctly mapped from UniProt subcellular location.
- term:
    id: GO:0051087
    label: protein-folding chaperone binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation for protein-folding chaperone binding inferred from InterPro
      domain IPR015310. AHSA1 directly binds HSP90, which is a protein-folding
      chaperone, so this annotation is accurate. The binding is well characterized:
      the N-terminal domain of AHSA1 binds the middle domain of HSP90
      (PMID:12604615), and the C-terminal domain stabilizes the dimerized N-terminal
      domains of HSP90 (PMID:33808352).
    action: ACCEPT
    reason: >-
      AHSA1 is a well-established HSP90-binding co-chaperone. The InterPro-to-GO
      mapping correctly captures the chaperone binding function. This is supported
      by multiple experimental studies (PMID:12504007, PMID:12604615, PMID:27353360,
      PMID:29127155).
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:12604615
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Lotz et al. 2003, which
      demonstrated that Aha1 binds to the middle domain of Hsp90 using biochemical
      approaches including co-immunoprecipitation and direct binding assays. The
      WITH column indicates interaction with HSP90AA1 (P07900). While the
      interaction is real and well-characterized, "protein binding" is uninformative
      since the specific interaction is better captured by GO:0051879 (Hsp90 protein
      binding).
    action: MODIFY
    reason: >-
      The underlying interaction with HSP90 is the core function of AHSA1 and is
      well-documented (PMID:12604615). However, GO:0005515 (protein binding) is
      too vague. The interaction is more precisely captured by GO:0051879 (Hsp90
      protein binding), which is already annotated from other evidence.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:12604615
        supporting_text: >-
          We have identified Aha1 (activator of Hsp90 ATPase) and its relative
          Hch1 (high copy Hsp90 suppressor) as binding partners of Hsp90 in
          Saccharomyces cerevisiae. By using genetic and biochemical approaches,
          the middle domain of Hsp90 (amino acids 272-617) was found to mediate
          the interaction with Aha1 and Hch1.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16696853
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Swick and Kapatos 2006,
      which identified AHSA1 as interacting with GCH1 (GTP cyclohydrolase I,
      P30793) using yeast two-hybrid screen and validated by GST pull-down assay.
      The authors note that "the physiological relevance of the Aha1-GCH1
      interaction requires further study" and speculate it may recruit GCH1 into
      the eNOS/Hsp90 complex. This is a secondary, possibly indirect interaction.
    action: KEEP_AS_NON_CORE
    reason: >-
      The interaction with GCH1 was detected by yeast two-hybrid and validated by
      GST pull-down (PMID:16696853), but its physiological relevance is uncertain.
      The authors themselves state the relevance "requires further study." This
      may reflect indirect bridging through HSP90 rather than a direct functional
      interaction. Keeping as non-core since the interaction was validated but
      may not represent a core AHSA1 function.
    supported_by:
      - reference_id: PMID:16696853
        supporting_text: >-
          The interaction of one of these clones, Activator of Heat Shock 90 kDa
          Protein (Aha1), with GCH1 was validated by glutathione-s-transferase
          (GST) pull-down assay. Although the physiological relevance of the
          Aha1-GCH1 interaction requires further study
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19875381
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Gano and Simon 2010,
      which identified AHSA1 as a component of HSP90 complexes by tandem affinity
      purification and LC-MS/MS. The WITH column indicates interaction with
      HSP90AA1 (P07900). This is a high-throughput proteomics study that
      characterized the nucleotide-dependent HSP90 interactome. The AHSA1-HSP90
      interaction is well established.
    action: MODIFY
    reason: >-
      The interaction with HSP90 is the core function of AHSA1. GO:0005515
      (protein binding) is uninformative; the specific interaction is better
      captured by GO:0051879 (Hsp90 protein binding).
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:19875381
        supporting_text: >-
          We identified 52 known and novel components of HSP90 complexes that are
          regulated by these ligands, including several co-chaperones.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:20618441
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Stankiewicz et al.
      2010. The WITH column indicates interaction with HSP90AB1 (P08238). This
      study investigated CHIP-mediated ubiquitination and tested the influence
      of Aha1 on HSP90 ATPase activity in the context of CHIP. AHSA1 is mentioned
      as an HSP90 co-chaperone tested for its effect on CHIP function. The
      interaction with HSP90 is incidental to the main focus of the paper.
    action: MODIFY
    reason: >-
      The interaction detected is between AHSA1 and HSP90AB1, which is the core
      binding partner. GO:0005515 is too vague; GO:0051879 (Hsp90 protein binding)
      is the appropriate specific term.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:20618441
        supporting_text: >-
          CHIP did not influence the ATPase cycle of Hsp90 in the absence of
          co-chaperones or in the presence of the Hsp90 cochaperones Aha1 or p23.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25036637
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Taipale et al. 2014,
      a large-scale quantitative chaperone interaction network study. The WITH
      column indicates interaction with HSP90AB1 (P08238). This is a systematic
      proteomics study mapping chaperone-client interactions. AHSA1 was identified
      as part of the HSP90 interaction network.
    action: MODIFY
    reason: >-
      The interaction detected is with HSP90AB1. GO:0005515 is too vague for
      AHSA1, whose binding to HSP90 is its core function. GO:0051879 (Hsp90
      protein binding) is the appropriate specific term.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:30382094
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Oroz et al. 2018,
      which determined the solution structure of the human Hsp90/FKBP51/Tau
      complex. The WITH column indicates interaction with HSP90AB1 (P08238).
      AHSA1 is mentioned as restoring HSP90 ATPase activity that was decreased
      by FKBP51 binding. This is a functional assay demonstrating AHSA1's role
      as an HSP90 ATPase activator.
    action: MODIFY
    reason: >-
      The interaction is with HSP90AB1 and reflects AHSA1's core function as an
      HSP90 co-chaperone. GO:0005515 is uninformative; GO:0051879 (Hsp90 protein
      binding) properly captures this interaction.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:30382094
        supporting_text: >-
          Hsp90 ATPase activity was restored by the addition of Aha1, a strong
          enhancer of Hsp90 ATPase activity through compaction of the Hsp90
          conformation
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:35271311
  review:
    summary: >-
      IPI protein binding annotation from IntAct, based on Cho et al. 2022
      (OpenCell project), which used endogenous tagging and mass spectrometry
      to map protein-protein interactions across the human proteome. The WITH
      column indicates interactions with HSP90AA1 (P07900) and HSP90AB1 (P08238).
      This is a large-scale systematic study confirming the known AHSA1-HSP90
      interaction.
    action: MODIFY
    reason: >-
      The interactions detected are with both HSP90 isoforms (HSP90AA1 and
      HSP90AB1), consistent with AHSA1's core function. GO:0005515 is too
      vague; GO:0051879 (Hsp90 protein binding) properly captures this.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
- term:
    id: GO:0051879
    label: Hsp90 protein binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      IEA annotation for Hsp90 protein binding from combined automated methods
      (ARBA, mouse ortholog, Ensembl). This is well-supported by extensive
      experimental evidence: AHSA1 directly binds HSP90AA1 and HSP90AB1
      (PMID:12504007, PMID:12604615, PMID:25486457, PMID:27353360, PMID:29127155).
      HSP90 binding is the core molecular interaction of AHSA1.
    action: ACCEPT
    reason: >-
      Hsp90 protein binding is the core molecular interaction of AHSA1. The
      automated annotation is correct and supported by multiple independent
      experimental studies. AHSA1 binds both HSP90 isoforms through its
      N-terminal and C-terminal domains.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  review:
    summary: >-
      IDA annotation for cytosol localization from Human Protein Atlas (HPA)
      based on curation of immunofluorescence data. This is consistent with
      AHSA1's role as a cytosolic HSP90 co-chaperone and is supported by
      UniProt subcellular location annotation (PMID:11554768).
    action: ACCEPT
    reason: >-
      Cytosol is the primary localization of AHSA1, directly demonstrated by
      immunofluorescence (HPA) and consistent with its function as an HSP90
      co-chaperone in the cytoplasm.
- term:
    id: GO:0036506
    label: maintenance of unfolded protein
  evidence_type: EXP
  original_reference_id: PMID:37486705
  review:
    summary: >-
      EXP annotation for maintenance of unfolded protein from DisProt, based on
      Tang et al. 2023, which demonstrated that the N-terminal extension (M1-R16)
      of human Aha1 confers holdase activity in vitro. The holdase activity
      prevents aggregation of heat-denatured MBP but is abolished by high NaCl
      concentration. This activity is mediated by the N-terminal extension unique
      to higher eukaryote Aha1 proteins and is absent from the conserved core
      domains.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The holdase activity demonstrated in vitro maps to the N-terminal extension
      (M1-R16) unique to higher eukaryotes, not the conserved Aha1 core domains.
      The activity is abolished by high NaCl concentration, suggesting
      electrostatic-driven non-specific interactions (PMID:37486705). No in vivo
      evidence supports this as a physiological function. AHSA1's core function
      is HSP90 ATPase activation, not autonomous chaperone activity.
    supported_by:
      - reference_id: PMID:37486705
        supporting_text: >-
          the highly conserved N-terminal extension spanning M1 to R16 in Aha1
          from higher eukaryotes is responsible for the holdase activity of the
          protein
      - reference_id: PMID:37486705
        supporting_text: >-
          since the high concentration of NaCl could abolish the holdase activity
          of Aha1, the electrostatic interactions mediated by those charged
          residues in Aha1's N-terminal extension are thus indicated to play a
          crucial role in the substrate recognition
- term:
    id: GO:0036506
    label: maintenance of unfolded protein
  evidence_type: IDA
  original_reference_id: PMID:37486705
  review:
    summary: >-
      IDA annotation for maintenance of unfolded protein from DisProt, based on
      the same study as above (Tang et al. 2023). This is a duplicate annotation
      with different evidence code (IDA vs EXP) from the same reference, and the
      same concerns apply.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Same rationale as the EXP annotation from the same reference. The holdase
      activity is an in vitro observation dependent on the non-conserved N-terminal
      extension (M1-R16) and abolished by high salt, suggesting non-specific
      electrostatic interactions. Not established as a physiological function
      of AHSA1.
    supported_by:
      - reference_id: PMID:37486705
        supporting_text: >-
          the highly conserved N-terminal extension spanning M1 to R16 in Aha1
          from higher eukaryotes is responsible for the holdase activity of the
          protein
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: EXP
  original_reference_id: PMID:33808352
  review:
    summary: >-
      EXP annotation for protein folding chaperone from DisProt, based on Hu et al.
      2021, which demonstrated using NMR HSQC titrations and ThT assays that
      full-length human Aha1 interacts with alpha-synuclein and inhibits its
      aggregation in vitro. The chaperone-like activity requires the N-terminal
      extension (M1-W27) and/or C-terminal RLF motif, which are peripheral to
      the conserved core domains (PMID:33808352). The core construct Aha128-335
      showed no significant interaction with alpha-synuclein.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The protein folding chaperone annotation for AHSA1 represents an
      over-annotation. The chaperone-like activity is an in vitro observation
      dependent on peripheral regions (N-terminal extension and C-terminal RLF
      motif) not present in the conserved Aha1 core. The core construct
      Aha128-335 showed no significant interaction with alpha-synuclein
      (PMID:33808352). No in vivo evidence supports autonomous chaperone function.
      AHSA1's established role is as an HSP90 ATPase activator co-chaperone, not
      as an independent protein folding chaperone.
    supported_by:
      - reference_id: PMID:33808352
        supporting_text: >-
          Without the presence of M1-W27 fragment and RLF (R336L337F338) motif,
          no significant chemical shift perturbations were observed for the NMR
          resonances of the residues in Aha128-335 upon the addition of
          alpha-synuclein
      - reference_id: PMID:33808352
        supporting_text: >-
          extensive in vivo studies need to be conducted to precisely decipher the
          functional roles of Aha1 under different physiological and pathological
          conditions
- term:
    id: GO:0044183
    label: protein folding chaperone
  evidence_type: IDA
  original_reference_id: PMID:33808352
  review:
    summary: >-
      IDA annotation for protein folding chaperone from DisProt, based on the same
      study (Hu et al. 2021). This is a duplicate annotation with different
      evidence code (IDA vs EXP) from the same reference, and the same concerns
      apply.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Same rationale as the EXP annotation from the same reference. The
      chaperone-like activity depends on peripheral regions absent from the
      conserved Aha1 core, has no in vivo validation, and does not represent
      a core function of AHSA1.
    supported_by:
      - reference_id: PMID:33808352
        supporting_text: >-
          Without the presence of M1-W27 fragment and RLF (R336L337F338) motif,
          no significant chemical shift perturbations were observed for the NMR
          resonances of the residues in Aha128-335 upon the addition of
          alpha-synuclein
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: EXP
  original_reference_id: PMID:33808352
  review:
    summary: >-
      GO:0051082 (unfolded protein binding) is being obsoleted (go-ontology#30962).
      The annotation from DisProt is based on PMID:33808352 (Hu et al. 2021), which
      used NMR HSQC titrations and ThT assays to show that full-length human Aha1
      interacts with intrinsically disordered alpha-synuclein and inhibits its
      aggregation in vitro. However, the core construct Aha128-335 (lacking the
      N-terminal M1-W27 extension and C-terminal RLF motif) showed no significant
      interaction with alpha-synuclein, indicating this is not a property of the
      conserved Aha1 core domains. The authors themselves describe this as
      "chaperone-like activity" and note that in vivo significance remains
      unestablished. AHSA1's primary, evolutionarily conserved function is as an
      HSP90 ATPase activator, not as an independent chaperone. The unfolded protein
      binding annotation is an over-annotation that conflates an in vitro
      observation with a core molecular function. Furthermore, GO:0051082 is
      scheduled for obsoletion. The holdase/chaperone-like activity demonstrated
      in vitro is better captured by the existing GO:0044183 (protein folding
      chaperone) and GO:0036506 (maintenance of unfolded protein) annotations
      already present on this gene.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      AHSA1 is primarily an HSP90 co-chaperone whose core function is stimulating
      HSP90 ATPase activity (PMID:12504007, PMID:29127155). The unfolded protein
      binding annotation is based on in vitro NMR titration experiments showing
      that full-length Aha1 interacts with alpha-synuclein (PMID:33808352), but
      this interaction depends on the N-terminal extension (M1-W27) and C-terminal
      RLF motif, which are peripheral to the conserved Aha1 domains and absent in
      lower eukaryotes. The authors state that "extensive in vivo studies need to
      be conducted to precisely decipher the functional roles of Aha1 under
      different physiological and pathological conditions" (PMID:33808352). A
      follow-up study (PMID:37486705) confirmed the holdase activity maps to the
      N-terminal extension (M1-R16) and is driven by electrostatic interactions
      that can be abolished by high NaCl concentration, suggesting non-specific
      binding. Additionally, GO:0051082 is being obsoleted. The chaperone-like
      activity is already captured by GO:0044183 and GO:0036506 annotations on
      this gene from the same research group.
    supported_by:
      - reference_id: PMID:33808352
        supporting_text: >-
          Without the presence of M1-W27 fragment and RLF (R336L337F338) motif,
          no significant chemical shift perturbations were observed for the NMR
          resonances of the residues in Aha128−335 upon the addition of
          α-synuclein
      - reference_id: PMID:33808352
        supporting_text: >-
          In particular, since Aha1 has been reported to drive the production of
          pathological tau aggregates by acting as Hsp90's co-chaperone [59],
          which is in opposite to the inhibition effect of Aha1 on
          α-synuclein's aggregation observed by us, extensive in vivo studies
          need to be conducted to precisely decipher the functional roles of
          Aha1 under different physiological and pathological conditions.
      - reference_id: PMID:37486705
        supporting_text: >-
          the highly conserved N-terminal extension spanning M1 to R16 in Aha1
          from higher eukaryotes is responsible for the holdase activity of the
          protein
      - reference_id: PMID:37486705
        supporting_text: >-
          since the high concentration of NaCl could abolish the holdase activity
          of Aha1, the electrostatic interactions mediated by those charged
          residues in Aha1's N-terminal extension are thus indicated to play a
          crucial role in the substrate recognition
- term:
    id: GO:0001671
    label: ATPase activator activity
  evidence_type: IDA
  original_reference_id: PMID:29127155
  review:
    summary: >-
      IDA annotation for ATPase activator activity based on Woodford et al. 2017,
      which showed that AHSA1 activates HSP90 ATPase activity and competes with
      the inhibitory co-chaperone TSC1 for binding to the HSP90 middle domain.
      Phosphorylation of Aha1-Y223 increases its affinity for HSP90 and displaces
      TSC1, providing a regulatory switch for the chaperone cycle. This is direct
      experimental evidence for the core function of AHSA1.
    action: ACCEPT
    reason: >-
      This is strong direct assay evidence for the core molecular function of
      AHSA1. The study demonstrates ATPase activation of HSP90 by AHSA1 and
      elucidates the regulatory mechanism involving phosphorylation of Y223
      (PMID:29127155).
    supported_by:
      - reference_id: PMID:29127155
        supporting_text: >-
          phosphorylation of Aha1-Y223 increases its affinity for Hsp90 and
          displaces Tsc1, thereby providing a mechanism for equilibrium between
          binding of these two co-chaperones to Hsp90
- term:
    id: GO:0032781
    label: positive regulation of ATP-dependent activity
  evidence_type: IDA
  original_reference_id: PMID:29127155
  review:
    summary: >-
      IDA annotation for positive regulation of ATP-dependent activity based on
      Woodford et al. 2017. AHSA1 stimulates the ATPase activity of HSP90,
      which is an ATP-dependent molecular chaperone. This annotation captures
      the regulatory biological process aspect of AHSA1's core function. The
      term is appropriate as AHSA1 positively regulates HSP90's ATP-dependent
      chaperone cycle.
    action: ACCEPT
    reason: >-
      This annotation accurately captures the biological process corresponding
      to AHSA1's core molecular function. AHSA1 positively regulates HSP90's
      ATP-dependent chaperone cycle by stimulating its ATPase activity
      (PMID:29127155, PMID:12504007, PMID:27353360).
    supported_by:
      - reference_id: PMID:29127155
        supporting_text: >-
          Tsc1 is a new co-chaperone for Hsp90 that inhibits its ATPase
          activity ... prevents the activating co-chaperone Aha1 from binding
          the middle domain of Hsp90. Conversely, phosphorylation of Aha1-Y223
          increases its affinity for Hsp90 and displaces Tsc1
- term:
    id: GO:0045296
    label: cadherin binding
  evidence_type: HDA
  original_reference_id: PMID:25468996
  review:
    summary: >-
      HDA annotation for cadherin binding from BHF-UCL, based on Guo et al. 2014,
      a large-scale quantitative proteomics study of the E-cadherin interactome
      using proximity biotinylation. AHSA1 was identified among 561 proteins in
      the vicinity of the cytoplasmic tail of E-cadherin. This is a high-throughput
      proximity labeling approach that detects proteins near E-cadherin, not
      necessarily direct binding partners. AHSA1 has no known functional
      relationship to cadherins, and its presence may reflect its abundance as a
      cytoplasmic chaperone co-factor.
    action: REMOVE
    reason: >-
      Cadherin binding is not a plausible molecular function for AHSA1, an HSP90
      co-chaperone. The HDA evidence comes from proximity biotinylation proteomics
      (PMID:25468996), which identifies proteins in the general vicinity of
      E-cadherin, not direct binding partners. AHSA1 is an abundant cytosolic
      protein and likely a background hit. There is no functional relationship
      between AHSA1 and cadherin biology.
- term:
    id: GO:0051879
    label: Hsp90 protein binding
  evidence_type: IPI
  original_reference_id: PMID:29127155
  review:
    summary: >-
      IPI annotation for Hsp90 protein binding based on Woodford et al. 2017, with
      interaction partner HSP90AA1 (P07900). This study demonstrated that AHSA1
      binds to the middle domain of HSP90, and this interaction is enhanced by
      phosphorylation of Aha1-Y223. AHSA1 competes with TSC1 for HSP90 binding.
      This is the core molecular interaction of AHSA1.
    action: ACCEPT
    reason: >-
      Hsp90 protein binding is the core molecular interaction of AHSA1, directly
      demonstrated by biochemical assays including binding competition with TSC1
      (PMID:29127155). The interaction is mediated by the N-terminal domain of
      AHSA1 binding the HSP90 middle domain, and the C-terminal domain
      stabilizing the HSP90 N-terminal domain dimer.
    supported_by:
      - reference_id: PMID:29127155
        supporting_text: >-
          The C-terminal domain of Tsc1 (998-1,164 aa) forms a homodimer and
          binds to both protomers of the Hsp90 middle domain. This ensures
          inhibition of both subunits of the Hsp90 dimer and prevents the
          activating co-chaperone Aha1 from binding the middle domain of Hsp90.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25486457
  review:
    summary: >-
      IPI protein binding annotation from UniProt, based on Synoradzki and
      Bieganowski 2015, which demonstrated that Aha1 interacts preferentially
      with HSP90alpha (HSP90AA1) over HSP90beta (HSP90AB1), with the distinction
      depending on the middle domain of HSP90. The WITH column indicates
      interactions with both HSP90AA1 (P07900) and HSP90AB1 (P08238). This study
      provides insight into isoform-specific binding of AHSA1 to HSP90.
    action: MODIFY
    reason: >-
      The interactions detected are with both HSP90 isoforms, which is the core
      binding function of AHSA1. GO:0005515 is too vague; GO:0051879 (Hsp90
      protein binding) is the appropriate specific term.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:25486457
        supporting_text: >-
          the Hsp90 co-chaperone Aha1 interacts preferentially with Hsp90alpha.
          The distinction depends on the middle domain of Hsp90.
- term:
    id: GO:0001671
    label: ATPase activator activity
  evidence_type: IDA
  original_reference_id: PMID:27353360
  review:
    summary: >-
      IDA annotation for ATPase activator activity based on Woodford et al. 2016,
      which showed that AHSA1 competes with the inhibitory FNIP co-chaperones for
      HSP90 binding, providing a reciprocal regulatory mechanism. The paper
      demonstrates AHSA1's role as an ATPase activator in the context of
      competition with FNIP1/FNIP2.
    action: ACCEPT
    reason: >-
      Direct experimental evidence for AHSA1's core function as an HSP90 ATPase
      activator. The study demonstrates that FNIPs compete with the activating
      co-chaperone Aha1 for binding to HSP90 (PMID:27353360), confirming AHSA1's
      role in activating the HSP90 ATPase cycle.
    supported_by:
      - reference_id: PMID:27353360
        supporting_text: >-
          FNIPs compete with the activating co-chaperone Aha1 for binding to
          Hsp90, thereby providing a reciprocal regulatory mechanism for
          chaperoning of client proteins
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:27353360
  review:
    summary: >-
      IPI protein binding annotation from UniProt, based on Woodford et al. 2016.
      The WITH column indicates interactions with HSP90AA1 (P07900) and FNIP1
      (Q8NFG4). The interaction with HSP90AA1 is AHSA1's core function. The
      interaction with FNIP1 reflects the competitive binding relationship at the
      HSP90 middle domain rather than a direct AHSA1-FNIP1 interaction.
    action: MODIFY
    reason: >-
      The primary interaction detected is with HSP90AA1, which is AHSA1's core
      binding partner. GO:0005515 is too vague. The HSP90 binding is captured
      by GO:0051879. The interaction with FNIP1 (Q8NFG4) is indirect, reflecting
      competitive binding to HSP90 rather than direct AHSA1-FNIP1 binding.
    proposed_replacement_terms:
      - id: GO:0051879
        label: Hsp90 protein binding
    supported_by:
      - reference_id: PMID:27353360
        supporting_text: >-
          FNIPs compete with the activating co-chaperone Aha1 for binding to
          Hsp90, thereby providing a reciprocal regulatory mechanism for
          chaperoning of client proteins
- term:
    id: GO:0051087
    label: protein-folding chaperone binding
  evidence_type: IDA
  original_reference_id: PMID:27353360
  review:
    summary: >-
      IDA annotation for protein-folding chaperone binding based on Woodford et al.
      2016, which demonstrated AHSA1 binding to HSP90. Since HSP90 is a
      protein-folding chaperone, this annotation is accurate and represents AHSA1's
      core interaction with HSP90.
    action: ACCEPT
    reason: >-
      AHSA1 directly binds HSP90, which is a protein-folding chaperone. This
      annotation correctly captures AHSA1's core interaction and is supported by
      direct biochemical evidence (PMID:27353360).
    supported_by:
      - reference_id: PMID:27353360
        supporting_text: >-
          FNIPs compete with the activating co-chaperone Aha1 for binding to
          Hsp90, thereby providing a reciprocal regulatory mechanism for
          chaperoning of client proteins
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19056867
  review:
    summary: >-
      HDA annotation for extracellular exosome localization based on Gonzales et al.
      2009, a large-scale proteomics study of urinary exosomes that identified 1132
      proteins. AHSA1 was detected among these proteins. This is a high-throughput
      proteomics study, and the presence of AHSA1 in exosomes likely reflects its
      high abundance as a cytoplasmic protein rather than a specific exosome
      localization.
    action: KEEP_AS_NON_CORE
    reason: >-
      AHSA1 was detected in urinary exosomes by mass spectrometry (PMID:19056867).
      While the detection is real, exosome localization is not a core function or
      localization of AHSA1. Many abundant cytoplasmic proteins are detected in
      exosome proteomics studies. This is a minor, non-functional localization.
- term:
    id: GO:0001671
    label: ATPase activator activity
  evidence_type: IDA
  original_reference_id: PMID:12504007
  review:
    summary: >-
      IDA annotation for ATPase activator activity based on Panaretou et al. 2002,
      the foundational study that identified Aha1 as an activator of Hsp90 ATPase.
      This study demonstrated that Aha1 binds directly to Hsp90 and stimulates its
      ATPase activity in vitro, and is required for in vivo Hsp90-dependent
      activation of clients such as v-Src. This is the primary publication
      establishing AHSA1's core function.
    action: ACCEPT
    reason: >-
      This is the foundational study that defined AHSA1's core function as an
      HSP90 ATPase activator (PMID:12504007). Direct experimental evidence
      including in vitro ATPase assays and in vivo client activation assays.
    supported_by:
      - reference_id: PMID:12504007
        supporting_text: >-
          A ubiquitous family of stress-regulated proteins have been identified
          (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90 and are
          required for the in vivo Hsp90-dependent activation of clients such as
          v-Src, implicating them as cochaperones of the Hsp90 system. In vitro,
          Aha1 and its shorter homolog, Hch1, stimulate the inherent ATPase
          activity of yeast and human Hsp90.
- term:
    id: GO:0051087
    label: protein-folding chaperone binding
  evidence_type: IDA
  original_reference_id: PMID:12504007
  review:
    summary: >-
      IDA annotation for protein-folding chaperone binding based on Panaretou et al.
      2002, which demonstrated that Aha1 binds directly to Hsp90. Since HSP90 is a
      protein-folding chaperone, this annotation correctly captures the core binding
      interaction of AHSA1.
    action: ACCEPT
    reason: >-
      AHSA1 directly binds HSP90, a protein-folding chaperone, as demonstrated
      by the foundational study (PMID:12504007). This is a core interaction of
      AHSA1.
    supported_by:
      - reference_id: PMID:12504007
        supporting_text: >-
          A ubiquitous family of stress-regulated proteins have been identified
          (Aha1, activator of Hsp90 ATPase) that bind directly to Hsp90
core_functions:
- molecular_function:
    id: GO:0001671
    label: ATPase activator activity
  description: >-
    AHSA1 (AHA1) is the most potent known stimulator of HSP90 ATP hydrolysis,
    functioning as an HSP90 co-chaperone that accelerates the HSP90
    conformational/ATPase cycle via conserved NxNNWHW and RKxK motifs in its
    N-terminal domain, which engage the HSP90 middle domain and catalytic loop
    (residues 370-390). The C-terminal domain stabilizes the dimerized HSP90
    N-terminal domains. AHSA1 can bind asymmetrically (one molecule sufficient
    for stimulation) or as two molecules per HSP90 dimer with distinct functional
    outcomes (DOI:10.1016/j.bpj.2023.07.020). A metazoan-specific ICD (aa ~1-20)
    confers HSP90-independent holdase activity and negatively regulates ATPase
    stimulation (DOI:10.1038/s44319-024-00193-8). AHSA1 competes with inhibitory
    co-chaperones TSC1 and FNIP1/FNIP2 for HSP90 binding, and phosphorylation of
    Aha1-Y223 modulates the competition by increasing affinity for HSP90. AHSA1
    influences maturation of diverse HSP90 clients including kinases, steroid
    receptors, and Dicer1 (affecting microRNA biogenesis)
    (DOI:10.1093/nar/gkac528).
  directly_involved_in:
    - id: GO:0006457
      label: protein folding
    - id: GO:0032781
      label: positive regulation of ATP-dependent activity
  locations:
    - id: GO:0005829
      label: cytosol
  supported_by:
    - reference_id: PMID:12504007
      supporting_text: >-
        A ubiquitous family of stress-regulated proteins have been identified (Aha1,
        activator of Hsp90 ATPase) that bind directly to Hsp90 and are required for
        the in vivo Hsp90-dependent activation of clients such as v-Src.
    - reference_id: PMID:12604615
      supporting_text: >-
        Aha1 but not Hch1 stimulated the intrinsic ATPase activity of Hsp90 5-fold.
    - reference_id: PMID:29127155
      supporting_text: >-
        phosphorylation of Aha1-Y223 increases its affinity for Hsp90 and displaces
        Tsc1, thereby providing a mechanism for equilibrium between binding of these
        two co-chaperones to Hsp90.
    - reference_id: PMID:27353360
      supporting_text: >-
        FNIPs compete with the activating co-chaperone Aha1 for binding to Hsp90,
        thereby providing a reciprocal regulatory mechanism for chaperoning of client
        proteins.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:12504007
  title: Activation of the ATPase activity of hsp90 by the stress-regulated cochaperone
    aha1.
  findings: []
- id: PMID:12604615
  title: Aha1 binds to the middle domain of Hsp90, contributes to client protein activation,
    and stimulates the ATPase activity of the molecular chaperone.
  findings: []
- id: PMID:16696853
  title: A yeast 2-hybrid analysis of human GTP cyclohydrolase I protein interactions.
  findings: []
- id: PMID:19056867
  title: Large-scale proteomics and phosphoproteomics of urinary exosomes.
  findings: []
- id: PMID:19875381
  title: A proteomic investigation of ligand-dependent HSP90 complexes reveals CHORDC1
    as a novel ADP-dependent HSP90-interacting protein.
  findings: []
- id: PMID:20618441
  title: CHIP participates in protein triage decisions by preferentially ubiquitinating
    Hsp70-bound substrates.
  findings: []
- id: PMID:25036637
  title: A quantitative chaperone interaction network reveals the architecture of
    cellular protein homeostasis pathways.
  findings: []
- id: PMID:25468996
  title: E-cadherin interactome complexity and robustness resolved by quantitative
    proteomics.
  findings: []
- id: PMID:25486457
  title: Middle domain of human Hsp90 isoforms differentially binds Aha1 in human
    cells and alters Hsp90 activity in yeast.
  findings: []
- id: PMID:27353360
  title: The FNIP co-chaperones decelerate the Hsp90 chaperone cycle and enhance drug
    binding.
  findings: []
- id: PMID:29127155
  title: Tumor suppressor Tsc1 is a new Hsp90 co-chaperone that facilitates folding
    of kinase and non-kinase clients.
  findings: []
- id: PMID:30382094
  title: Structure and pro-toxic mechanism of the human Hsp90/PPIase/Tau complex.
  findings: []
- id: PMID:33808352
  title: Aha1 Exhibits Distinctive Dynamics Behavior and Chaperone-Like Activity.
  findings: []
- id: PMID:35271311
  title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.'
  findings: []
- id: PMID:37486705
  title: Human Aha1's N-terminal extension confers it holdase activity in vitro.
  findings: []
- id: DOI:10.1038/s44319-024-00193-8
  title: Recruitment of Ahsa1 to Hsp90 is regulated by a conserved peptide that inhibits
    ATPase stimulation.
  findings:
  - statement: >-
      AHSA1 contains a metazoan-specific intrinsic chaperone domain (ICD, aa
      ~1-20) that dampens ATPase stimulation by interfering with NxNNWHW
      function and controls regulated recruitment to HSP90
  - statement: >-
      NxNNWHW and RKxK are conserved motifs central to AHSA1 ATPase
      stimulatory function; RKxK stabilizes the HSP90 catalytic loop to
      facilitate ATP hydrolysis
- id: DOI:10.1016/j.bpj.2023.07.020
  title: Aha1 regulates Hsp90's conformation and function in a stoichiometry-dependent
    way.
  findings:
  - statement: >-
      One or two Aha1 molecules can bind per HSP90 dimer, with binding
      stoichiometry differentially regulating HSP90 conformational properties
      and ATPase activity
- id: DOI:10.1093/nar/gkac528
  title: HSP90 and Aha1 modulate microRNA maturation through promoting the folding of
    Dicer1.
  findings:
  - statement: >-
      HSP90 and AHA1 modulate microRNA maturation by promoting folding and
      stability of Dicer1; AHA1 depletion reduces Dicer1 protein and mature
      miRNA levels
- id: DOI:10.1186/s13046-021-02220-1
  title: AHSA1 is a promising therapeutic target for cellular proliferation and proteasome
    inhibitor resistance in multiple myeloma.
  findings:
  - statement: >-
      AHSA1 binds bufalin at K137; the inhibitor KU-177 targets the same
      site, reduces AHSA1-HSP90A interaction, and reverses proteasome
      inhibitor resistance in multiple myeloma
- id: DOI:10.3389/fnmol.2024.1509280
  title: The role of Aha1 in cancer and neurodegeneration.
  findings:
  - statement: >-
      Review framing Aha1 dysregulation as a driver of disease phenotypes in
      cancer, cystic fibrosis, and neurodegeneration via HSP90 proteostasis
      network imbalance
  - statement: >-
      HSP90/Aha1 protein-protein interaction interface proposed as a
      therapeutic target with potential to spare essential baseline HSP90
      functions
- id: DOI:10.1038/s41580-023-00640-9
  title: Structural and functional complexity of HSP90 in cellular homeostasis and disease.
  findings:
  - statement: >-
      Authoritative review noting co-chaperone regulation by AHA1 contributes
      to HSP90 functional complexity in disease and discusses chemical
      inhibition of the Aha1-HSP90 complex

AHSA1

Gene Description

Existing Annotations Review

Core Functions

References

📚 Additional Documentation

Deep Research Falcon

Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Output

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

Target Gene/Protein Identity (from UniProt):

MANDATORY VERIFICATION STEPS:

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

Research Target:

Citations

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