GRPEL1

Summary: GRPEL1 is a nucleotide exchange factor (NEF) for mitochondrial HSP70 (HSPA9). It accelerates ADP release and ATP rebinding on HSPA9, which is the defining biochemical activity of an adenyl-nucleotide exchange factor. Quantitative binding data show GRPEL1 binds HSPA9 with KD ~39 nM and inhibits single-turnover ATP hydrolysis at saturating concentrations, consistent with NEF activity (Srivastava et al., JBC 2017). The IBA annotation is phylogenetically well-supported, as GrpE family members across bacteria and eukaryotes share this conserved NEF function. This is the core molecular function of GRPEL1.

Reason: Adenyl-nucleotide exchange factor activity is the defining molecular function of GRPEL1. This is strongly supported by in vitro biochemical data (Srivastava et al., JBC 2017) showing direct NEF activity on HSPA9, as well as by conserved domain architecture (GrpE family) and phylogenetic inference.

Summary: GRPEL1 is a component of the PAM (presequence translocase-associated motor) complex. Co-immunoprecipitation experiments in mitochondrial lysates show GRPEL1 associates with HSPA9 and TIMM44 (Srivastava et al., JBC 2017). UniProt lists GRPEL1 as a component of the TIM23 complex (ComplexPortal CPX-6129 and CPX-6130). The IBA annotation is phylogenetically well-supported from the yeast Mge1p ortholog.

Reason: Multiple lines of evidence confirm GRPEL1 as a PAM complex component: co-immunoprecipitation with HSPA9 and TIMM44 (Srivastava et al., JBC 2017), functional conservation with yeast Mge1p in the import motor, and ComplexPortal entries for TIM23 complexes listing GRPEL1.

Summary: GRPEL1 participates in protein import into the mitochondrial matrix as the NEF for HSPA9 within the PAM/TIM23 import motor. By cycling HSPA9 between ADP- and ATP-bound states, GRPEL1 enables the ratchet mechanism that drives preprotein translocation through the TIM23 channel (Srivastava et al., JBC 2017; Havalova et al., IJMS 2021). The IBA annotation from yeast Mge1p is well-supported.

Reason: GRPEL1 is directly involved in mitochondrial protein import as the NEF component of the PAM import motor. This is the primary biological process in which GRPEL1 functions, supported by its PAM complex membership and essential role in HSPA9 cycling during preprotein translocation.

Summary: This annotation attributes direct unfolded protein binding to GRPEL1. However, GRPEL1 is a nucleotide exchange factor, not a substrate-binding chaperone. The unfolded protein binding activity in the Hsp70 system resides with HSPA9 itself (via its substrate-binding domain) and J-protein co-chaperones that deliver substrates. GRPEL1 acts on the nucleotide-binding domain of HSPA9 to accelerate ADP release, which indirectly causes substrate release. There is no evidence that GRPEL1 directly binds unfolded polypeptide substrates. The IBA annotation propagates from an IDA (PMID:11311562) that conflated the co-chaperone's role in the chaperone cycle with direct substrate binding.

Reason: GRPEL1 is a nucleotide exchange factor that acts on the ATPase domain of HSPA9; it does not directly bind unfolded protein substrates. The chaperone system as a whole handles unfolded proteins, but GRPEL1's specific role is nucleotide exchange. This annotation confuses the system-level function with the molecular-level activity of this specific component. The core MF annotation should be GO:0000774 (adenyl-nucleotide exchange factor activity). The underlying IDA (PMID:11311562) does not demonstrate direct unfolded protein binding by GRPEL1.

Summary: IEA annotation from InterPro domain mapping (IPR000740, GrpE domain). The GrpE domain is the defining structural feature of nucleotide exchange factors for DnaK/Hsp70 chaperones. This electronic annotation is fully consistent with the IBA annotation and experimental evidence.

Reason: Correct IEA mapping. The GrpE domain (IPR000740) is a reliable predictor of adenyl-nucleotide exchange factor activity, and this is confirmed by both phylogenetic (IBA) and experimental evidence for GRPEL1.

Summary: IEA annotation from UniProtKB subcellular location vocabulary mapping. GRPEL1 is annotated in UniProt as localizing to the mitochondrial matrix based on PMID:11311562. This electronic annotation is consistent with the IDA annotation from the same publication.

Reason: Correct IEA mapping consistent with experimental IDA evidence. Mitochondrial matrix localization is well established for GRPEL1 by subcellular fractionation and immunocytochemistry (PMID:11311562) and by the Srivastava et al. (JBC 2017) study.

Summary: IEA annotation from InterPro domain mapping. GrpE family proteins participate in the Hsp70 chaperone cycle, which assists protein folding. GRPEL1 contributes to protein folding indirectly by enabling HSPA9 to cycle through substrate binding and release, supporting folding of newly imported and misfolded matrix proteins. However, GRPEL1 is not itself a folding chaperone; it is a co-chaperone (NEF) that regulates the chaperone cycle.

Reason: While GRPEL1 participates in the protein folding process through its NEF function in the HSPA9 chaperone cycle, protein folding is not its core evolved function. Its core function is nucleotide exchange factor activity (GO:0000774) within the mitochondrial protein import system. Protein folding is a downstream consequence of HSPA9 cycling that GRPEL1 facilitates, making it a non-core annotation.

Summary: IEA annotation from ARBA machine learning. GRPEL1 does form homo-oligomers as shown by size exclusion chromatography and native gel experiments (Srivastava et al., JBC 2017), though the physiologically relevant form appears to be the GRPEL1-GRPEL2 hetero-oligomer (~150-160 kDa). The IPI annotation from PMID:32296183 provides experimental support for self-association.

Reason: GRPEL1 self-association is experimentally supported by IPI data (PMID:32296183) and by biochemical studies showing GRPEL1 homo-oligomerization (Srivastava et al., JBC 2017), though the heteromeric complex with GRPEL2 is the predominant physiological form.

Summary: IEA annotation from InterPro domain mapping. GrpE family members are known to dimerize. Bacterial GrpE forms a homodimer. GRPEL1 can self-associate as shown by co-IP and Y2H data (PMID:32296183), and purified GRPEL1 forms oligomers (Srivastava et al., JBC 2017), though the predominant physiological form is the GRPEL1-GRPEL2 hetero-oligomer.

Reason: Homodimerization is a conserved property of GrpE family proteins, and there is experimental evidence that GRPEL1 can self-associate. However, note that in vivo the GRPEL1-GRPEL2 hetero-oligomer may be the predominant species.

Summary: IEA annotation from InterPro domain mapping. GRPEL1 directly binds the protein-folding chaperone HSPA9/mortalin with high affinity (KD ~39 nM; Srivastava et al., JBC 2017). The original characterization paper (PMID:11311562) also showed GRPEL1 binding to HSP70, HSC70, and HSJ1b. This annotation accurately captures the binding interaction between GRPEL1 and its chaperone partner HSPA9.

Reason: GRPEL1 is a co-chaperone that directly binds the protein-folding chaperone HSPA9 with high affinity. This binding is central to its nucleotide exchange factor function. Supported by multiple lines of evidence including co-IP, biolayer interferometry, and functional assays.

Summary: High-throughput interactome study. The WITH/FROM field indicates interaction with HSPA9 (P38646), which is the known physiological partner of GRPEL1. However, protein binding is uninformative as a GO term; the specific interaction is better captured by GO:0051087 (protein-folding chaperone binding) and GO:0000774 (adenyl-nucleotide exchange factor activity).

Reason: Protein binding (GO:0005515) is uninformative per GO curation guidelines. The GRPEL1-HSPA9 interaction is better captured by more specific terms already annotated (GO:0051087 and GO:0000774).

Summary: High-throughput crosslinking mass spectrometry study of histone interactions in cell nuclei. The WITH/FROM field indicates interaction with HSPA9 (P38646). The study context (histone interaction landscapes in nuclei) is not directly relevant to GRPEL1's mitochondrial function. The GRPEL1-HSPA9 interaction is well established but protein binding is uninformative.

Reason: Protein binding (GO:0005515) is uninformative. The GRPEL1-HSPA9 interaction is real but already captured by more specific terms. The study context (nuclear histone interactions) is tangential to GRPEL1's mitochondrial function.

Summary: Reference map of the human binary protein interactome (HuRI). Multiple interactors detected for GRPEL1 including POLR1C (O15160), GALT (P07902), CCDC57 (Q2TAC2-2), and SPG21 (Q9NZD8). These are high-throughput Y2H interactions. Some may represent artifacts from overexpression; GALT and POLR1C are not established mitochondrial matrix proteins. Protein binding is uninformative.

Reason: Protein binding (GO:0005515) is uninformative per GO curation guidelines. Several of the detected interactors (GALT, POLR1C, CCDC57, SPG21) are from high-throughput Y2H and may not represent physiologically relevant interactions for a mitochondrial matrix protein.

Summary: Dual proteome-scale network study detecting GRPEL1 interaction with HSPA9 (P38646). This confirms the well-established GRPEL1-HSPA9 interaction, but protein binding is uninformative as a GO term.

Reason: Protein binding (GO:0005515) is uninformative. The GRPEL1-HSPA9 interaction is the core functional interaction already captured by more specific terms (GO:0051087, GO:0000774).

Summary: The HuRI binary interactome study detected GRPEL1 self-interaction (WITH/FROM: Q9HAV7). This is consistent with the known oligomeric behavior of GrpE family proteins. Srivastava et al. (JBC 2017) showed that purified GRPEL1 forms oligomers, though the physiologically predominant form is the GRPEL1-GRPEL2 hetero-oligomer.

Reason: Experimental IPI evidence for GRPEL1 self-interaction from a binary interactome study. This is consistent with known GrpE family homodimerization and biochemical evidence of GRPEL1 oligomerization.

Summary: IDA annotation from HPA immunofluorescence data. GRPEL1 mitochondrial localization is well established by multiple methods including subcellular fractionation and immunocytochemistry (PMID:11311562), and microscopy with mitochondrial markers (Srivastava et al., JBC 2017). This is a broader term than GO:0005759 (mitochondrial matrix) but still correct.

Reason: Mitochondrial localization is firmly established for GRPEL1. While GO:0005759 (mitochondrial matrix) is more specific, this broader IDA annotation from HPA immunofluorescence is also correct and independently confirms the localization.

Summary: NAS annotation from ComplexPortal based on Bauer et al. (1999) which characterized the human TIM23 translocase. The paper describes hTim44 localization in the matrix and TIM23/TIM17 in the inner membrane. GRPEL1 itself was not characterized in this paper (it was identified later in 2001). GRPEL1 is a soluble matrix protein that associates with the inner membrane via its interaction with the PAM/TIM23 complex. The annotation likely reflects its association with the inner membrane-bound TIM23 complex rather than intrinsic membrane localization.

Reason: GRPEL1 is primarily a soluble mitochondrial matrix protein that peripherally associates with the inner membrane through its interactions with the PAM/TIM23 complex. The annotation is not wrong per se, as GRPEL1 does associate with inner membrane complexes, but GO:0005759 (mitochondrial matrix) is the more accurate primary localization. The cited paper (PMID:10339406) predates the identification of GRPEL1 and describes the TIM23 translocase in general.

Summary: NAS annotation from ComplexPortal. GRPEL1 is involved in intracellular protein transport as part of the mitochondrial import machinery. However, this term is overly broad; the more specific term GO:0030150 (protein import into mitochondrial matrix) better captures GRPEL1's role. The cited paper (PMID:10339406) describes the TIM23 translocase but predates GRPEL1's identification.

Reason: While GRPEL1 is involved in intracellular protein transport, this term is too general. The more specific term GO:0030150 (protein import into mitochondrial matrix) precisely describes GRPEL1's role in the TIM23/PAM-dependent import of preproteins into the matrix, and is already annotated via IBA.

Proposed replacements: protein import into mitochondrial matrix

Summary: HTP annotation from a quantitative high-confidence human mitochondrial proteome study. GRPEL1 was identified in the mitochondrial proteome with high confidence. This is consistent with all other localization evidence.

Reason: High-throughput proteomic confirmation of GRPEL1 mitochondrial localization. Consistent with all other evidence (IDA from HPA, IDA from PMID:11311562, and functional studies).

Summary: This IDA annotation claims direct assay evidence for GRPEL1 binding unfolded proteins based on PMID:11311562 (Choglay et al., 2001). However, careful reading of the paper reveals that the experiments demonstrated GRPEL1 (HMGE) binding to Hsp70 proteins (DnaK, Hsc70, Mt-Hsp70) and inhibiting HSJ1b-enhanced Hsc70 ATPase activity, not direct binding to unfolded protein substrates. The study characterized GRPEL1 as a GrpE homolog that interacts with Hsp70 chaperones, which is nucleotide exchange factor activity, not unfolded protein binding. The annotation appears to be a misinterpretation of the co-chaperone's role in the chaperone cycle as direct substrate binding.

Reason: The cited paper (PMID:11311562) does not demonstrate direct binding of GRPEL1 to unfolded proteins. The experiments showed GRPEL1 binding to Hsp70 chaperones (DnaK, Hsc70, Mt-Hsp70) and inhibiting co-chaperone-stimulated ATPase activity. These are hallmarks of nucleotide exchange factor activity (GO:0000774), not unfolded protein binding. GRPEL1 acts on the nucleotide-binding domain of Hsp70 to catalyze ADP/ATP exchange; it does not directly engage unfolded polypeptide substrates. This annotation likely arose from conflating the system-level function (chaperone cycle handles unfolded proteins) with the specific molecular activity of this component.

Summary: IDA annotation based on subcellular fractionation and immunocytochemistry from the original characterization paper (Choglay et al., 2001). The paper demonstrates GRPEL1 (HMGE) is a mitochondrial protein using antisera and subcellular fractionation. This is further confirmed by Srivastava et al. (JBC 2017) showing matrix localization by microscopy with mitochondrial markers.

Reason: Well-supported by direct experimental evidence from subcellular fractionation and immunocytochemistry in the original characterization paper. Independently confirmed by Srivastava et al. (JBC 2017). Mitochondrial matrix is the specific subcellular location where GRPEL1 performs its NEF function within the PAM/TIM23 complex.