PCSK1N encodes ProSAAS (also called Proprotein convertase 1 inhibitor, pro-SAAS), a small neuroendocrine secretory precursor that is processed at paired basic sites in the regulated secretory pathway into multiple bioactive peptides. ProSAAS is expressed in neurons throughout the brain and in pituitary, adrenal, and pancreas, and is stored in dense-core secretory granules of the trans-Golgi network before regulated exocytosis. Its C-terminal Big PEN-LEN domain is an endogenous inhibitor of proprotein convertase 1/3, while full-length ProSAAS and an N-terminal fragment act as extracellular anti-aggregant chaperones that bind and prevent fibrillation of amyloidogenic clients including amyloid-beta, alpha-synuclein, and TDP-43. ProSAAS does not refold proteins or disaggregate preformed fibrils. In vivo, ProSAAS overexpression rescues dopaminergic neurons from alpha-synuclein toxicity and blocks transsynaptic alpha-synuclein spread in rodent Parkinson disease models. Processed PEN and Big LEN peptides function as endogenous neuropeptide ligands for GPR83 and GPR171, respectively.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004866 endopeptidase inhibitor activity | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA annotation supported by clear experimental evidence that the C-terminal Big PEN-LEN region of ProSAAS inhibits PCSK1/PC1/3 (PMID:10632593, PMID:11435430). However, PCSK1 is a subtilisin-family serine endopeptidase, so the more specific child term GO:0004867 serine-type endopeptidase inhibitor activity (already on the GOA record as IEA) is preferable. Reason: Generic parent of the more informative GO:0004867 already present. Retain to preserve the IBA phylogenetic annotation but treat as non-core in the synthesis since GO:0004867 captures the same activity at a more specific level. Supporting Evidence: PMID:10632593 Purified proSAAS inhibits prohormone convertase 1 activity with an IC(50) of 590 n m but does not inhibit prohormone convertase 2. |
| GO:0004866 endopeptidase inhibitor activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: IEA from InterPro (IPR010832 ProSAAS) recapitulating the IBA above. Same considerations apply: parent of the more specific GO:0004867. Reason: InterPro2GO mapping is correct but redundant with the more specific GO:0004867 already present. Retained but not core. Supporting Evidence: PMID:10632593 Purified proSAAS inhibits prohormone convertase 1 activity with an IC(50) of 590 n m but does not inhibit prohormone convertase 2. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: IEA combined-method annotation. ProSAAS is a regulated-secretory pathway protein that is released into the extracellular space. The annotation is correct; GO has obsoleted GO:0005615 extracellular space and replaced it with this term, so it also covers the released pool. Reason: ProSAAS is genuinely secreted via the regulated pathway (PMID:10632593; UniProt SUBCELLULAR LOCATION). The IEA annotation is well supported. Supporting Evidence: PMID:10632593 When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. |
| GO:0005794 Golgi apparatus | IEA GO_REF:0000044 | ACCEPT | Summary: IEA from UniProt SubCellular Location vocabulary (SL-0132). ProSAAS is processed by paired-basic cleavage in the Golgi (PMID:12914799). The child term GO:0005802 trans-Golgi network is more specific and is also annotated. Both can be retained. Reason: Golgi localization is consistent with the regulated-secretory-pathway biology and with the documented proteolytic processing of ProSAAS in this compartment. Parent of the more specific trans-Golgi network annotation. Supporting Evidence: PMID:10632593 proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. |
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000107 | ACCEPT | Summary: IEA from Ensembl Compara orthology to mouse Q9QXV0. The C-terminal Big PEN-LEN region of ProSAAS is a specific inhibitor of PCSK1/PC1/3, a subtilisin-family serine endopeptidase (MEROPS S8). Alanine scanning of the LLRVKR hexapeptide (residues 239-244) shows that single substitutions at K243 or R244 abolish inhibition (PMID:11435430). The 2000 paper measured an IC50 of 590 nM for purified ProSAAS against PCSK1 and showed no inhibition of PCSK2 (PMID:10632593). This is the most informative and well-supported MF annotation on the record and represents one of the two core molecular functions. Reason: Strongly supported by direct biochemical evidence (PMID:10632593, PMID:11435430) and by orthology. Most specific available MF term for the convertase-inhibitory activity. Promote to a core function. Supporting Evidence: PMID:10632593 Purified proSAAS inhibits prohormone convertase 1 activity with an IC(50) of 590 n m but does not inhibit prohormone convertase 2. PMID:11435430 the decapeptide proSAAS-(235-244)( 235)VLGALLRVKR(244) is the most potent reversible competitive PC1-inhibitor (K(i) approximately 9 nm). file:human/PCSK1N/PCSK1N-deep-research-falcon.md The report summarizes proSAAS as an endogenous PC1/3 inhibitor that modulates prohormone-processing protease activity in the regulated secretory pathway. |
| GO:0005802 trans-Golgi network | IEA GO_REF:0000107 | ACCEPT | Summary: IEA from Ensembl Compara orthology to mouse. UniProt curation places ProSAAS in the Golgi apparatus, trans-Golgi network. ProSAAS is processed by paired-basic cleavage at this location. Well-supported localization annotation. Reason: ProSAAS is processed in the Golgi/TGN of the regulated secretory pathway (UniProt SUBCELLULAR LOCATION). Specific and informative. Supporting Evidence: PMID:10632593 proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. |
| GO:0030141 secretory granule | IEA GO_REF:0000107 | ACCEPT | Summary: IEA from Ensembl Compara orthology to mouse. ProSAAS is stored in dense-core secretory granules of neuroendocrine cells (PMID:10632593, PMID:24102330) consistent with its regulated-secretory-pathway routing. Well-supported localization. Reason: ProSAAS storage in regulated secretory granules is documented in the original characterization paper (PMID:10632593) and recapitulated by multiple subsequent studies. Supporting Evidence: PMID:10632593 When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway and is also processed at paired-basic cleavage sites into smaller peptides. |
| GO:0005576 extracellular region | ISS GO_REF:0000024 | ACCEPT | Summary: ISS from manual transfer of the experimentally-verified mouse annotation (UniProtKB:Q9QXV0). Equivalent to the IEA above. Retain as supporting. Reason: Same as the IEA annotation: ProSAAS is secreted via the regulated pathway, well documented in PMID:10632593. Supporting Evidence: PMID:10632593 When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway. |
| GO:0004866 endopeptidase inhibitor activity | TAS PMID:10632593 Identification and characterization of proSAAS, a granin-lik... | KEEP AS NON CORE | Summary: TAS from the original ProSAAS characterization paper. Same considerations as the IBA/IEA above: GO:0004867 (serine-type endopeptidase inhibitor activity) is the more specific and informative child term given that PCSK1 is a serine endopeptidase, and that more specific term is already present on the GOA record. Reason: Generic parent term. The more specific GO:0004867 child is already annotated and serves as the core MF; this entry is retained but considered non-core to keep the trio of GO:0004866 annotations consistent in their treatment. Supporting Evidence: PMID:10632593 Purified proSAAS inhibits prohormone convertase 1 activity with an IC(50) of 590 n m but does not inhibit prohormone convertase 2. |
| GO:0005102 signaling receptor binding | TAS PMID:10632593 Identification and characterization of proSAAS, a granin-lik... | REMOVE | Summary: TAS from the 2000 ProSAAS paper (PMID:10632593). The cited paper does NOT directly demonstrate receptor binding; it focuses on PCSK1 inhibition and tissue distribution. The annotation is presumably intended to capture the activity of the processed peptides Big LEN (GPR171 ligand, PMID:21795693 Wardman et al., not cited here) and PEN (GPR83 ligand, PMID:23690615 Gomes et al., not cited here). Even granting the underlying biology, the term is non-specific and the cited reference does not support it. The activity is also peptide-product-specific (Big LEN, PEN), not a property of full-length ProSAAS. Reason: The peptide-receptor binding biology may be real for processed peptide products such as Big LEN and PEN, but the cited PMID does not demonstrate receptor binding and instead supports PCSK1 inhibition and regulated secretion of the precursor. Remove this unsupported precursor-level TAS annotation; processed peptide ligands should be re-annotated separately using direct GPR171/GPR83 evidence if appropriate. Supporting Evidence: PMID:10632593 Taken together, proSAAS may represent an endogenous inhibitor of prohormone convertase 1. |
| GO:0005576 extracellular region | TAS PMID:10632593 Identification and characterization of proSAAS, a granin-lik... | ACCEPT | Summary: TAS from the original ProSAAS characterization paper, which documents regulated secretion of ProSAAS from AtT-20 cells. Well-supported localization. Reason: Directly demonstrated in the cited paper. Supporting Evidence: PMID:10632593 When expressed in AtT-20 cells, proSAAS is secreted via the regulated pathway. |
| GO:0051787 misfolded protein binding | IDA PMID:24102330 A novel function for proSAAS as an amyloid anti-aggregant in... | NEW | Summary: Proposed NEW annotation. Direct biochemical evidence shows that ProSAAS binds amyloid-beta and prevents its fibrillation in vitro at substoichiometric ratios; ProSAAS co-immunoprecipitates with AΞ² in APdE9 mouse brain. Additional evidence in PMID:27457957 (alpha-synuclein) and PMID:35527562 (alpha-synuclein in vivo). This MF (or its more specific child GO:0001540 amyloid-beta binding) is the appropriate primary term for the "anti-aggregant chaperone" activity highlighted in upstream go-annotation issue #6407, since ProSAAS does not refold clients (so GO:0044183 is inappropriate) and does not perform a documented carrier/handoff (so GO:0140309 unfolded protein holdase activity is at best a partial fit). The CLU/clusterin curation in this repository uses the same MF for an analogous extracellular anti-aggregant chaperone. Reason: Captures the secreted anti-aggregant chaperone activity of ProSAAS established in PMID:24102330, PMID:27457957, PMID:35527562 (and PMID:35549000 for cytoplasmic capacity). Supporting Evidence: PMID:24102330 We here describe a novel anti-aggregant chaperone function for the neuroendocrine protein proSAAS, an abundant secretory polypeptide that is widely expressed within neural and endocrine tissues PMID:27457957 Recombinant proSAAS potently inhibits the fibrillation of Ξ±-synuclein in an in vitro assay; residues 158-180, containing a largely conserved element, are critical to this bioactivity. file:human/PCSK1N/PCSK1N-deep-research-falcon.md The report summarizes proSAAS as a secretory chaperone/anti-aggregant that can limit fibrillation or aggregation of amyloid-beta, alpha-synuclein, and islet amyloid polypeptide. |
| GO:0001540 amyloid-beta binding | IPI PMID:24102330 A novel function for proSAAS as an amyloid anti-aggregant in... | NEW | Summary: Proposed NEW annotation. PMID:24102330 provides amyloid-beta-specific physical-interaction evidence: ProSAAS co-immunoprecipitates with AΞ² from APdE9 mouse brain lysates and recombinant ProSAAS prevents AΞ²(1-42) fibrillation in vitro. This child term captures the amyloid-beta component of the broader misfolded-protein-binding activity. Reason: The notes explicitly identify amyloid-beta binding as a supported, more-specific MF term for the AΞ² co-IP and anti-fibrillation evidence. Add it alongside GO:0051787 rather than relying only on the broader misfolded protein binding parent. Supporting Evidence: PMID:24102330 Immunoreactive proSAAS co-immunoprecipitated with AΞ² immunoreactivity in lysates from APdE9 mouse brains. PMID:24102330 In vitro, proSAAS efficiently prevented the fibrillation of AΞ²(1-42) at molar ratios of 1 : 10, and this anti-aggregation effect was dose dependent. |
| GO:1905907 negative regulation of amyloid fibril formation | IDA PMID:24102330 A novel function for proSAAS as an amyloid anti-aggregant in... | NEW | Summary: Proposed NEW BP annotation. ProSAAS prevents amyloid-beta fibrillation in vitro at substoichiometric ratios (PMID:24102330) and inhibits alpha-synuclein fibrillation in vitro (PMID:27457957), with in vivo neuroprotective consequences in rodent models of Parkinson's disease (PMID:35527562). GO:1902430 (negative regulation of amyloid-beta formation) is NOT recommended because that term refers to the proteolytic generation of AΞ² from APP, not to its aggregation. Reason: Best-fitting BP term for the anti-fibrillation activity directly demonstrated in vitro and in vivo for two amyloidogenic clients. Supporting Evidence: PMID:24102330 In vitro, proSAAS efficiently prevented the fibrillation of AΞ²(1-42) at molar ratios of 1 : 10, and this anti-aggregation effect was dose dependent. PMID:27457957 Recombinant proSAAS potently inhibits the fibrillation of Ξ±-synuclein in an in vitro assay |
| GO:0050821 protein stabilization | IDA PMID:24102330 A novel function for proSAAS as an amyloid anti-aggregant in... | NEW | Summary: Proposed NEW BP annotation, parent of holdase-like client stabilization. The CLU/clusterin curation uses this BP alongside GO:1905907 to capture the extracellular maintenance of misfolded clients in soluble non-aggregating states. The same logic applies to ProSAAS. Reason: Captures the chaperone-mediated stabilization of misfolded amyloidogenic clients in soluble (non-aggregating) form. Supporting Evidence: PMID:24102330 We here describe a novel anti-aggregant chaperone function for the neuroendocrine protein proSAAS PMID:27457957 the small secretory chaperone known as proSAAS (named after four residues in the amino terminal region) has many attractive properties. |
| GO:0010955 negative regulation of protein processing | IDA PMID:10632593 Identification and characterization of proSAAS, a granin-lik... | NEW | Summary: Proposed NEW BP annotation. ProSAAS overexpression slows processing of proopiomelanocortin (a PCSK1 substrate) in AtT-20 cells. The convertase- inhibitory activity of ProSAAS exerts its biological effect by negatively regulating prohormone processing; this is the appropriate BP for the core MF GO:0004867 serine-type endopeptidase inhibitor activity. Reason: The molecular function (PCSK1 inhibition) acts upon prohormone processing reactions; this BP captures the consequence in the regulated secretory pathway. Supporting Evidence: PMID:10632593 Overexpression of proSAAS in the AtT-20 cells substantially reduces the rate of processing of the endogenous prohormone proopiomelanocortin. |
| GO:0060570 negative regulation of peptide hormone processing | IDA PMID:10632593 Identification and characterization of proSAAS, a granin-lik... | NEW | Summary: Proposed NEW BP annotation. The substrate of ProSAAS-inhibited PCSK1 is proopiomelanocortin (a peptide-hormone precursor) and other prohormones, making this a more specific and biologically informative BP than GO:0010955. Reason: Specific child of GO:0010955 capturing the prohormone-processing context that is the actual physiological role. Supporting Evidence: PMID:10632593 Overexpression of proSAAS in the AtT-20 cells substantially reduces the rate of processing of the endogenous prohormone proopiomelanocortin. |
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Download this section (compressed HTML)Q: Should an extracellular anti-aggregant chaperone that prevents fibrillation but does not refold clients and does not perform a documented carrier/handoff be annotated to GO:0051787 misfolded protein binding (plus BP GO:1905907 negative regulation of amyloid fibril formation), or to GO:0140309 unfolded protein holdase activity? The CLU/clusterin curation in this repository uses the GO:0051787 + GO:1905907 + GO:0050821 combination for an analogous activity; the SCG5/7B2 review takes a similar approach. Is this the agreed convention for "anti-aggregant" chaperones, or is a new MF term warranted?
Suggested experts: I. Lindberg
Q: The TDP-43-encapsulating sphere phenotype reported in PMID:35549000 was obtained by deliberately removing ProSAAS's signal peptide to force cytoplasmic expression. Should this evidence be used to support a normal cellular MF (holdase activity) or restricted to "demonstrates capacity for prion-like client binding" with no associated cellular component annotation? ProSAAS does not normally reach the cytoplasm.
Suggested experts: I. Lindberg, J. Shorter
Q: Are PEN (GPR83 ligand) and Big LEN (GPR171 ligand) better captured by annotating the cleavage products as separate functional isoforms with their own MF (peptide-receptor ligand activity) and BP (G protein-coupled receptor signaling pathway) annotations, rather than by the current GO:0005102 signaling receptor binding annotation on the precursor?
Suggested experts: L. Devi, L. Fricker
Experiment: PCSK1N knockout (or neuron-specific conditional KO) crossed with AAV-alpha-synuclein PD models, with quantification of nigrostriatal TH loss, motor asymmetry, and rostral spread of alpha-synuclein after vagal AAV-aSyn injection. Compares the gain-of-function PMID:35527562 result against the loss-of-function condition.
Hypothesis: Endogenous (rather than transgenic) ProSAAS limits alpha-synuclein fibrillation and transsynaptic spread in vivo.
Type: in vivo mouse genetics + viral aggregation models
Experiment: Surface plasmon resonance and microscale thermophoresis with purified ProSAAS and AΞ²(1-42), alpha-synuclein, IAPP at defined assembly stages (monomer, oligomer, protofibril, mature fibril). Combine with AFM/EM to correlate binding modes with fibrillation outcome. Establishes whether the activity is on-pathway sequestration (holdase-like) versus passive co-aggregation.
Hypothesis: Recombinant ProSAAS binds amyloidogenic clients with stoichiometry and kinetics consistent with classical sub-stoichiometric "holdase" chaperones (e.g. small heat shock proteins, clusterin) and the binding is to monomers/early oligomers rather than mature fibrils.
Type: biophysical binding + fibrillation kinetics
Experiment: Immuno-EM and antibody mapping of N-terminal SAAS fragments in human AD and Pick's disease brain combined with PCSK1N gain- and loss-of-function in iPSC-derived tauopathy organoids; assess whether ProSAAS modulates tau fibrillation in vitro.
Hypothesis: The N-terminal SAAS deposits in tau inclusions of Pick's disease and AD are a consequence of proteostatic dysregulation rather than ProSAAS actively driving tau pathology.
Type: human neuropathology + iPSC functional study
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