SPCS1 (signal peptidase complex subunit 1, also SPC12, the microsomal signal peptidase 12 kDa subunit) is a small (169 aa) multi-pass endoplasmic reticulum membrane protein that is one of the three non-catalytic accessory subunits (with SPCS2 and SPCS3) of the eukaryotic ER signal peptidase complex (SPC). The SPC removes N-terminal signal sequences from secretory and membrane preproteins as they are translocated into the ER lumen; its catalytic activity resides in the SEC11A or SEC11C subunits, and SPCS1 is dispensable for enzymatic cleavage. SPCS1 spans the ER membrane twice with both termini facing the cytosol and only minimal lumenal exposure, and together with the other subunits it shapes a transmembrane "window" that locally thins the lipid bilayer, contributing to substrate selectivity for signal-peptide hydrophobic regions shorter than about 18-20 residues. Beyond its housekeeping role in signal peptide processing, SPCS1 acts as a host factor exploited by flaviviruses (West Nile, Dengue, Zika, yellow fever, Japanese encephalitis viruses) and hepatitis C virus, where it promotes post-translational processing of viral structural proteins and assembly of infectious virions through interactions with viral membrane proteins such as HCV NS2/E2 and flaviviral NS2B.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005787 signal peptidase complex | IBA GO_REF:0000033 | ACCEPT | Summary: SPCS1 is a constitutive accessory subunit of the ER signal peptidase complex; phylogenetic assignment is correct and reflects the core identity of the protein. Reason: Core complex membership, strongly supported experimentally (cryo-EM, ComplexPortal) and by UniProt. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt Component of the signal peptidase complex paralog A (SPC-A) composed of a catalytic subunit SEC11A and three accessory subunits SPCS1, SPCS2 and SPCS3 |
| GO:0045047 protein targeting to ER | IBA GO_REF:0000033 | REMOVE | Summary: SPCS1 does not target nascent proteins to the ER (that is the role of SRP and the Sec61 translocon); it acts after translocation as part of the signal peptidase complex that cleaves the signal peptide. The phylogenetic term is mis-specified; the accurate process is signal peptide cleavage, i.e. protein processing, which this gene already carries as an ACCEPTed IDA annotation (GO:0016485, PMID:34388369). Reason: The SPC functions downstream of ER targeting, on already-translocating substrates, to cleave the signal peptide. UniProt/ComplexPortal annotate this activity as signal peptide processing (GO:0006465). GO has since obsoleted GO:0006465 because it represents the molecular function signal peptidase activity (GO:0009003, captured in core_functions); its live process-level parent is protein processing (GO:0016485), which is already an ACCEPTed IDA annotation on this gene (PMID:34388369). A MODIFY to GO:0016485 would duplicate that row, so the mis-specified IBA term is removed rather than replaced. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt catalyzes the cleavage of N-terminal signal sequences from nascent proteins as they are translocated into the lumen of the endoplasmic reticulum |
| GO:0005787 signal peptidase complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic assignment to the signal peptidase complex, consistent with experimental evidence. Reason: Correct core complex membership; redundant with experimental IPI/IBA evidence. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt Component of the signal peptidase complex paralog A (SPC-A) composed of a catalytic subunit SEC11A and three accessory subunits SPCS1, SPCS2 and SPCS3 |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: SPCS1 is a multi-pass ER membrane protein; the subcellular-location-based electronic annotation is correct and reflects the core localization. Reason: Core localization, supported by experimental topology (PMID:8632014) and structure (PMID:34388369). Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0016020 membrane | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Generic membrane localization, a parent of the more informative ER membrane term. Reason: Correct but uninformative; the specific GO:0005789 (ER membrane) better captures SPCS1 localization. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt Multi-pass membrane protein |
| GO:0005515 protein binding | IPI PMID:24009510 Signal peptidase complex subunit 1 participates in the assem... | KEEP AS NON CORE | Summary: IntAct capture of SPCS1 interaction with hepatitis C virus NS2; a real host-pathogen interaction but the bare protein binding term is uninformative. Reason: Real experimental interaction (HCV NS2/E2) but bare protein binding is uninformative per curation guidelines and reflects a secondary viral role. Supporting Evidence: PMID:24009510 SPCS1 was found to interact with both NS2 and E2 |
| GO:0005787 signal peptidase complex | IPI PMID:34388369 Structure of the human signal peptidase complex reveals the ... | ACCEPT | Summary: ComplexPortal annotation of SPCS1 as a component of the signal peptidase complex, based on the cryo-EM structure of the human SPC. Reason: Definitive experimental (structural) evidence for SPC membership; this is the core identity of SPCS1. Supporting Evidence: PMID:34388369 the human SPC exists in two functional paralogs with distinct proteolytic subunits |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:34388369 Structure of the human signal peptidase complex reveals the ... | ACCEPT | Summary: Direct (structural) evidence places SPCS1 in the ER membrane as part of the SPC; correct core localization. Reason: IDA-supported ER membrane localization consistent with the multi-pass topology and the membrane-embedded SPC structure. Supporting Evidence: PMID:34388369 a transmembrane window collectively formed by all subunits locally thins the bilayer |
| GO:0016485 protein processing | IDA PMID:34388369 Structure of the human signal peptidase complex reveals the ... | ACCEPT | Summary: SPCS1 participates in proteolytic processing of preproteins as part of the signal peptidase complex; more specifically, the SPC cleaves N-terminal signal peptides (the former GO:0006465 signal peptide processing, now obsolete in GO, for which protein processing is the live process term). Reason: Correct involvement in the SPC's proteolytic processing of substrates; supported by the structural study and ComplexPortal curation. The core process is signal peptide cleavage, which GO now represents with this protein processing term (GO:0016485) after obsoleting GO:0006465 signal peptide processing. Supporting Evidence: PMID:34388369 the determinants for signal peptide cleavage |
| GO:0005789 endoplasmic reticulum membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity transfer of ER membrane localization from an ortholog; consistent with stronger experimental evidence. Reason: Correct core localization, redundant with IDA/IEA evidence. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005515 protein binding | IPI PMID:34388369 Structure of the human signal peptidase complex reveals the ... | KEEP AS NON CORE | Summary: Interactions with the other SPC subunits (SEC11A/SEC11C, SPCS2, SPCS3) captured during the structural study; the bare protein binding term is uninformative although the underlying intra-complex interactions are genuine and functionally relevant. Reason: Real intra-complex interactions (SPCS2, SPCS3, SEC11A/C) but bare protein binding is uninformative; the SPC membership is already captured by GO:0005787. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt Within the complex, interacts with SPCS2 and SPCS3 |
| GO:0005515 protein binding | IPI PMID:29593046 Host Factor SPCS1 Regulates the Replication of Japanese Ence... | KEEP AS NON CORE | Summary: IntAct capture of SPCS1 interaction with flaviviral NS2B; a real host-pathogen interaction but the bare protein binding term is uninformative. Reason: Real experimental interaction (flavivirus NS2B) but bare protein binding is uninformative and reflects a secondary viral role. Supporting Evidence: PMID:29593046 SPCS1 was found to interact with nonstructural protein 2B (NS2B) |
| GO:0019068 virion assembly | IMP PMID:24009510 Signal peptidase complex subunit 1 participates in the assem... | KEEP AS NON CORE | Summary: SPCS1 knockdown markedly reduces production of infectious HCV by impairing formation of the membrane-associated NS2-E2 complex; a real experimental phenotype but a host-pathogen (secondary) role rather than SPCS1's core cellular function. Reason: Experimentally supported viral host-factor phenotype; not the core housekeeping function of SPCS1. Supporting Evidence: PMID:24009510 SPCS1 plays a key role in the formation of the membrane-associated NS2-E2 complex via its interaction with NS2 and E2 |
| GO:0019068 virion assembly | IMP PMID:29593046 Host Factor SPCS1 Regulates the Replication of Japanese Ence... | KEEP AS NON CORE | Summary: SPCS1 loss reduces JEV virion assembly through interaction with NS2B; a real experimental viral phenotype but a secondary host-pathogen role. Reason: Experimentally supported viral host-factor phenotype; not the core cellular function. Supporting Evidence: PMID:29593046 The loss of SPCS1 function markedly reduced intracellular virion assembly and the production of infectious JEV particles |
| GO:0019082 viral protein processing | IMP PMID:27383988 A CRISPR screen defines a signal peptide processing pathway ... | KEEP AS NON CORE | Summary: A genome-wide CRISPR screen showed SPCS1 is required for proper cleavage of flavivirus structural proteins (prM and E); a real experimental phenotype but a secondary host-pathogen role. Reason: Experimentally supported viral host-factor role in processing flaviviral structural proteins; secondary to SPCS1's core function in cellular signal peptide processing. Supporting Evidence: PMID:27383988 a subset of endoplasmic reticulum-associated signal peptidase complex (SPCS) proteins was necessary for proper cleavage of the flavivirus structural proteins (prM and E) and secretion of viral particles |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-422051 | ACCEPT | Summary: Reactome curation of SPCS1 ER membrane localization (preproghrelin signal peptide cleavage context). Reason: Correct core localization; redundant with experimental evidence. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-9918795 | ACCEPT | Summary: Reactome curation of SPCS1 ER membrane localization (flavivirus signalase cleavage context). Reason: Correct core localization; redundant with experimental evidence. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-9918871 | ACCEPT | Summary: Reactome curation of SPCS1 ER membrane localization (signalase cleavage context). Reason: Correct core localization; redundant with experimental evidence. Supporting Evidence: file:human/SPCS1/SPCS1-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005787 signal peptidase complex | TAS PMID:8632014 Membrane topology of the 12- and the 25-kDa subunits of the ... | ACCEPT | Summary: Early biochemical/topology study identifying SPC12 (SPCS1) as the 12-kDa subunit of the mammalian signal peptidase complex. Reason: Correct core complex membership; the historical reference that cloned and characterized SPC12 as an SPC subunit. Supporting Evidence: PMID:8632014 We have determined the cDNA sequence of the remaining 12-kDa subunit (SPC12) |
| GO:0005789 endoplasmic reticulum membrane | IDA PMID:8632014 Membrane topology of the 12- and the 25-kDa subunits of the ... | ACCEPT | Summary: Direct topology study placing SPC12 (SPCS1) in the ER (microsomal) membrane, spanning it twice with cytosol-facing termini. Reason: IDA-supported ER membrane localization with experimentally determined topology. Supporting Evidence: PMID:8632014 Both polypeptides span the membrane twice with their N and C termini facing the cytosol |
| GO:0006508 proteolysis | TAS PMID:8632014 Membrane topology of the 12- and the 25-kDa subunits of the ... | KEEP AS NON CORE | Summary: General proteolysis annotation derived from SPCS1 being a subunit of the signal peptidase complex. SPCS1 itself is non-catalytic, and the cytosol-facing topology argues it is not directly involved in the lumenal cleavage; the specific process is signal peptide processing. Reason: Generic parent term; SPCS1 is a non-catalytic accessory subunit and the cited paper explicitly concluded it is likely involved in processes other than enzymatic signal-sequence cleavage. The informative process term is protein processing (GO:0016485; GO:0006465 signal peptide processing is now obsolete). Supporting Evidence: PMID:8632014 SPC12 and SPC25 are likely to be involved in processes other than the enzymatic cleavage of the signal sequence |
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Download this section (compressed HTML)Q: Does SPCS1 modulate the substrate range or efficiency of the signal peptidase complex (e.g., selectivity for particular signal-peptide hydrophobic-region lengths), or is it purely structural?
Q: Is the flavivirus/HCV dependence on SPCS1 a direct consequence of its signal-peptidase accessory role, or a separable function mediated by direct binding to viral transmembrane proteins (NS2B, NS2/E2)?
Experiment: Reconstitute the SEC11A- and SEC11C-paralog signal peptidase complexes in vitro with and without SPCS1 and measure cleavage kinetics across a panel of substrates with varying signal-peptide h-region lengths to define SPCS1's contribution to activity and selectivity.
Experiment: Targeted SPCS1 degradation (e.g., degron) followed by quantitative N-terminomics to identify cellular preproteins whose signal-peptide cleavage depends on SPCS1.
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