SSR1 encodes Translocon-associated protein subunit alpha (TRAP-alpha), a single-pass type I ER membrane glycoprotein that is a core component of the heterotetrameric TRAP complex (TRAP-alpha/beta/gamma/delta, encoded by SSR1-4). The TRAP complex stably associates with the Sec61 protein-conducting channel and ribosomes at the ER membrane, facilitating co-translational translocation of secretory and membrane proteins, particularly those with weak or atypical signal peptides. TRAP-alpha features a large N-terminal luminal domain positioned beneath the Sec61 channel exit site and a single long transmembrane helix that contacts Sec61. The complex coordinates with the oligosaccharyltransferase (OST) to couple translocation with N-glycosylation. TRAP-alpha also participates in ER quality control and ERAD pathways.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005783 endoplasmic reticulum | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for ER localization based on phylogenetic inference from PANTHER. TRAP-alpha is well-established as an ER-resident protein across eukaryotes. Recent cryo-EM structures confirm TRAP as a core ER translocon component [SSR1-deep-research-falcon.md]. Reason: Core localization annotation. TRAP-alpha is definitively an ER membrane protein. The IBA annotation is well-supported by extensive structural and biochemical evidence showing TRAP-alpha as a core component of the ER translocon complex [Pauwels et al. 2023; Karki et al. 2023; PMID:36697828]. Supporting Evidence: PMID:8050590 The alpha-subunit of the TRAP complex (TRAP alpha) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane. file:human/SSR1/SSR1-deep-research-falcon.md model: Edison Scientific Literature |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation based on InterPro TRAP_alpha domain and UniProt subcellular location annotation. This is the specific membrane localization. Reason: Core localization annotation. TRAP-alpha is a single-pass type I membrane protein anchored in the ER membrane. UniProt confirms this topology with a luminal N-terminal domain and cytoplasmic C-terminus. Structural studies confirm ER membrane integration [PMID:36697828]. Supporting Evidence: PMID:36697828 near-complete atomic model of the most abundant ER translocon variant comprising the protein-conducting channel SEC61, TRAP and the oligosaccharyltransferase complex A (OSTA) reveals specific interactions of TRAP with other translocon components |
| GO:0005515 protein binding | IPI PMID:17380188 Simultaneous induction of the four subunits of the TRAP comp... | MODIFY | Summary: IPI annotation based on interaction with SERPINA1/alpha1-antitrypsin, calnexin (CANX), and SEC61B detected in the context of ER stress and ERAD studies. Reason: While the protein binding annotation reflects genuine interactions, it is uninformative. The study shows TRAP complex involvement in ERAD and interaction with misfolded substrates like alpha1-antitrypsin NHK variant. A more specific term capturing the functional interaction would be appropriate. Proposed replacements: protein localization to ribosome-translocon complex Supporting Evidence: PMID:17380188 The TRAP complex bound preferentially to misfolded proteins rather than correctly folded wild-type substrates. Thus, the TRAP complex induced by the unfolded protein response pathway might discriminate ERAD substrates from correctly folded substrates, accelerating degradation. |
| GO:0005515 protein binding | IPI PMID:22314232 Palmitoylated calnexin is a key component of the ribosome-tr... | MODIFY | Summary: IPI annotation for interaction with palmitoylated calnexin (CANX). The study demonstrates calnexin association with the ribosome-translocon complex including TRAP components. Reason: The protein binding term is too generic. The interaction with calnexin is functionally significant - palmitoylated calnexin associates with the ribosome-translocon complex to facilitate glycoprotein folding. A more specific MF term should be used. Proposed replacements: protein localization to ribosome-translocon complex Supporting Evidence: PMID:22314232 Palmitoylation mediates the association of calnexin with the ribosome-translocon complex (RTC) leading to the formation of a supercomplex that recruits the actin cytoskeleton, leading to further stabilization of the assembly. |
| GO:0005515 protein binding | IPI PMID:29568061 An AP-MS- and BioID-compatible MAC-tag enables comprehensive... | MODIFY | Summary: IPI annotation for interaction with calnexin detected by AP-MS and BioID proximity labeling methods. Reason: High-throughput interaction study confirming calnexin-TRAP association. The generic protein binding term is uninformative - should be captured by more specific translocon-related terms. Proposed replacements: protein localization to ribosome-translocon complex Supporting Evidence: PMID:29568061 An AP-MS- and BioID-compatible MAC-tag enables comprehensive mapping of protein interactions and subcellular localizations. |
| GO:0005515 protein binding | IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce... | MODIFY | Summary: IPI annotation from OpenCell study detecting interactions with calnexin and SEC61B via endogenous tagging. Reason: High-throughput proteomics confirming established TRAP complex interactions. The generic protein binding is uninformative for understanding function. These represent structural associations within the translocon complex. Proposed replacements: protein localization to ribosome-translocon complex Supporting Evidence: PMID:35271311 2022 Mar 11. OpenCell: Endogenous tagging for the cartography of human cellular organization. |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000107 | ACCEPT | Summary: IEA annotation transferred from mouse ortholog via Ensembl Compara. Reason: Redundant with IBA annotation but correct. ER localization is core to TRAP-alpha function and well-conserved across mammals. |
| GO:0005783 endoplasmic reticulum | IDA GO_REF:0000052 | ACCEPT | Summary: IDA annotation based on Human Protein Atlas immunofluorescence data. Reason: Direct experimental evidence for ER localization. HPA immunofluorescence confirms ER residence of SSR1/TRAP-alpha. |
| GO:0005789 endoplasmic reticulum membrane | NAS PMID:36697828 Visualization of translation and protein biogenesis at the E... | ACCEPT | Summary: NAS annotation from ComplexPortal based on cryo-ET structural study visualizing the SEC61-TRAP-OSTA translocon complex. Reason: Core localization. The structural study provides near-atomic resolution of TRAP within the ER membrane translocon complex [Gemmer et al. 2023]. Supporting Evidence: PMID:36697828 An AlphaFold-based model of TRAP could be fitted unambiguously into the SEC61-OSTA-TRAP translocon map, requiring only minor repositioning of single transmembrane helices |
| GO:0031204 post-translational protein targeting to membrane, translocation | NAS PMID:36697828 Visualization of translation and protein biogenesis at the E... | MODIFY | Summary: NAS annotation from ComplexPortal. However, this term refers to post-translational translocation, whereas TRAP is primarily involved in co-translational translocation. Reason: The term is inaccurate. TRAP/SSR1 is primarily involved in co-translational protein targeting, not post-translational. The structural study and all functional evidence show TRAP associating with translating ribosomes at the ER. Should be replaced with co-translational targeting term. Proposed replacements: cotranslational protein targeting to membrane Supporting Evidence: PMID:36697828 The dynamic ribosome-translocon complex, which resides at the endoplasmic reticulum (ER) membrane, produces a major fraction of the human proteome |
| GO:0005789 endoplasmic reticulum membrane | TAS Reactome:R-HSA-1791164 | ACCEPT | Summary: TAS annotation from Reactome pathway "Expression of SSR1 (Trap alpha)" indicating ER membrane localization. Reason: Core localization annotation. Reactome pathway information is consistent with established TRAP-alpha function. |
| GO:0005783 endoplasmic reticulum | TAS PMID:8050590 The N-terminal region of the alpha-subunit of the TRAP compl... | ACCEPT | Summary: TAS annotation from original characterization paper describing TRAP-alpha as an ER protein. Reason: Primary literature confirming ER localization. This paper established the widespread conservation of TRAP-alpha in eukaryotes. Supporting Evidence: PMID:8050590 The alpha-subunit of the TRAP complex (TRAP alpha) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane. |
| GO:0006613 cotranslational protein targeting to membrane | TAS PMID:8050590 The N-terminal region of the alpha-subunit of the TRAP compl... | ACCEPT | Summary: TAS annotation indicating involvement in co-translational protein targeting based on the original characterization. Reason: Core function annotation. TRAP-alpha is established as a component of the ribosome-translocon complex facilitating co-translational translocation of nascent polypeptides into the ER. Supporting Evidence: PMID:8050590 The alpha-subunit of the TRAP complex (TRAP alpha) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane. |
| GO:0008284 positive regulation of cell population proliferation | TAS PMID:10437777 Translocon-associated protein alpha transcripts are induced ... | KEEP AS NON CORE | Summary: TAS annotation based on observation that TRAP-alpha transcripts are induced by GM-CSF and more abundant in growing vs quiescent cells. Reason: This is a pleiotropic effect rather than a core function. The study shows TRAP-alpha mRNA is upregulated in proliferating cells stimulated by GM-CSF, but this reflects increased demand for protein secretion capacity rather than direct regulation of proliferation. TRAP-alpha's primary function is in protein translocation, not proliferation control. Supporting Evidence: PMID:10437777 Its messenger RNA was more abundant in growing cells than in either factor-deprived cells or quiescent cells |
| GO:0016020 membrane | TAS PMID:8050590 The N-terminal region of the alpha-subunit of the TRAP compl... | MARK AS OVER ANNOTATED | Summary: TAS annotation for generic membrane localization. Reason: Too general. GO:0005789 (endoplasmic reticulum membrane) is more specific and accurate for TRAP-alpha localization. This generic term adds no additional information. Supporting Evidence: PMID:8050590 The N-terminal region of the alpha-subunit of the TRAP complex has a conserved cluster of negative charges. |
| GO:0036503 ERAD pathway | TAS PMID:17380188 Simultaneous induction of the four subunits of the TRAP comp... | NEW | Summary: Proposed new annotation based on evidence that TRAP complex participates in ERAD by preferentially binding misfolded proteins. Reason: TRAP complex is induced by ER stress and preferentially binds misfolded proteins, facilitating their degradation via ERAD. This represents an additional function beyond co-translational translocation. Supporting Evidence: PMID:17380188 The TRAP complex bound preferentially to misfolded proteins rather than correctly folded wild-type substrates. Thus, the TRAP complex induced by the unfolded protein response pathway might discriminate ERAD substrates from correctly folded substrates, accelerating degradation. |
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Download this section (compressed HTML)Q: What is the precise mechanism by which TRAP-alpha recognizes and assists translocation of substrates with weak signal peptides?
Q: Does TRAP-alpha have substrate specificity or does it function equivalently for all translocon-dependent substrates?
Q: What is the functional significance of TRAP-alpha phosphorylation at Ser-268 during mitosis?
Experiment: Structure-function studies with TRAP-alpha luminal domain mutants to determine residues critical for nascent chain interaction
Experiment: Quantitative proteomics comparing translocation efficiency of defined substrates in TRAP-knockdown vs control cells
Experiment: Investigation of TRAP-alpha post-translational modifications and their regulation during cell cycle or ER stress
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