PIGBOS1 (PIGB opposite strand 1, PIGBOS) is a 54-amino-acid microprotein encoded by a small open reading frame on the strand opposite the GPI-anchor biosynthesis gene PIGB. It is widely expressed and has no paralogs. PIGBOS1 is a signal-anchored, single-pass protein of the mitochondrial outer membrane: a short N-terminal segment faces the intermembrane space, a single transmembrane helix anchors it, and a C-terminal region of about 25 residues faces the cytosol. Through this cytosolic region (residues 30-36 are critical) it binds CLCC1, an endoplasmic reticulum chloride channel, and the interaction is concentrated where the ER lies close to mitochondria. PIGBOS1 does not tether the two organelles; loss of the protein leaves ER-mitochondria contacts unchanged. Its best-supported role is to dampen the ER unfolded protein response: cells lacking PIGBOS1 respond to ER stressors with stronger XBP1 splicing, ATF6 activity and UPR target-gene induction and undergo more ER stress-induced apoptosis, overexpression has the opposite effect, and rescue depends on the CLCC1-binding region. PIGBOS1 self-associates. A more recent study reports that the PIGBOS1-CLCC1 interaction also promotes ER calcium release, ER-to-mitochondria calcium transfer, store-operated calcium entry and mitochondrial respiration.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:31653868 Regulation of the ER stress response by a mitochondrial micr... | MODIFY | Summary: Interaction with CLCC1 (UniProtKB:Q96S66), shown by IP-MS with SAINT/CRAPome filtering, APEX proximity labelling, reciprocal co-immunoprecipitation and split-GFP complementation, and mapped to PIGBOS1 residues 30-36. GOA carries this row twice (UniProt and IntAct); both are covered here. Reason: The interaction is very well supported, but bare protein binding says nothing about it. CLCC1 is an ER chloride channel (it carries GO:0005254 chloride channel activity by IDA in GOA), so GO:0044325 transmembrane transporter binding states what was shown. The interaction is also functionally necessary: CLCC1-binding-deficient mutants fail to rescue the UPR phenotype of PIGBOS1-depleted cells. Proposed replacements: transmembrane transporter binding Supporting Evidence: PMID:31653868 Immunoprecipitation experiments identified amino acids 30–36 of PIGBOS to be critical for CLCC1 binding since mutation of these amino acids to alanine resulted in decreased CLCC1 enrichment PMID:31653868 CLCC1 is a putative chloride channel localized to the ER |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: PIGBOS1 is included in the MitoCoP high-confidence human mitochondrial proteome. Reason: Correct and consistent with the targeted localization work, though less informative than the mitochondrial outer membrane annotations this gene also carries. Supporting Evidence: PMID:34800366 the small open reading frames (smORF)-encoded proteins SMIM26, SLC35A4, and PIGBOS1 |
| GO:0005741 mitochondrial outer membrane | IDA PMID:31653868 Regulation of the ER stress response by a mitochondrial micr... | ACCEPT | Summary: Mitochondrial fractionation, immunofluorescence with Tom20, a proteinase K protection assay on intact mitochondria and live-cell split-GFP all place PIGBOS1 in the mitochondrial outer membrane with its C terminus in the cytosol. Reason: Correct and specific primary location; the core location of the protein. Supporting Evidence: PMID:31653868 Proteolysis of isolated mitochondria with proteinase K under conditions that retain MOM integrity led to the degradation of PIGBOS—an identical result to that of the MOM protein Tom20—indicating that PIGBOS is a MOM microprotein PMID:31653868 PIGBOS’s topology indicates that it belongs to a group of MOM proteins called signal-anchor proteins |
| GO:0005741 mitochondrial outer membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Combined electronic annotation, drawing on the UniProt subcellular location and the rat ortholog; agrees with the direct experimental evidence. Reason: Correct; duplicates the IDA annotation from PMID:31653868. Supporting Evidence: PMID:31653868 Proteolysis of isolated mitochondria with proteinase K under conditions that retain MOM integrity led to the degradation of PIGBOS—an identical result to that of the MOM protein Tom20—indicating that PIGBOS is a MOM microprotein |
| GO:1900101 regulation of endoplasmic reticulum unfolded protein response | IMP PMID:31653868 Regulation of the ER stress response by a mitochondrial micr... | MODIFY | Summary: siRNA knockdown or CRISPR knockout of PIGBOS1 heightens the response to tunicamycin, thapsigargin and brefeldin A (XBP1 splicing, ATF6 reporter, UPR target genes, ATF4 and CHOP protein); siRNA-resistant PIGBOS1 rescues, and overexpression dampens the UPR. Mitochondrial UPR is unaffected. Reason: The annotation is sound and captures the core biological role of PIGBOS1, but the evidence is directional in both senses - loss increases and gain decreases UPR output, with rescue - so the more specific child term GO:1900102 negative regulation of endoplasmic reticulum unfolded protein response is supported. PIGBOS1 participates as a regulator (its CLCC1-binding region is required for rescue), not merely as a necessary bystander. Proposed replacements: negative regulation of endoplasmic reticulum unfolded protein response Supporting Evidence: PMID:31653868 Overexpression of PIGBOS in WT cells resulted in the desensitization of cells to UPR with decreased XBP1 splicing PMID:31653868 Upon UPR induction with TM, the loss of PIGBOS led to dramatic increases in the levels of all UPR target genes measured, indicating increased UPR signaling across all the branches (IRE1, PERK, and ATF6) PMID:31653868 Only full-length PIGBOS microprotein reversed the XBP1 splicing phenotype |
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Download this section (compressed HTML)Q: Is PIGBOS1 a regulator of CLCC1 channel activity? CLCC1 has chloride channel activity and a CLCC1-binding-dependent effect of PIGBOS1 on ER calcium release, ER-to-mitochondria calcium transfer and store-operated calcium entry has been reported (PMID:42351194), but whether PIGBOS1 changes CLCC1 conductance directly has not been measured. A positive answer would justify a channel regulator activity term in place of transporter binding.
Q: Should PIGBOS1 be annotated to GO:0044233 mitochondria-associated ER membrane contact site? The PIGBOS1-CLCC1 split-GFP signal lies between ER and outer membrane markers, and GOA gives CLCC1 this term from the same paper, but PIGBOS1 is not a tether and only a fraction of it is bound to CLCC1; outer-membrane contact-site partners such as RMDN3 and MFN2 do not carry the term either.
Q: How does an outer membrane signal-anchor protein with about four intermembrane-space residues copurify HIGD2A and complex III/IV subunits in digitonin-solubilized mitochondria (PMID:34800366)? Is this a post-lysis association or evidence of a second pool?
Experiment: Electrophysiology of reconstituted CLCC1 (planar bilayer or ER-targeted patch) with and without PIGBOS1 or its 30-36 alanine mutant, to test whether PIGBOS1 directly modulates CLCC1 conductance.
Experiment: Endogenous-level proximity labelling or super-resolution imaging of PIGBOS1 to measure what fraction resides at ER-mitochondria contacts, and whether the fraction changes under ER stress.
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