VgrG1b (PA0095) is a valine-glycine repeat G (VgrG) spike protein encoded in the HSI-I region of Pseudomonas aeruginosa, close to vgrG1a, within a seven-gene cluster (PA0095-PA0101) and co-regulated with the H1 type VI secretion system through the RetS/Gac-Rsm pathway. It is closely related to VgrG1a (64% identity), but it behaves differently from the other two co-regulated VgrGs: its own release into the medium does not require any of the three ClpV ATPases, and it is dispensable for Hcp1 secretion and for secretion of VgrG1a and VgrG1c. Later work showed that VgrG1b nevertheless forms a spike capped by a PAAR-domain protein encoded in its own cluster and that delivery of that cargo to neighbouring bacteria requires the H1-T6SS cluster, so VgrG1b is a cluster-specific spike rather than a core component of the Hcp1 apparatus.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005576 extracellular region | EXP PMID:21325275 Type VI secretion system in Pseudomonas aeruginosa: secretio... | KEEP AS NON CORE | Summary: VgrG1b is detected in culture supernatants with VgrG1b-specific antibodies, by a route independent of the three T6SS clusters. Reason: Directly shown, and the authors excluded lysis and periplasmic leakage, so the location is sound. It is peripheral to the function, and the export route is unidentified. Supporting Evidence: PMID:21325275 Thus, VgrG1b secretion is independent of the three known T6SS clusters. |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Combined IEA (ARBA plus the UniProt "Secreted" subcellular location). Reason: Same claim as the EXP row, which shows it directly. Supporting Evidence: PMID:21325275 Thus, VgrG1b secretion is independent of the three known T6SS clusters. |
| GO:0033103 protein secretion by the type VI secretion system | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT IBA from PTN002088178 (a VgrG clade, 13 leaf IBAs in P. aeruginosa, Yersinia pestis and Salmonella), seeded by VgrG1a, VgrG1b and VgrG1c. Reason: The inherited claim holds for VgrG1b, although not through the route its own PseudoCAP seed cites. VgrG1b forms a spike topped by a PAAR protein and delivery of that cargo to target cells depends on the H1-T6SS cluster, so VgrG1b does part of the work of T6SS-mediated export. The node term is right for a clade of VgrG spike proteins, and the IBD rests on the sound VgrG1a and VgrG1c seeds in any case. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike PMID:24794869 Finally, VgrG1b-dependent killing is detectable upon complementation of a triple vgrG1abc mutant. |
| GO:0033103 protein secretion by the type VI secretion system | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of the same process. Reason: Same claim as the IBA row; supported for VgrG1b by the PAAR-capped spike work, not by the 2011 secretion analysis. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike |
| GO:0033103 protein secretion by the type VI secretion system | IMP PMID:21325275 Type VI secretion system in Pseudomonas aeruginosa: secretio... | ACCEPT | Summary: PseudoCAP IMP citing Hachani et al. 2011, which reports the opposite for this gene: the vgrG1b mutant secretes Hcp1, VgrG1a and VgrG1c normally, and VgrG1b's own release needs none of the three T6SSs. Reason: The term is true of VgrG1b, so the row is kept, but not on this evidence. The cited paper's vgrG1b mutant phenotype is explicitly negative, so an IMP row pointing at it misrepresents the result (full text read). The claim is established instead by the 2018 work showing a PAAR-capped VgrG1b spike whose cargo delivery requires the H1-T6SS cluster, and by the 2014 work in which vgrG1b alone restores activity to a triple vgrG1abc mutant. The reference needs correcting; see the PSEUDOCAP defect. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike PMID:24794869 Finally, VgrG1b-dependent killing is detectable upon complementation of a triple vgrG1abc mutant. |
| GO:0033104 type VI protein secretion system complex | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: PAINT IBA from PTN002088178, seeded by VgrG1a, VgrG1b and VgrG1c. Reason: VgrG1b is a spike protein and its cargo delivery depends on the H1-T6SS cluster, so membership of a T6SS complex is defensible. But it is not a core component of the Hcp1 apparatus: the Hcp1 tube is assembled and fired without it, and VgrG1b is not needed for VgrG1a or VgrG1c secretion. Whether a VgrG1b-capped spike is a distinct assembly has not been resolved, so the term is kept as a non-core claim. Propagation Review Root cause: NO FAILURE NON CORE Failure modes: CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN002088178 · PTN002088178 SUPPORTS TRANSFER VgrG clade (13 leaf IBAs, 3 taxa: P. aeruginosa VgrGs plus Yersinia pestis and Salmonella homologues). The node term suits spike proteins. UniProtKB:Q9I741 · vgrG1a (PA0091) SUPPORTS TRANSFER IDA and IMP seeds sound; required with VgrG1c for Hcp1 secretion. UniProtKB:Q9I0F3 · vgrG1c (PA2685) SUPPORTS TRANSFER IDA and IMP seeds sound; required with VgrG1a for Hcp1 secretion. UniProtKB:Q9I737 · vgrG1b (self) SOURCE WEAK OR INFERRED Expected self-seed, but the cited 2011 paper reports a negative phenotype for this gene; the claim rests on later work instead. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike |
| GO:0033104 type VI protein secretion system complex | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA prediction of complex membership. Reason: Same claim as the IBA row, and non-core for the same reason. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike |
| GO:0033104 type VI protein secretion system complex | IMP PMID:21325275 Type VI secretion system in Pseudomonas aeruginosa: secretio... | KEEP AS NON CORE | Summary: PseudoCAP IMP citing Hachani et al. 2011, in which the vgrG1b mutant has no effect on H1-T6SS assembly or secretion and VgrG1b release is T6SS-independent. Reason: Membership of a T6SS complex is defensible for a spike protein whose cargo delivery needs the H1-T6SS cluster, and non-core because the Hcp1 apparatus works without it. The cited paper's result for this gene is negative and cannot be its IMP support (full text read); the reference needs correcting, as recorded in the PSEUDOCAP defect on the GO:0033103 IMP row, which covers both rows. Supporting Evidence: PMID:30455305 Tse7 sits on the tip of the VgrG1b spike PMID:21325275 Overall, these results indicate that although vgrG1b is co-expressed with H1-T6SS genes, VgrG1b neither impacts secretion of components of the H1-T6SS nor requires H1-T6SS components for its own secretion. |
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Download this section (compressed HTML)Q: Does VgrG1b cap a sheath-driven apparatus of its own, or does it reach the tip of the H1-T6SS machine only when its PAAR partner is present?
Experiment: Track fluorescently tagged VgrG1b alongside a tagged H1-T6SS sheath in single cells, with and without its PAAR partner, and test whether its release depends on the sheath and baseplate genes rather than on the ClpV ATPases.
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The route by which VgrG1b reaches the cell exterior is unidentified. Its release requires none of the three ClpV ATPases, so no known T6SS accounts for it, and no alternative export pathway has been shown.
BIOLOGY
What is known: Established: VgrG1b is genuinely exported (not released by lysis or periplasmic leakage), its release is unaffected by clpV1, clpV2 and clpV3 deletions, and delivery of its PAAR-capped cargo to target bacteria depends on the H1-T6SS cluster.
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