mucB (algN, PA0764) encodes MucB, a periplasmic RseB-family protein that acts as a negative regulator of the extracytoplasmic-function sigma factor AlgU (AlgT, sigma22). It is encoded in the algU-mucA-mucB-mucC-mucD operon and is exported across the inner membrane by a cleavable signal peptide. In the periplasm MucB binds tightly to the C-terminal periplasmic domain of the transmembrane anti-sigma factor MucA and protects it from cleavage by the site-1 protease AlgW, the first step of the regulated intramembrane proteolysis that releases AlgU. Crystal structures of MucB alone and in complex with the MucA periplasmic domain show a hydrophobic cavity proposed to sense lipid signals that dissociate the complex, and a disulfide bond that affects its oligomeric state. Loss of mucB derepresses AlgU, activating the algD promoter and converting P. aeruginosa to the alginate-overproducing mucoid form typical of cystic fibrosis isolates.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:19226327 Use of cell wall stress to characterize sigma 22 (AlgT/U) ac... | MODIFY | Summary: IPI with MucA (P38107). Wood and Ohman showed MucB is part of the MucA sigma-sequestration complex. Reason: Bare protein binding is uninformative. The partner, MucA, is the anti-sigma factor for AlgU, so the informative MF is antisigma factor binding, which the gene already carries by IBA. Proposed replacements: antisigma factor binding Supporting Evidence: PMID:19226327 Here we showed that MucB, encoded by the algT-mucABCD operon, interacts with MucA in the sigma-sequestration complex. |
| GO:0005515 protein binding | IPI PMID:19298369 Control of Pseudomonas aeruginosa AlgW protease cleavage of ... | MODIFY | Summary: IPI with MucA (P38107) from purified-protein binding and cleavage-inhibition assays (Cezairliyan and Sauer 2009). Reason: Bare protein binding is uninformative. MucB binds the periplasmic domain of the anti-sigma factor MucA, so antisigma factor binding is the informative replacement. Proposed replacements: antisigma factor binding Supporting Evidence: PMID:19298369 We find that MucB binds tightly to MucA and inhibits its cleavage by AlgW. |
| GO:0030288 outer membrane-bounded periplasmic space | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN002215378 (RseB family), seeded by E. coli RseB. Reason: MucB has a Sec signal peptide and was localised to the periplasm by Western blotting and phoA fusions. It acts there on the periplasmic domain of MucA. The IBD is sound for this gene. Supporting Evidence: PMID:9171421 By Western blotting and phoA fusion analyses, the MucB antagonist of sigma22 was found to localize to the periplasm of the cell. file:PSEAE/mucB/mucB-uniprot.txt SIGNAL 1..21 |
| GO:0032885 regulation of polysaccharide biosynthetic process | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: IBA from PTN002215378, whose GO:0032885 IBD is seeded only by this gene's own PseudoCAP IMP row. The target appearing in its own WITH/FROM is expected. Reason: True of MucB: through AlgU it controls the algD alginate operon, so it regulates alginate (polysaccharide) biosynthesis. This is a lineage-specific output of the AlgU regulon, not the core function, which is binding the anti-sigma factor MucA. The alginate route itself does not propagate, because the algD operon is confined to Pseudomonas and Azotobacter; the only other leaf of the node with experimental annotation, E. coli RseB, is annotated to antisigma factor binding and protein stabilization and to nothing concerning polysaccharides. Whether the broad term holds for those leaves by another route is unresolved, so the IBD is queried rather than refuted (see source defect). Supporting Evidence: PMID:8412698 Insertional inactivation of mucB on the chromosome of the standard genetic strain PAO resulted in mucoid phenotype, and in a strong transcriptional activation of algD. PMID:19298369 RseB (a MucB homolog), which is a protein that inhibits DegS cleavage of RseA |
| GO:0032885 regulation of polysaccharide biosynthetic process | IMP PMID:9171421 Posttranslational control of the algT (algU)-encoded sigma22... | KEEP AS NON CORE | Summary: PseudoCAP IMP; sole seed of the PTN002215378 GO:0032885 IBD. Mathee et al. showed that an algT clone made PAO1 mucoid only when the coupled mucB was disrupted. Reason: The abstract supports the claim for this gene: an intact mucB keeps cells nonmucoid, and chromosomal mucB inactivation independently causes mucoidy with algD activation (PMID:8412698). The regulation is negative and acts via the sigma factor, but GO has no negative child of this term, and AlgU directly transcribes algD, so the term is acceptable. It is peripheral to the conserved molecular role. The PseudoCAP row is not a defect; the problem is its use as a family-wide seed. Supporting Evidence: PMID:9171421 However, the bacterial cells remained nonmucoid if the transcriptionally coupled mucB on the clone remained intact. PMID:8412698 Thus, a loss of mucB function is sufficient to cause conversion of P. aeruginosa into the mucoid phenotype. |
| GO:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping to periplasm. Reason: Consistent with the experimental periplasmic localisation and the signal peptide. The more specific outer membrane-bounded periplasmic space is also carried by IBA; both are correct. Supporting Evidence: PMID:9171421 By Western blotting and phoA fusion analyses, the MucB antagonist of sigma22 was found to localize to the periplasm of the cell. |
| GO:0045152 antisigma factor binding | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN002215378, seeded by E. coli RseB, which binds the anti-sigma factor RseA. Reason: MucB binds the periplasmic domain of the anti-sigma factor MucA tightly, shown with purified proteins and by crystal structure of the complex. This is the conserved RseB-family function and the core activity of MucB. Supporting Evidence: PMID:19298369 We find that MucB binds tightly to MucA and inhibits its cleavage by AlgW. PMID:31297938 MucB binds MucA in the periplasm and inhibits proteolysis of MucA and subsequent release of AlgU. |
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Download this section (compressed HTML)Q: Should the PTN002215378 regulation of polysaccharide biosynthetic process IBD be removed or moved to a Pseudomonas-only node, given that the other RseB leaves regulate sigmaE rather than polysaccharide synthesis?
Q: Which natural lipid ligands occupy the MucB hydrophobic cavity in vivo to release MucA for AlgW cleavage?
Experiment: Identify lipids co-purifying with periplasmic MucB from envelope-stressed cells, and test cavity mutants for loss of signal-induced MucA cleavage and AlgU activation.
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