McpB (Aer2, Aer-2, TlpG; PA0176) is a soluble, cytoplasmic methyl-accepting chemoreceptor of Pseudomonas aeruginosa, encoded in chemotaxis gene cluster II (che2) together with McpA and the CheA2/CheW2/CheY2/CheR2/CheB2 proteins. It lacks transmembrane segments. Its N-terminal half has three HAMP domains, then a PAS domain that binds penta-coordinated b-type heme, then two further HAMP domains and a conserved kinase control (MCP signalling) module ending in a GWEEF pentapeptide that recruits the CheR2 methyltransferase and the CheB2 methylesterase. The PAS-heme domain binds O2 (Kd about 16 uM), and also CO and NO, and ligand binding switches the receptor to a signal-on state; expressed in Escherichia coli, McpB mediates repellent responses to these gases. In P. aeruginosa an mcpB mutant has general chemotaxis defects and reduced aerotaxis, aerotaxis is lost in an aer mcpB double mutant, mcpB expression is RpoS-dependent, and the gene contributes to virulence in several infection models.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004888 transmembrane signaling receptor activity | IBA GO_REF:0000033 | MODIFY | Summary: Propagated from the E. coli transmembrane MCPs (Trg, Tar, Tap) and Aer at the MCP node PTN002079588. McpB is a receptor, but a soluble one with no transmembrane segment. Reason: The receptor part of the claim holds: McpB binds O2, CO and NO at its PAS-heme domain and signals to the chemotaxis kinase. The transmembrane part does not: McpB has no membrane-spanning segment and is cytoplasmic. GO:0004888 is_a GO:0038023 signaling receptor activity, which keeps what is true. The specific sensor activity is added as NEW GO:0019826 oxygen sensor activity. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH FUNCTIONAL DIVERGENCE Sources checked: PANTHER:PTN002079588 · PTN002079588 SUPPORTS SOURCE BUT NOT TARGET MCP node seeded by E. coli transmembrane chemoreceptors. McpB descends from it but has lost the transmembrane architecture and senses an intracellular signal. UniProtKB:P07017 · E. coli Tar SUPPORTS SOURCE BUT NOT TARGET Transmembrane receptor with periplasmic ligand-binding domain. Proposed replacements: signaling receptor activity Supporting Evidence: PMID:21255112 here we show that Aer-2 can interact with the chemotaxis system of Escherichia coli to mediate repellent responses to oxygen, carbon monoxide and nitric oxide PMID:14987771 A hydropathy plot of TlpG indicated that hydrophobic membrane-spanning regions are missing in TlpG. |
| GO:0004888 transmembrane signaling receptor activity | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the chemotaxis transducer signature, which covers soluble as well as transmembrane MCPs. Reason: Same claim as the IBA row. McpB is a soluble signalling receptor, so the transmembrane qualifier is wrong for it; signaling receptor activity (GO:0038023, the is_a parent) is what holds. Proposed replacements: signaling receptor activity Supporting Evidence: PMID:21255112 PAS/poly-HAMP signalling in Aer-2, a soluble haem-based sensor. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: McpB is a soluble cytoplasmic chemoreceptor without transmembrane segments. Reason: The sequence has no membrane-spanning region, and the full-length protein is soluble when expressed. The cytoplasm is where it senses gases and signals. Supporting Evidence: PMID:14987771 A hydropathy plot of TlpG indicated that hydrophobic membrane-spanning regions are missing in TlpG. PMID:21255112 coli, Aer-2 is stable, soluble, and amenable to structural and functional analyses |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | REMOVE | Summary: Propagated from the E. coli transmembrane MCPs. McpB is a soluble cytoplasmic receptor. Reason: McpB lacks the transmembrane segments that anchor the seed receptors in the inner membrane, and it is soluble. This is a lineage-specific loss of the membrane architecture inside the MCP clade, so the plasma membrane IBA does not apply to this leaf. Chemoreceptor arrays are membrane-proximal, but no evidence places McpB in the plasma membrane. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN002079588 · PTN002079588 SUPPORTS SOURCE BUT NOT TARGET The node location is right for the transmembrane MCPs that seed it, not for the soluble McpB. Supporting Evidence: PMID:14987771 A hydropathy plot of TlpG indicated that hydrophobic membrane-spanning regions are missing in TlpG. PMID:21255112 PAS/poly-HAMP signalling in Aer-2, a soluble haem-based sensor. |
| GO:0006935 chemotaxis | IBA GO_REF:0000033 | ACCEPT | Summary: Chemotaxis IBD at the MCP node PTN002079588. McpB is one of six seeds, through its own PseudoCAP aerotaxis IMP; the other five are E. coli chemoreceptors. Reason: McpB appearing in its own WITH/FROM is expected, not circular. An mcpB mutant is defective in chemotaxis in swarm assays with many attractants and is complemented in trans, and the mutant has reduced aerotaxis. The receptor signals through a CheA kinase, as the clade does. Supporting Evidence: PMID:12142407 An mcpB mutant was defective in chemotaxis when assayed in dilute rich soft-agar swarm medium or in minimal-medium swarm plates containing any 1 of 60 chemoattractants. PMID:14987771 Mutant strains which contained deletions in the methyl-accepting chemotaxis protein-like genes tlpC and tlpG showed decreased aerotaxis. |
| GO:0006935 chemotaxis | IEA GO_REF:0000120 | ACCEPT | Summary: Automatic chemotaxis annotation from the chemotaxis transducer signature. Reason: Consistent with the IBA row and the mutant evidence. Supporting Evidence: PMID:12142407 The mutant phenotype could be complemented by the addition of mcpB in trans. |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | MODIFY | Summary: Correct but general. McpB is a cytoplasmic receptor that relays an intracellular gas signal to the chemotaxis kinase. Reason: The signal (O2, CO or NO bound by the cytoplasmic PAS-heme domain) is sensed and transmitted entirely inside the cell, through the HAMP domains to the kinase control module, so intracellular signal transduction (GO:0035556, an is_a descendant) is the precise term. Proposed replacements: intracellular signal transduction Supporting Evidence: PMID:28167524 The signal then propagates from the PAS domain to regulate the C-terminal HAMP and kinase control domains, ultimately modulating a cellular response. |
| GO:0009454 aerotaxis | IMP PMID:14987771 Chemotaxis proteins and transducers for aerotaxis in Pseudom... | TOO GENERAL | Summary: PseudoCAP IMP seed for the PTN002079588 chemotaxis IBD (with the CACAO positive aerotaxis IMP row). The abstract states that the tlpG (mcpB) mutant showed decreased aerotaxis and that the tlpC tlpG double mutant was aerotaxis-negative. Reason: Seed verdict SOUND. A transducer mutant that loses aerotaxis shows the gene's own process, and aerotaxis is_a chemotaxis, so the seed supports the node term. The row is entailed by GO:0052131 positive aerotaxis (is_a aerotaxis), accepted on this gene from the same paper. Supporting Evidence: PMID:14987771 Mutant strains which contained deletions in the methyl-accepting chemotaxis protein-like genes tlpC and tlpG showed decreased aerotaxis. PMID:14987771 A double mutant deficient in tlpC and tlpG was negative for aerotaxis. |
| GO:0016020 membrane | IEA GO_REF:0000002 | REMOVE | Summary: InterPro2GO membrane mapping from HAMP and MCP signatures. McpB has no membrane-spanning segment. Reason: The HAMP and MCP signal-domain signatures occur in soluble receptors too, and the mapping is demonstrably wrong for McpB, which is soluble and has no hydrophobic membrane-spanning region. Supporting Evidence: PMID:14987771 A hydropathy plot of TlpG indicated that hydrophobic membrane-spanning regions are missing in TlpG. PMID:20399181 The crystal structure of a three-unit poly-HAMP chain from the Pseudomonas aeruginosa soluble receptor Aer2 |
| GO:0042802 identical protein binding | IPI PMID:20399181 Structure of concatenated HAMP domains provides a mechanism ... | KEEP AS NON CORE | Summary: The crystal structure of the N-terminal poly-HAMP region (residues 1-172) is a symmetric dimer. Reason: Homodimerisation is real and is the normal state of chemoreceptors, but it is structural rather than the receptor's sensing or signalling activity. Supporting Evidence: PMID:20399181 The structure of Aer2 1-172 is a symmetric dimer containing three HAMP domains (Figure 2), each similar to the NMR structure of Af1503. |
| GO:0042802 identical protein binding | IPI PMID:23424282 HAMP domain conformers that propagate opposite signals in ba... | KEEP AS NON CORE | Summary: Structural work on Aer2 HAMP conformers, building on the dimeric poly-HAMP structure. Reason: Same claim as the other IPI row; true, but structural rather than core. Supporting Evidence: PMID:23424282 We subsequently determined the structure of a poly-HAMP domain composed of three concatenated HAMP units from the Pseudomonas aeruginosa soluble receptor Aer2 |
| GO:0052131 positive aerotaxis | IMP PMID:14987771 Chemotaxis proteins and transducers for aerotaxis in Pseudom... | ACCEPT | Summary: CACAO IMP; the tlpG (mcpB) deletion mutant shows decreased aerotaxis, and the aer mcpB double mutant has none. Reason: Aerotaxis assays measure migration towards the preferred oxygen concentration, and the receptor mutant is defective. McpB is an O2-binding receptor, so a direct role is plausible. In E. coli McpB mediates repellent responses, so the sign of its contribution in P. aeruginosa may depend on context, but the PAO1 phenotype is a loss of positive aerotaxis. Supporting Evidence: PMID:14987771 Mutant strains which contained deletions in the methyl-accepting chemotaxis protein-like genes tlpC and tlpG showed decreased aerotaxis. PMID:14987771 A double mutant deficient in tlpC and tlpG was negative for aerotaxis. |
| GO:0020037 heme binding | IDA PMID:21255112 PAS/poly-HAMP signalling in Aer-2, a soluble haem-based sens... | NEW | Summary: The purified McpB PAS domain binds penta-coordinated b-type heme, coordinated by His234. Reason: Direct spectroscopic evidence from purified protein, confirmed by mutagenesis of the heme pocket. Heme is the cofactor of the gas sensor, so this is part of the core mechanism. Supporting Evidence: PMID:21255112 we determined that the Aer-2 PAS domain binds penta-co-ordinated b-type haem PMID:28167524 H234 coordinated heme |
| GO:0019826 oxygen sensor activity | IDA PMID:28167524 Gas sensing and signaling in the PAS-heme domain of the Pseu... | NEW | Summary: The PAS-heme domain binds O2 with a Kd of about 16 uM, O2 appears to be the native ligand, and O2 binding initiates the signalling conformational change. Reason: Two lines agree. Purified PAS-heme binds O2 and mutants that bind O2 signal, while those that cannot stabilise O2 do not; and full-length McpB mediates O2-dependent responses through a chemotaxis system. This is the receptor's own sensing activity, absent from GOA. Supporting Evidence: PMID:28167524 O2 appeared to be the native Aer2 ligand PMID:28167524 With one exception, mutants that bound O2 could signal, whereas many mutants that bound CO could not. |
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Download this section (compressed HTML)Q: What is the physiological output of the McpB/Che2 pathway in P. aeruginosa? McpB mediates gas repellent responses in E. coli, yet the PAO1 mutant loses positive aerotaxis, and the che2 cluster is RpoS-regulated.
Q: Should PAINT record a loss of the transmembrane architecture (plasma membrane, transmembrane signaling receptor activity) for the soluble McpB leaf under the MCP node PTN002079588?
Experiment: Temporal aerotaxis and O2-gradient capillary assays with PAO1, mcpB deletion and heme-pocket point mutants (H234A, W283A) in stationary-phase cells, when the RpoS-dependent che2 cluster is expressed, to define the sign and context of the McpB response.
Type: behavioural assay
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