Abstract
Many bacteria possess multiple chemosensory pathways that are composed of homologous signaling proteins. These pathways appear to be functionally insulated from each other, but little information is available on the corresponding molecular basis. We report here a novel mechanism that contributes to pathway insulation. We show that, of the four CheB paralogs of Pseudomonas aeruginosa PAO1, only CheB2 recognizes a pentapeptide at the C-terminal extension of the McpB (Aer2) chemoreceptor (KD = 93 µM). McpB is the sole chemoreceptor that stimulates the Che2 pathway, and CheB2 is the methylesterase of this pathway. Pectobacterium atrosepticum SCRI1043 has a single CheB, CheB_Pec, and 19 of its 36 chemoreceptors contain a C-terminal pentapeptide. The deletion of cheB_Pec abolished chemotaxis, but, surprisingly, none of the pentapeptides bound to CheB_Pec. To determine the corresponding structural basis, we solved the 3D structure of CheB_Pec. Its structure aligned well with that of the pentapeptide-dependent enzyme from Salmonella enterica. However, no electron density was observed in the CheB_Pec region corresponding to the pentapeptide-binding site in the Escherichia coli CheB. We hypothesize that this structural disorder is associated with the failure to bind pentapeptides. Combined data show that CheB methylesterases can be divided into pentapeptide-dependent and independent enzymes.
Highlights
Chemosensory pathways are among the most abundant prokaryotic signal transduction mechanisms [1]
Of the four CheB paralogs of Pseudomonas aeruginosa PAO1, only CheB2 recognizes a pentapeptide at the C-terminal extension of the McpB (Aer2) chemoreceptor (KD = 93 μM)
We showed previously that exclusively CheR2 but not any of the remaining three CheR homologs of P. aeruginosa binds to the McpB pentapeptide [13]
Summary
Chemosensory pathways are among the most abundant prokaryotic signal transduction mechanisms [1]. Tar possesses a C-terminal pentapeptide that is tethered to the C-terminal end of the chemoreceptor signaling domain via an unstructured linker [7] This pentapeptide, NWETF, represents an additional binding site for CheR and CheB [8,9]. Typhimurium [20,21,22,23] that share 95% of their amino acid sequence identity (Figure S1) Both species possess a single chemosensory pathway, a single pentapeptide-dependent CheB and two chemoreceptors, Tar and Tsr, that contain a C-terminal pentapeptide. UUppppeerrppaanneel:l: rraaww ttiittrraattiioonn dataa;; lower panel: iinntteeggrraatteedd,, ddiilluuttiioonn heeaatt--ccoorrrreecctteedd and concentration-nnoorrmmaalliizzeedd ppeeaakk aarreeaass ooff tthhee titraattiioonn dattaa for CheB22..DDaattaawweerreeffiittteedduussiinnggtthhee““oonneebbiinnddiinnggssiitteemmooddeell””oofftthhee MMiiccrrooCCaallvveerrssiioonn ooff OORRIIGGIINN This indicated that the CheR and CheB homologs of the Che pathway interact with the only chemoreceptor that feeds into this pathway, McpB (Figure 1). The sequence alignment of pentapeptide-dependent and independent CheB did not reveal any obvious conserved sequence features
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