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Publication Abstract from PubMed

Bacteria sense environmental chemicals using chemosensor proteins, most of which are present in the cytoplasmic membrane. Canonical chemoreceptors bind their specific ligands in their periplasmic domain, and the ligand binding creates a molecular stimulus that is transmitted into the cytoplasm, leading to various cellular responses, such as chemotaxis and specific gene expression. Vibrio cholerae, the causative agent of cholera, contains about 44 putative sensor proteins, which are homologous to methyl-accepting chemotaxis proteins involved in chemotaxis. Two of them, Mlp24 and Mlp37, have been identified as chemoreceptors that mediate chemotactic responses to various amino acids. Although most of the residues of Mlp37 involved in ligand binding are conserved in Mlp24, these chemoreceptors bind the same ligands with different affinities. Moreover, they have distinct cellular roles. Here we determined a series of ligand complex structures of the periplasmic domains of Mlp24 (Mlp24p). The structures revealed that Ca(2+) binds to the loop that forms the upper wall of the ligand-binding pocket. Ca(2+) does not bind to the corresponding loop of Mlp37, implying that the structural difference of the loop may cause the ligand affinity difference. ITC measurements indicated that Ca(2+) changes the ligand binding affinity of Mlp24p. Furthermore, Ca(2+) affected chemotactic behaviors to various amino acids mediated by Mlp24. Thus, Ca(2+) is suggested to serve as a co-signal for the primary signal mediated by Mlp24p and V. cholerae fine-tunes its chemotactic behavior depending on the Ca(2+) concentration by modulating the ligand sensitivity of Mlp24.ImportanceMlp24 and Mlp37 are homologous chemoreceptors of Vibrio cholerae that bind various amino acids. Although most of the residues involved in ligand interaction are conserved, these chemoreceptors show different affinity for the same ligand and play different cellular roles. A series of ligand complex structures of the periplasmic region of Mlp24 (Mlp24p) and following ITC analysis revealed that Ca(2+) binds to the loop of Mlp24p and modulates the ligand binding affinity of Mlp24p. Moreover, Ca(2+) changes the chemotactic behaviors mediated by Mlp24. We propose that Ca(2+) acts as a co-signal that modulates the affinity of Mlp24 for the primary signal, thereby changes the chemotactic behavior of V. cholerae.

Calcium ion modulates amino acid sensing of the chemoreceptor Mlp24 of Vibrio cholerae.,Takahashi Y, Nishiyama SI, Sumita K, Kawagishi I, Imada K J Bacteriol. 2019 Feb 11. pii: JB.00779-18. doi: 10.1128/JB.00779-18. PMID:30745373^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.