Studies on the role of CheS in Sinorhizobium meliloti chemotaxis
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Chemotaxis is the ability of an organism to sense its environment and move towards attractants and away from repellents. The two-component system controlling chemotaxis in bacteria contains a histidine kinase CheA, which is autophosphorylated in response to a signal from a ligand-bound transmembrane methyl-accepting chemotaxis protein. CheA transfers the phosphate group to its cognate response regulator which modulates flagellar rotation. Signal termination by dephosphorylation of the response regulator is necessary for the organism to react rapidly to changes in the environment. The phosphorylated response regulator CheY in Escherichia coli is dephosphorylated by CheZ, a phosphatase; certain organisms, such as Sinorhizobium meliloti, that lack a CheZ homolog have developed alternate methods of signal termination. The signaling chain of S. meliloti contains two response regulators, CheY1 and CheY2, in which CheY2 modulates flagellar rotation and CheY1 causes signal termination by acting as a phosphate sink. In addition to known chemotaxis components, the second gene in the chemotaxis operon of S. meliloti codes a 97 amino acid protein, called CheS. The phenotype of a cheS deletion strain is similar to that of a cheY1 deletion strain. Therefore, the possibility that CheS causes signal termination was explored in this work. The derived amino acid sequence of CheS showed similarities with its orthologs from other °-proteobacteria. Sequence conservation was highest at the centrally located °4 and °5 helices. Earlier observations that CheS localizes at the polar chemotaxis cluster in a CheA-dependent manner were confirmed, and the co-localization of CheS with CheA was demonstrated by fluorescence microscopy. The stable expression of CheS in the presence of CheA was confirmed by immunoblot. The same approach was used to establish the stable expression of CheS only in the presence of the P2 domain of CheA, but not with the P1 or P345 domains. Limited proteolysis followed by mass spectrometry defined CheA163-256 as the CheS binding domain, and this domain overlapped the previously defined CheY2-binding domain, CheA174-316. The role of CheS in the phosphate flux in S. meliloti chemotaxis was analyzed by assays using radio-labeled [?-?°P]ATP. CheS does not play a role in the autophosphorylation of CheA. However, CheS accelerated the rate of CheY1~P dephosphorylation by almost two-fold, but did not affect the rate of CheY2~P dephosphorylation. CheS also does not seem to affect phosphate flow in the retrophosphorylation from CheY2~P to CheA using acetyl [?°P]phosphate as phosphodonor. Since CheS increases the rate of CheY1 dephosphorylation, it can be envisioned that it either increases the association of CheY1 to CheA, increasing the flow of phosphate from CheA to CheY1, or directly accelerates the dephosphorylation of CheY1~P. The presence of a STAS domain and a conserved serine residue in CheS also raises the possibility that CheS may be phosphorylated by a yet unknown kinase, in a mechanism similar to the phosphorylation of Bacillus subtilis SpoIIAA by its cognate kinase SpoIIAB. Phosphorylated CheS may then switch CheA between a kinase or phosphotransferase ON/OFF state or activated CheS may directly interact with CheY1. Further studies are needed to determine the association of CheY1 with CheS to elucidate the mechanism of CheY1 dephosphorylation. This work has confirmed the in vitro association of CheS with CheA, determined the CheS binding domain on CheA, and indicated that CheS accelerates the dephosphorylation of CheY1~P. This has advanced our understanding of the role of CheS in the chemotaxis signaling chain of S. meliloti.
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