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Vibrio cholerae  contains multiple copies of chemotaxis response regulator ( Vc CheY1– Vc CheY4) whose functions are elusive yet. Although previous studies suggested that only  Vc CheY3 directly switches the flagellar rotation, the involvement of  Vc CheY4 in chemotaxis could not be ruled out. None of these studies, however, focused on the structure, mechanism of activation or molecular basis of FliM binding of the  Vc CheYs. From the crystal structures of Ca 2+  and Mg 2+  bound  Vc CheY3 we proposed the presence of a conformational barrier composed of the hydrophobic packing of W61, M88 and V106 and a unique hydrogen bond between T90 and Q97 in  Vc CheY3. Lesser fluorescence quenching and higher  K m   value of  Vc CheY3, compared to its mutants  Vc CheY3-Q97A and  Vc CheY3-Q97A/E100A supported our proposition. Furthermore, aforesaid biochemical data, in conjunction with the structure of  Vc CheY3-Q97A, indicated that the coupling of T90 and Q97 restricts the movement of T90 toward the active site reducing the stabilization of the bound phosphate and effectively promoting autodephosphorylation of  Vc CheY3. The structure of BeF 3   −  activated  Vc CheY3 insisted us to argue that elevated temperature and/or adequacy of phosphate pool might break the barrier of the free-state  Vc CheY3 and the conformational changes, required for FliM binding, occur upon phosphorylation. Structure of  Vc CheY4 has been solved in the free and sulfated states.  Vc CheY4 sulf , containing a bound sulfate at the active site, appears to be more compact and stable with a longer α4 helix, shorter β4α4 loop and hydrogen bond between T82 and the sulfate compared to  Vc CheY4 free . While pull down assay of  Vc CheYs with  Vc FliM NM  showed that only activated  Vc CheY3 can interact with  Vc FliM NM  and  Vc CheY4 cannot, a knowledge based docking explained the molecular mechanism of the interactions between  Vc CheY3 and  Vc FliM and identified the limitations of  Vc CheY4 to interact with  Vc FliM even in its phosphorylated state.

Conformational Barrier of CheY3 and Inability of CheY4 to Bind FliM Control the Flagellar Motor Action in Vibrio cholerae