Molecular basis of ChvE function in sugar binding and virulence in Agrobacterium tumefaciens.

ChvE is a chromosomally encoded protein in Agrobacterium tumefaciens that mediates a sugar-related increase in virulence (vir ) gene expression through the activities of the VirA/VirG two-component system, and has been proposed to be involved in both sugar utilization and chemotaxis towards sugars. The ChvE protein has homology to several bacterial periplasmic sugar binding proteins such as ribose-binding protein (RBP) and galactose/glucose-binding protein (GGBP) of Escherichia coli. In this study, we characterized the role of the chvE operon in sugar-related processes other than virulence and examined the biophysical properties of the ChvE protein. Further, ChvE mutants with altered vir gene expression phenotypes have been isolated and characterized. Three distinct categories of mutants have been identified: those that cannot bind sugar, those that can bind sugar but have effects on both sugar utilization and vir gene expression and, finally, those that can bind and utilize sugar in a wild type fashion but affect vir gene expression. Examination of a homology model of ChvE shows the mutated residues associated with the latter two phenotypes lie in two overlapping solvent-exposed sites adjacent to the sugar-binding cleft where conformational changes associated with the binding of sugar might have a maximal affect on ChvE's interactions with VirA and other proteins. We propose that the third category of mutants define a surface of ChvE that interacts with the periplasmic domain of the sensor kinase, VirA.