Crystal structure of the CheA histidine phosphotransfer domain that mediates response regulator phosphorylation in bacterial chemotaxis

The x-ray crystal structure of the P1 or H domain of the Salmonella CheA pr otein has been solved at 2.1-Angstrom resolution. The structure is composed of an up-down up-down four-helix bundle that is typical of histidine phosp hotransfer or HPt domains such as Escherichia coli ArcB(C) and Saccharomyce s cerevisiae Ypd1. Loop regions and additional structural features distingu ish all three proteins. The CheA domain has an additional C-terminal helix that lies over the surface formed by the C and D helices. The phosphoaccept ing His-48 is located at a solvent-exposed position in the middle of the B helix where it is surrounded by several residues that are characteristic of other HPt domains. Mutagenesis studies indicate that conserved glutamate a nd lysine residues that are part of a hydrogen-bond network with His-48 are essential for the ATP-dependent phosphorylation reaction but not for the p hosphotransfer reaction with CheY. These results suggest that the CheA-P1 d omain may serve as a good model for understanding the general function of H Pt domains in complex two-component phosphorelay systems.