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 protein has been solved at 2.1-Å resolution. The structure is composed of an up-down up-down four-helix bundle that is typical of histidine phosphotransfer or HPt domains such as Escherichia coli ArcB_C and Saccharomyces cerevisiae Ypd1. Loop regions and additional structural features distinguish 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 phosphoaccepting 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 and 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 phosphotransfer reaction with CheY. These results suggest that the CheA-P1 domain may serve as a good model for understanding the general function of HPt domains in complex two-component phosphorelay systems.