The Histidine Kinase Family. Structures of Essential Building Blocks

The histidine kinase protein, the principal component of protein phosphotransfer in bacteria, plays a central role in the signaling pathways required for the environmental adaptation of these organisms. Histidine kinases are composed of a central dimerization domain and an ATP-binding domain (which is highly conserved among members of this protein family, but distinct in primary sequence from Ser/Thr/Tyr protein kinases) and other structural modules such as the sensor domain and the phosphotransfer domain. Advances in three-dimensional structural studies of these domains, or the essential building blocks, from the various histidine kinases have deciphered both a functional and an evolutionary link between the members of this protein family as well as between these and other proteins. Most notably, the a/ß sandwich fold identified in both EnvZ and CheA ATP-binding domains revealed a marked resemblance to the fold found in the GHL ATPase family containing Hsp90, DNA gyrase B, and MutL. This chapter discusses the structurally characterized building blocks that are essential for the activity and regulation of histidine kinases.