Protein Histidine Kinase

Abstract

Protein phosphorylation on histidine occurs in a large number of processes in prokaryotes and in an unknown number of processes in eukaryotes. The functions of protein phosphohistidine in prokaryotes are much more clearly understood (1,2. The methods presented here have been developed for the study of protein histidine phosphorylation in eukaryotes where phosphohistidine is found as an intermediate in some enzyme reactions and may also be involved in signaling mechanisms (3). In prokaryotes, two types of protein phosphorylation on histidine have been characterized in detail. The first, historically, is the phosphoenolpyruvate-sugar phosphotransferase system (4,5). In this system, phosphohistidine acts as a phosphate donor and the initial phosphate donor is phosphoenolpyruvate, so this is not a protein kinase system. The second prokaryotic system is the twocomponent regulatory system (1,2). The core of this system comprises three protein domains, which may or may not be on different polypeptides. The first domain is the protein kinase domain, the second is the histidine substrate for this kinase and the third contains an aspartate residue that removes the phosphate from phosphohistidine. Genes homologous to those of the two-component system have been identified in lower eukaryotes (1,6,7).