X-ray structure of HPr kinase: a bacterial protein kinase with a P-loop nucleotide-binding domain

HPr kinase/phosphatase (HprK/P) is a key regulatory enzyme controlling carb on metabolism in Grampositive bacteria. It catalyses the ATP-dependent phos phorylation of Ser46 in HPr, a protein of the phosphotransferase system, an d also its dephosphorylation. HprK/P is unrelated to eukaryotic protein kin ases, but contains the Walker motif A characteristic of nucleotide-binding proteins. We report here the X-ray structure of an active fragment of Lacto bacillus casei HprK/P at 2.8 resolution, solved by the multiwavelength anom alous dispersion method on a seleniated protein (PDB code 1jb1). The protei n is a hexamer, with each subunit containing an ATP-binding domain similar to nucleoside/nucleotide kinases, and a putative HPr-binding domain unrelat ed to the substrate-binding domains of other kinases. The Walker motif A fo rms a typical P-loop which binds inorganic phosphate in the crystal. We mod elled ATP binding by comparison with adenylate kinase, and designed a tenta tive model of the complex with HPr based on a docking simulation. The resul ts confirm that HprK/P represents a new family of protein kinases, first id entified in bacteria, but which may also have members in eukaryotes.