A comparison of the three-dimensional structures of three HPK families (HPK2, HPK3, and HPK9) shows the strong conservation of the catalytic domain structure. (a) The kinase core (dimerization and catalytic domains) of an HPK9, the Thermotoga maritima CheA (Protein Data Bank, PDB, entry 1B3Q) . (b) The catalytic domain of T. maritima CheA complexed with Mg2+ and the nucleotide analog ADPCP (PDB entry 1I58) . (c) The catalytic domain of the HPK2 E. coli EnvZ complexed with ADP (model 1 from PDB entry 1BXD) . (d) The catalytic domain of an HPK3, PhoQ of E. coli, complexed with Mg2+ and the nucleotide analog AMPNP (PDB entry 1ID0) . The figure was created with Swiss-PdbViewer 3.7 and rendered with POV-Ray 3.1.