This family represents the N-terminal region of Hpr Serine/threonine kinase PtsK. This kinase is the sensor in a multicomponent phospho-relay system in control of carbon catabolic repression in bacteria. This kinase in unusual in that it recognizes the tertiary structure of its target and is a member of a novel family unrelated to any previously described protein phosphorylating enzymes. X-ray analysis of the full-length crystalline enzyme from Staphylococcus xylosus at a resolution of 1.95 A shows the enzyme to consist of two clearly separated domains that are assembled in a hexameric structure resembling a three-bladed propeller. The blades are formed by two N-terminal domains each, and the compact central hub assembles the C-terminal kinase domains.