High Quality Content by WIKIPEDIA articles! Histidine Kinases (HK) are multifunctional, typically transmembrane, proteins of the transferase class that play a role in signal transduction across the cellular membrane. The vast majority of HKs are homodimers that exhibit autokinase, phosphotransfer, and phosphatase activity. HKs can act as cellular receptors for signaling molecules in a way analogous to tyrosine kinase receptors (RTK). Multifunctional receptor molecules such as HKs and RTKs typically have portions on the outside of the cell (extracellular domain) that bind to hormone- or growth factor-like molecules, portions that span the cell membrane (transmembrane domain), and portions within the cell (intracellular domain) that contain the enzymatic activity. In addition to kinase activity, the intracellular domains typically have regions that bind to a secondary effector molecule or complex of molecules that further propagate signal transduction within the cell. Distinct from other classes of protein kinases, HKs are parts of a two-component signal transduction mechanisms. Described in greater detail below, essentially a HK transfers a phosphate group from ATP to a histidine residue within the kinase, and then to an aspartate residue on a `receiver domain` on a different protein (or sometimes on the kinase itself) -- that aspartyl phosphate residue is thus activated for signaling.