 X-ray crystal structures have been obtained for several GRKs (GRK1, GRK2, GRK4, GRK5 and GRK6), alone or bound to ligands.Overall, GRKs share sequence homology and domain organization in which the central protein kinase catalytic domain is preceded by a domain with homology to the active domain of Regulator of G protein Signaling proteins, RGS proteins (the RGS-homology – RH – domain) and is followed by a variable carboxyl terminal tail regulatory region.In the folded proteins, the kinase domain forms a typical bi-lobe kinase structure with a central ATP-binding active site.