Differential interaction of GRK2 with members of the Gαq family

Regulators of G protein signaling (RGS) proteins bind to active Galpha subunits and accelerate the rate of GTP hydrolysis and/or block interaction with effector molecules, thereby decreasing signal duration and strength. RGS proteins are defined by the presence of a conserved 120-residue region termed the RGS domain. Recently, it was shown that the G protein-coupled receptor kinase 2 (GRK2) contains an RGS domain that binds to the active form of Go. Here, the ability of GRK2 to interact with other members of the Galpha(q) family, Galpha(11), Galpha(14), and Galpha(16), was tested. The signaling of all members of the Galpha(q) family, with the exception of Galpha(16), was inhibited by GRK2. Immunoprecipitation of full-length GRK2 or pull down of GST-GRK2-(45-178) resulted in the detection of Galpha(q), but not Galpha(16), in an activation-dependent manner. Moreover, activated Galpha(16) failed to promote plasma membrane (PM) recruitment of a GRK2-(45-178)-GFP fusion protein. Assays with chimeric Galpha(q-16) subunits indicated that the C-terminus of Galpha(q) mediates binding to GRK2. Despite showing no interaction with GRK2, Galpha(16) does interact with RGS2, in both inositol phosphate and PM recruitment assays. Thus, GRK2 is the first identified RGS protein that discriminates between members of the Gaq family, while another RGS protein, RGS2, binds to both Galpha(q) and Galpha(16).