LIPID-MEDIATED REGULATION OF G-PROTEIN-COUPLED RECEPTOR KINASE-2 AND KINASE-3

G protein coupled receptor-mediated signaling is attenuated by a proce ss referred to as desensitization, wherein agonist-dependent phosphory lation of receptors by G protein-coupled receptor kinases (GRKs) is pr oposed to be a hey initial event, However, mechanisms that activate GR Ks are not fully understood, In one scenario, beta gamma-subunits of G proteins (G(beta gamma)) activate certain GRKs (beta-adrenergic recep tor kinases 1 and 2, or GRK2 and GRK3), via a pleckstrin homology doma in in the COOH terminus, This interaction has been proposed to translo cate cytosolic beta-adrenergic receptor kinases (beta ARKs) to the pla sma membrane and facilitate interaction with receptor substrates, Here , we report a novel finding that membrane lipids modulate beta ARK act ivity in vitro in a manner that is analogous and competitive with G(be ta gamma). Several lipids, including phosphatidylserine (PS), stimulat ed, whereas phosphatidylinositol 4,5-bisphosphate inhibited, the abili ty of these GRKs to phosphorylate agonist occupied m2 muscarinic acety lcholine receptors. Furthermore, both PS and phosphatidylinositol 4,5- bisphosphate specifically bound to beta ARK1, whereas phosphatidylchol ine, a lipid that did not modulate beta ARK activity, did not bind to beta ARK1, The lipid regulation of beta ARKs did not occur via a modul ation of its autophosphorylation state, PS- and G(beta gamma)-mediated stimulation of beta ARK1 was compared and found strikingly similar; m oreover, their effects together were not additive (except at initial s tages of reaction), which suggests that PS and G(beta gamma) employed a common interaction and activation mechanism with the kinase, The eff ects of these lipids were prevented by two well known G(beta gamma)-bi nding proteins, phosducin and GST-beta ARK-(466-689) fusion protein, s uggesting that the G(beta gamma)-binding domain (possibly the pleckstr in homology domain) of the GRKs is also a site for lipid:protein inter action. We submit the intriguing possibility that both lipids and G pr oteins co-regulate the function of GRKs.