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

G protein coupled receptor-mediated signaling is attenuated by a process referred to as desensitization, wherein agonist-dependent phosphorylation of receptors by G protein-coupled receptor kinases (GRKs) is proposed to be a hey initial event, However, mechanisms that activate GRKs are not fully understood, In one scenario, beta gamma-subunits of G proteins (G(beta gamma)) activate certain GRKs (beta-adrenergic receptor kinases 1 and 2, or GRK2 and GRK3), via a pleckstrin homology domain in the COOH terminus, This interaction has been proposed to translocate cytosolic beta-adrenergic receptor kinases (beta ARKs) to the plasma membrane and facilitate interaction with receptor substrates, Here, we report a novel finding that membrane lipids modulate beta ARK activity in vitro in a manner that is analogous and competitive with G(beta gamma). Several lipids, including phosphatidylserine (PS), stimulated, whereas phosphatidylinositol 4,5-bisphosphate inhibited, the ability of these GRKs to phosphorylate agonist occupied m2 muscarinic acetylcholine receptors. Furthermore, both PS and phosphatidylinositol 4,5-bisphosphate specifically bound to beta ARK1, whereas phosphatidylcholine, 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 modulation of its autophosphorylation state, PS- and G(beta gamma)-mediated stimulation of beta ARK1 was compared and found strikingly similar; moreover, their effects together were not additive (except at initial stages of reaction), which suggests that PS and G(beta gamma) employed a common interaction and activation mechanism with the kinase, The effects of these lipids were prevented by two well known G(beta gamma)-binding proteins, phosducin and GST-beta ARK-(466-689) fusion protein, suggesting that the G(beta gamma)-binding domain (possibly the pleckstrin homology domain) of the GRKs is also a site for lipid:protein interaction. We submit the intriguing possibility that both lipids and G proteins co-regulate the function of GRKs.