STRUCTURE-FUNCTION ANALYSIS OF G PROTEIN-COUPLED RECEPTOR KINASE-5 - ROLE OF THE CARBOXYL-TERMINUS IN KINASE REGULATION

Many G protein-coupled receptors are phosphorylated and regulated by a distinct family of G protein-coupled receptor kinases (GRKs) that spe cifically target the activated form of the receptor. Recent studies ha ve revealed that the GRKs are also subject to post-translational regul ation. For example, GRK5 activity is strongly inhibited by protein kin ase C phosphorylation and by Ca2+-calmodulin binding. Ca2+-calmodulin binding also promotes GRK5 autophosphorylation, which further contribu tes to kinase inhibition. In this study we identify two important stru ctural domains in GRK5, a phospholipid binding domain (residues 552-56 2) and an autoinhibitory domain (residues 563-590), that significantly contribute to GRK5 localization and function. We demonstrate that the C-terminal region of GRK5 (residues 563-590) contains residues autoph osphorylated in the presence of calmodulin as well as the residues pho sphorylated by protein kinase C. Deletion of this domain increases the apparent affinity of GRK5 for receptor substrates 3-4-fold but has no effect on nonreceptor substrates. These findings define residues 563- 590 of GRK5 as an autoinhibitory domain with efficacy that is regulate d by phosphorylation. Another C-terminal domain in GRK5 that appears t o be functionally important is found between residues 552 and 562. Del etion of this region significantly inhibits kinase phosphorylation of membrane-bound receptor substrates but has no effect on soluble substr ates. Additional studies reveal that this domain is critical for GRK5 interaction with phospholipids and for the intracellular localization of the kinase. Interestingly, similar regions in GRK4 and GRK6 appear to be palmitoylated (and involved in membrane interaction), suggesting evolutionary conservation of the function of this domain.