G protein-coupled receptor kinases (GRKs) specifically phosphorylate and regulate the activated form of multiple G protein-coupled receptors. Recent studies have revealed that GRKs are also subject to regulation. For example, GRK2 and GRK5 can be phosphorylated and either activated or inhibited, respectively, by protein kinase C. Here we demonstrate that calmodulin is a potent inhibitor of GRK activity with a selectivity for GRK5 (ICs0 50 nM) > GRK6 >> GRK2 (ICso 2000 nM) >> GRK1. Calmodulin inhibition of GRK5 is mediated via a reduced kinase binding to both receptor and phospholipid. Interestingly, calmodulin activates autophosphorylation of GRK5 at sites distinct from the two major autophosphorylation sites on GRK5. Moreover. calmodulin-stimulated autophosphorylation of GRK5 directly inhibits GRK5 interaction with the receptor even in the absence of calmodulin. Using gluthatione-S-transferase-GRK5 fusion proteins to either inhibit calmodulinstimulated autophosphorylation or bind directly to calmodulin, we determined that an amino terminal domain of GRK5 (amino acids 20-39) is sufficient for calmodulin binding. This domain is abundant in basic and hydrophobic residues, characteristics typical of calmodulin binding sites, and is highly conserved in GRK4, GRK5, and GRK6. These studies suggest that calmodulin may serve a general role in mediating calcium-dependent regulation of GRK activity. Supported by grants from NIH, Dept. of the Navy, and the American Heart Association.
|Original language||English (US)|
|State||Published - Dec 1 1997|
ASJC Scopus subject areas
- Molecular Biology