Killing Two Kinases with One Inhibitor

Abstract

RKIP (Raf kinase inhibitor protein) associates with and inhibits the enzymatic activity of the protein kinase Raf, an upstream kinase in MAPK (mitogen-activated protein kinase) signaling cascades. RKIP appears to mediate crosstalk with other pathways, and phosphorylation of RKIP by protein kinase C (PKC) relieves inhibition of Raf-1. Now Lorenz et al. report a more extensive role of RKIP in coordinating signaling through G protein-coupled receptors (GPCRs). Their experiments show that RKIP is also an inhibitor of GRK-2 (G protein-coupled receptor kinase 2), a kinase that diminishes signaling through GPCRs by uncoupling them from G proteins and promoting their internalization. In transfected cells and various native tissues, the amount of RKIP associated with GRK-2 was increased after stimulation of appropriate GPCRs and correlated with phosphorylation of RKIP at a site phosphorylated by PKC. Such phosphorylation appeared to be necessary for inhibition of GRK-2 because mutants of RKIP lacking the phosphorylated serine residue were not effective inhibitors of GRK-2. The increased association of RKIP with GRK-2 coincided with reduced association of RKIP with Raf-1. Thus, in unstimulated cells, RKIP appears to hold Raf-1 in an inactive state. Depletion of RKIP from cardiomyocytes by electroporation in the presence of specific antibodies inhibited signaling and contraction in response to stimulation of β-adrenergic receptors. The authors propose that phosphorylation of RKIP by PKC after stimulation of GPCRs then contributes in two ways to GPCR signaling: Release of RKIP from Ras-1 promotes signaling through MAPK and the released protein then inhibits GRK-1, thereby preventing desensitization and internalization of GPCRs. K. Lorenz, M. J. Lohse, U. Quitterer, Protein kinase C switches the Raf kinase inhibitor from Raf-1 to GRK-2. Nature 426 , 574-579 (2003). [Online Journal]