G protein-coupled receptors (GPCRs) are regulated by multiple families of kinases including GPCR kinases (GRKs). GRK4 is constitutively active towards GPCRs, and polymorphisms of GRK4 gamma are linked to hypertension. We examined, through co-immunoprecipitation, the interactions between GRK4 gamma and the G alpha and G beta subunits of heterotrimeric G proteins. Because GRK4 has been shown to inhibit G alpha(s)-coupled GPCR signaling and lacks a PH domain, we hypothesized that GRK4 gamma would interact with active G alpha(s) but not G beta. Surprisingly, GRK4 gamma preferentially interacts with inactive G alpha(s) and G beta to a greater extent than active G alpha(s). GRK4 gamma also interacts with inactive G alpha(13) and G beta. Functional studies demonstrate that wild-type GRK4 gamma, but not kin se-dead GRK4 gamma, ablates isoproterenol-mediated cAMP production indicating that the kinase domain is responsible for GPCR regulation. This evidence suggests that binding to inactive G alpha(s) and G beta may explain the constitutive activity of GRK4 gamma towards G alpha(s)-coupled receptors. (C) 2007 Elsevier Inc. All rights reserved.