SPECIFIC ENHANCEMENT OF BETA-ADRENERGIC-RECEPTOR KINASE-ACTIVITY BY DEFINED G-PROTEIN BETA-SUBUNIT AND GAMMA-SUBUNIT

The beta and gamma subunits of heterotrimeric guanine nucleotide-binding proteins (G proteins) have recently been shown to play an active role in signal transduction. Among other effects they enable translocation of the beta-adrenergic receptor kinase (betaARK) from the cytosol to the plasma membrane and thus permit phosphorylation and ultimately desensitization of beta-adrenergic receptors and other G-protein-coupled receptors. To investigate the specificity of this effect, we have purified various combinations of recombinant beta and gamma subunits expressed in Sf9 cells and measured their effects on betaARK-catalyzed phosphorylation of beta2-adrenergic receptors and of rhodopsin. The combinations tested were beta1gamma2, beta1gamma3, beta2gamma2, beta2gamma3, and transducin betagamma (beta1gamma1). There were clear differences in enhancement of rhodopsin phosphorylation, with an order of efficacy beta2gamma2 > beta1gamma2 >> beta2gamma3 almost-equal-to beta1gamma3 almost-equal-to beta1gamma1. The first two combinations had larger effects than a mixed betagamma preparation from bovine brain. In enhancing phosphorylation of beta2-adrenergic receptors, betA1gamma2 Was more efficient and potent than all other combinations. These data suggest a twofold specificity of betagamma complexes in enhancing betaARK-catalyzed receptor phosphorylation: the gamma subunits may be important in interacting with betaARK, with gamma2 being more potent than gamma3, whereas the beta subunits may determine coupling to the receptors, with beta2 being more effective than beta1 for rhodopsin and beta1 being more effective than beta2 for beta2-adrenergic receptors.