Abstract
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 (beta ARK) 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 beta ARK-catalyzed
phosphorylation of beta 2-adrenergic receptors and of rhodopsin.
The combinations tested were beta 1 gamma 2, beta 1 gamma 3, beta
2 gamma 2, beta 2 gamma 3, and transducin beta gamma (beta 1 gamma
1). There were clear differences in enhancement of rhodopsin phosphorylation,
with an order of efficacy beta 2 gamma 2 > beta 1 gamma 2 >> beta
2 gamma 3 approximately beta 1 gamma 3 approximately beta 1 gamma
1. The first two combinations had larger effects than a mixed beta
gamma preparation from bovine brain. In enhancing phosphorylation
of beta 2-adrenergic receptors, beta 1 gamma 2 was more efficient
and potent than all other combinations. These data suggest a twofold
specificity of beta gamma complexes in enhancing beta ARK-catalyzed
receptor phosphorylation: the gamma subunits may be important in
interacting with beta ARK, with gamma 2 being more potent than gamma
3, whereas the beta subunits may determine coupling to the receptors,
with beta 2 being more effective than beta 1 for rhodopsin and beta
1 being more effective than beta 2 for beta 2-adrenergic receptors.
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