An Intact N Terminus of the Subunit Is Required for the G Stimulation of Rhodopsin Phosphorylation by Human -Adrenergic Receptor Kinase-1 but Not for Kinase Binding

Cleavage after lysine 32 in the G subtype and after lysine 36 in the G subtype of purified mixed brain G β by endoproteinase Lys-C blocks G β -mediated stimulation of phosphorylation of rhodopsin in urea-extracted rod outer segments by recombinant human β-adrenergic receptor kinase (h β ARK1) holoenzyme while h β ARK1 binding to rod outer segments is partially affected. This treatment does not attenuate the binding of the treated G β to C-terminal fragments of h β ARK1 containing the pleckstrin homology domain. Lys-C proteolysis also does not alter the association of the G β with phospholipids, its ability to support pertussis toxin-catalyzed Gα /Gα ADP-ribosylation, or its ability to inhibit forskolin-stimulated platelet adenylate cyclase. The G β subunit remains noncovalently associated with the cleaved G fragments. Thus, in addition to recruiting h β ARK1 to its receptor substrate, G contributes secondary and/or tertiary structural features to activate the kinase.