Role of C-terminal domains of the G protein beta subunit in the activation of effectors.

The prenyl group on the G protein gamma subunit is an important determinant of protein-protein interactions between the betagamma dimer and its targets, such as alpha subunits, receptors, and effectors. In an effort to identify domains of the beta subunit important for the activation of effectors, we have prepared two types of mutants, one set in the domain suggested to form a hydrophobic prenyl-binding pocket for the gamma subunit's prenyl group (prenyl pocket mutants) and the other set in a domain between Gly(306) and Gly(319) in the beta propeller, which undergoes a conformational change when the dimer binds to phosducin (conformational change mutants). Recombinant baculoviruses for each set of mutants were prepared, and the nine mutant beta subunits were overexpressed with either the gamma(2) subunit (modified with geranylgeranyl) or the gamma(2-L71S) subunit (gamma(2) with altered CAAX sequence and modified with farnesyl). The purified dimers were tested for their ability to couple Galpha(i1) to the A1 adenosine receptor and to activate phospholipase C-beta or type II adenylyl cyclase. All dimers containing mutant beta subunits were indistinguishable from wild-type beta(1)gamma(2) or beta(1)gamma(2-L71S) in coupling the receptor to Galpha(i1). The prenyl pocket mutants expressed with gamma(2) were 10-fold less potent in activating phospholipase C-beta and adenylyl cyclase than beta(1)gamma(2) and had similar activities to beta(1)gamma(2-L71S). The conformational change mutants caused a 15- to 23-fold decrease in EC(50) values for activation of these two effectors. Overall, the results suggest that the sites in Gbeta identified by these mutants are very important in the activation of effectors. Furthermore, the nature of the prenyl group on Ggamma may play an important role in the conformational change leading to the activity of Gbetagamma on effectors.