G protein βγ subunits synthesized in Sf9 cells: Functional characterization and the significance of prenylation of γ

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) consist of a nucleotide-binding a subunit and a high-affinity complex of β and γ subunits. There is molecular heterogeneity of β and γ, but the significance of this diversity is poorly understood. Different G protein β and γ subunits have been expressed both singly and in combinations in Sf9 cells. Although expression of individual subunits is achieved in all cases, βγ subunit activity (support of pertussis toxin-catalyzed ADP-ribosylation of rGiα1) is detected only when β and γ are expressed concurrently. Of the six combinations of βγ tested (β1 or β2 with γ1, γ2, or γ3), only one, β2γ1, failed to generate a functional complex. Each of the other five complexes has been purified by subunit exchange chromatography using G_oα-agarose as the chromatographic matrix. We have detected differences in the abilities of the purified proteins to support ADP-ribosylation of G_iα1, these differences are attributable to the 7 component of the complex. When assayed for their ability to inhibit calmodulin-stimulated type-I adenylylcyclase activity or to potentiate G_8α-stimulated type-II adenylylcyclase, recombinant β1γ1 and transducin βγ are approximately 10 and 20 times less potent, respectively, than the other complexes examined. Prenylation and/or further carboxylterminal processing of γ are not required for assembly of the βγ subunit complex but are indispensable for high affinity interactions of βγ with either G protein α subunits or adenylylcyclases.