PHOSPHORYLATION OF C-Z-ALPHA BY PROTEIN-KINASE-C BLOCKS INTERACTION WITH THE BETA-GAMMA COMPLEX

G(Z alpha) is a G protein alpha subunit with biochemical properties that distinguish it from other members of the G protein alpha subunit family. One such property is its ability to be stoichiometrically phosphorylated by protein kinase C (PRC), both in vitro and in intact cells. The site of this phosphorylation has been mapped to a region near the N terminus of G(Z alpha), but no functional significance of the modification has been established. To investigate this question, we have developed a baculovirus/Sf9 cell expression system to produce G(Z alpha). The protein purified from Sf9 cells is functional as assessed by its ability both to bind guanine nucleotide in a Mg2+-sensitive fashion and to serve as a substrate for phosphorylation by PKC. Furthermore, addition of the G protein beta gamma complex purified from bovine brain inhibits phosphorylation of G(Z alpha) in a dose-dependent manner. Conversely, phosphorylation of G(Z alpha) inhibits its ability to interact with beta gamma subunits. These results establish a functional consequence for PKC-catalyzed phosphorylation of G(Z alpha) and suggest a mechanism for regulation of signaling through G(Z) by preventing reassociation of its subunits.