Protein kinase C phosphorylates G(12 alpha) and inhibits its interaction with G(beta gamma)

Of nine G protein alpha subunits examined, only alpha(12) and alpha(z) served as substrates for phosphorylation by various iso forms of protein kinase C in vitro. A close homolog of alpha(12), alpha(13), was not phosphorylated. Exposure of NIH 3T3 cells that stably express alpha(12) to phorbol 12-myristate 13-acetate also resulted in phosphorylation of the protein. Phosphorylation in vitro occurred near the amino terminus (probably Ser(38)), and approximately 1 mol of phosphate was incorporated per mol of alpha(12). Although G protein heterotrimers containing either alpha(12) or alpha(z) were poor substrates for phosphorylation, the isolated a subunits were phosphorylated equally well in their GDP- or GTP gamma S bound forms. The guanine nucleotide binding properties of purified alpha(12) and alpha(z) were unaltered by phosphorylation, as was the capacity of alpha(z) to inhibit type V adenylyl cyclase. However, phosphorylation of either protein greatly reduced its affinity for G protein beta gamma subunits, consistent with the newly determined crystal structure of a G protein heterotrimer. We suggest that protein kinase C regulates alpha(12)- and alpha(z)-mediated signaling pathways by preventing their association with beta gamma.