Protein kinase C phosphorylates G12 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 isoforms of protein kinase C in vitro. A close homolog of alpha 12, alpha 12 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 Ser38), and approximately 1 mol of phosphate was incorporated per mol of alpha 12. Although G protein heterotrimers containing either alpha 12 or a z were poor substrates for phosphorylation, the isolated alpha 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.