We have determined the primary structure of a novel γ subunit (γ12, previously designated γ(S1)) of G protein purified from bovine spleen. The mature γ12 protein composed of 68 amino acids had acetylated serine at the N terminus and geranylgeranylated/carboxylmethylated cysteine at the C terminus. This was consistent with the C-terminal prenylation signal in the amino acid sequence, which was predicted from γ12 cDNA isolated from a bovine spleen cDNA library. Western blots with the specific antibody against γ12 showed that γ12 is present in all tissues examined. Among various γ subunits (γ1, γ2, γ3, γ7, and γ12), γ12 has a unique property to be phosphorylated by protein kinase C. The phosphorylated amino acid residue was Ser1 (or Ser2). The phosphorylated βγ12 associated with G(o)α more tightly than the unphosphorylated form. Exposure of Swiss 3T3 and aortic smooth muscle cells to phorbol 12-myristate 13-acetate and NaF induced phosphorylation of γ12. Stimulation of aortic smooth muscle cells with natural vasoactive agents such as angiotensin II and vasopressin also induced phosphorylation of γ12. The extent of phosphorylation of βγ12 in vitro was suppressed by a complex formation with G(o)α, which was relieved by the addition of guanosine 5'-O-(3-thiotriphosphate) or aluminum fluoride. These results strongly suggest that γγ12 is phosphorylated by protein kinase C during activation of receptor(s) and G protein(s) in living cells.
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