MULTIMERIZATION AND TRANSCRIPTIONAL ACTIVATION OF THE PHOSPHOPROTEIN (P) OF VESICULAR STOMATITIS-VIRUS BY CASEIN KINASE-II

Casein kinase-II (CK-II) is a widely distributed protein kinase, which plays numerous roles in the regulation of transcription through modif ication of transacting transcription factors. Phosphorylation of vesic ular stomatitis virus (VSV) P protein by CK-II was found to be both ne cessary and sufficient for transcriptional activation. Upon treatment of P by CK-II, activity was acquired faster (t(1/2) = 3.7 min) than we re total phosphates (t(1/2) = 7.4 min). Stoichiometry was 2 mol phosph ate/mol P, indicating activation by phosphorylation at either one or b oth of two independent sites. The sites were identified by substitutin g aspartate (D) residues at either S60 or T62, producing proteins that were partly active without phosphorylation, but were fully active at higher concentrations; CK-II added only a single phosphate group to ea ch of these, and conferred full activity. P protein doubly substituted with D at S60 and T62 was fully active without phosphorylation, and w as not a substrate for CK-II. Active P protein, whether CK-II treated or doubly substituted, was shown by gel filtration and crosslinking to exist as a discretely multimeric, probably tetrameric, structure. The singly substituted mutants were partly multimeric, becoming fully so after CK-II treatment. Phosphorylation by CK-II thus mediates the self -association of P into the multimeric, transcriptionally active form.