Activation of transcription factor SAF involves its phosphorylation by protein kinase C

The transcription factor serum amyloid A (SAA)-activating factor (SAF), a f amily of zinc finger proteins, plays a significant role in the induced expr ession of the SAA gene. Activity of SAF is regulated by a phosphorylation e vent involving serine/threonine protein kinase (Ray, A, Schatten, H., and R ay, B. K. (1999) J. Biol. Chem. 274, 4300-4308; Ray, A., and Ray, B. K. (19 98) Mol. Cell. Biol. 18, 7327-7335). However, the identity of the protein k inase has so far remained unknown, Induction of SAA by phorbol 12-myristate 13-acetate, a known agonist of protein kinase C (PKC), suggested a potenti al role of the PHC signaling pathway in the activation process. The DNA bin ding activity of endogenous SAF was increased by agonists of PKC. In vitro phosphorylation of SAF-1 by PRC-beta markedly increased its DNA binding abi lity. Consistent with these findings, treatment of cells with activators of PKC or overexpression of PKC-beta II in transfected cells increased expres sion of an SAF-regulated promoter. Further analysis with a GAL4 reporter sy stem indicated that PKC-mediated phosphorylation mostly increases the DNA b inding activity of SAF-1. Together these data indicated that the PKC signal ing pathway plays a major role in controlling expression of SAF-regulated g enes by increasing the interaction between promoter DNA and phosphorylated SAF.