Evidence of a functional interaction between serine 3 and serine 25 Mos phosphorylation sites. A dominant inhibitory role of serine 25 phosphorylation on Mos protein kinase.

The Journal of Biological Chemistry
Y YangB Singh

Abstract

Recently, we identified the major in vivo phosphorylation site on v-Mos as Ser-56, which is phosphorylated by cyclic AMP dependent protein kinase (PKA). Others have shown that c-Mos phosphorylation at Ser-3 (equivalent to Ser-34 in v-Mos) is important for the interaction of c-Mos with its substrate MEK and for its stability and cytostatic factor activity in eggs. To investigate the role of Ser-56 phosphorylation, we generated site-directed mutants of v-Mos that would mimic phosphorylation in terms of charge at positions 56 and 34. After mutating serine (S) residues with alanine (A) or glutamic acid (E) in different combinations, various v-Mos mutants were expressed in a rabbit reticulocyte lysate in vitro translation system and in COS-1 or NIH/3T3 cells. The effect of mutations on Mos function was evaluated by in vitro protein kinase assays and by the ability of Mos to cause neoplastic transformation of NIH/3T3 cells. The S56E but not the S56A mutation inhibited v-Mos kinase activity suggesting that Ser-56 phosphorylation has an inhibitory role. As predicted from Xenopus c-Mos studies, S34A but not S34E mutation inhibited v-Mos activity. Studies with the double mutants showed that the S56E mutation but not S56A mutation inhibit...Continue Reading

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