14-3-3 proteins are required for maintenance of Raf-1 phosphorylation and kinase activity

By binding to serine-phosphorylated proteins, 14-3-3 proteins function as effecters of serine phosphorylation. The exact mechanism of their action is, however, still largely unknown. Here we demonstrate a requirement for 14-3-3 for Raf-l kinase activity and phosphorylation. Expression of dominant negative forms of 14-3-3 resulted in the loss of a critical Raf-l phosphorylation, while overexpression of 14-3-3 resulted in enhanced phosphorylation of this site. 14-3-3 levels, therefore, regulate the stoichiometry of Raf-l phosphorylation and its potential activity in the cell. Phosphorylation of Raf-l, however, was insufficient by itself for kinase activity. Removal of 14-3-3 from phosphorylated Raf abrogated kinase activity, whereas addition of 14-3-3 restored it. This supports a paradigm in which the effects of phosphorylation on serine as well as tyrosine residues are mediated by inducible protein-protein interactions.