BCR AND RAF FORM A COMPLEX IN-VIVO VIA 14-3-3-PROTEINS

In a yeast two-hybrid screen we identified a member of the 14-3-3 family of proteins that can bind to Bcr. 14-3-3 beta binds to the serine/threonine rich region B in the kinase domain encoded by the first exon, In this paper we show by co-immunoprecipitation that Bcr binds to Raf in vivo and we argue that this interaction is mediated by 14-3-3 dimers, based on the following findings. First, 14-3-3 isoforms bind to both Raf and Bcr. Second, Bcr does not bind to Raf directly in the two-hybrid system, but co-expression of 14-3-3 beta allows complex formation. Third, Bcr, 14-3-3 proteins and Raf co-elute in gel filtration and in sequential ion exchange chromatography and the three proteins can be co-immunoprecipitated from the separate fractions, indicating that they are present in a ternary complex. Moreover, similar to 10 times more Raf is bound to Bcr, and vice versa, in the membrane fraction (where Raf is activated) than in the cytosolic fraction. We suggest a new function for 14-3-3 proteins as a novel type of adaptor which acts by dimerization and binding to different proteins.