14-3-3 is phosphorylated by casein kinase I on residue 233 - Phosphorylation at this site in vivo regulates Raf/14-3-3 interaction

14-3-3 proteins mediate interactions between proteins involved in signal transduction and cell cycle regulation, Phosphorylation of target proteins as well as 14-3-3 are important for protein-protein interactions, Here, we describe the purification of a protein kinase from porcine brain that phosphorylates 14-3-3 zeta on Thr-233, This protein kinase has been identified as casein kinase I alpha (CKI alpha) by peptide mapping analysis and sequencing, Among mammalian 14-3-3, only 14-3-3 tau possesses a phosphorylatable residue at the same position (Ser-233), and we show that this residue is also phosphorylated by CKI. In addition, we show that 14-3-3 zeta is exclusively phosphorylated on Thr-233 in human embryonic kidney 293 cells. The residue 233 is located within a region shown to be important for the association of 14-3-3 to target proteins, me showed previously that, in 293 cells, only the unphosphorylated form of 14-3-3 zeta associates with the regulatory domain of c-Raf, me have now shown that in vivo phosphorylation of 14-3-3 zeta at the CKI alpha site (Thr-233) negatively regulates its binding to c-Raf, and may be important in Raf-mediated signal transduction.