文摘
C-RAF kinase is a central component of the Ras-RAF-MEK (mitogen©\activated protein kinase/extracellular signal©\regulated kinase)-ERK (extracellular signal©\regulated kinase) pathway, which has been shown to be activated in 30 % of human tumors. 14-3-3 proteins inactivate C-RAF by binding to the two N-terminal phosphorylation-dependent binding sites surrounding S233 and S259. 14-3-3 proteins can bind two target sequences located on one polypeptide chain simultaneously, thereby increasing binding affinity compared to single©\site binding and possibly allowing regulated 14-3-3 binding through gatekeeper phosphorylation. To date, it was unclear whether 14-3-3 proteins can bind the two N-terminal phosphorylation-dependent binding sites of C-RAF simultaneously. Fluorescence polarization using phosphorylated peptides demonstrated that S233 is the low-affinity and S259 is the high-affinity binding site, while simultaneous engagement of both sites by 14-3-3¦Æ enhances affinity compared to single©\site binding. Determination of a 1:1 stoichiometry for the di-phosphorylated peptide binding to one 14-3-3¦Æ dimer with isothermal titration calorimetry was supported by the crystal structure of the 14-3-3¦Æ/C-RAFpS233,pS259 complex. Cellular localization studies validate the significance of these sites for cytoplasmic retention of C-RAF, suggesting an extended mechanism of RAF regulation by 14-3-3 proteins.
