Biophysical Characterization of Essential Phosphorylation at the Flexible C-Terminal Region of C-Raf with 14-3-3ζ Protein

Phosphorylation at the C-terminal flexible region of the C-Raf protein plays an important role in regulating its biological activity. Auto-phosphorylation at serine 621 (S621) in this region maintains C-Raf stability and activity. This phosphorylation mediates the interaction between C-Raf and scaffold protein 14-3-3 zeta to activate the downstream MEK kinase pathway. In this study, we have defined the interaction of C-terminal peptide sequence of C-Raf with 14-3-3 zeta protein and determined the possible structural adaptation of this region. Biophysical elucidation of the interaction was carried out using phosphopeptide (residue number 615-630) in the presence of 14-3-3 zeta protein. Using isothermal titration calorimetry (ITC), a high binding affinity with micro-molar range was found to exist between the peptide and 14-3-3 zeta protein, whereas the non-phosphorylated peptide did not show any appreciable binding affinity. Further interaction details were investigated using several biophysical techniques such as circular dichroism (CD), fluorescence, and nuclear magnetic resonance (NMR) spectroscopy, in addition to molecular modeling. This study provides the molecular basis for C-Raf C-terminal-derived phosphopeptide interaction with 14-3-3 zeta protein as well as structural insights responsible for phosphorylated S621-mediated 14-3-3 zeta binding at an atomic resolution.