Analysis of the Modulation of RAF Signaling by 14-3-3 Proteins

The regulation of cellular biochemical signaling reactions includes the modulation of protein activity through a variety of processes. For example, signaling by the RAF kinases, which are key transmitters of extracellular growth signals downstream from the RAS GTPases, is modulated by dimerization, protein conformational changes, post-translational modifications, and protein-protein interactions. 14-3-3 proteins are known to play an important role in RAF signal regulation, and have the ability to stabilize both inactive (monomeric) and active (dimeric) states of RAF. It is poorly understood how these antagonistic roles ultimately modulate RAF signaling. To investigate, we develop a mathematical model of RAF activation with both roles of 14-3-3, perform algebraic and numeric analyses, and compare with available experimental data. We derive the conditions necessary to explain experimental observations that 14-3-3 overexpression activates RAF, and we show that strong binding of 14-3-3 to Raf dimers alone is not generally sufficient to explain this observation. Our integrated analysis also suggests that RAF-14-3-3 binding is relatively weak for the reasonable range of parameter values, and suggests the Raf dimer-14-3-3 interactions are stabilized primarily by avidity. Lastly we find that in the limit of paired weak/avidity driven interactions between RAF and 14-3-3, the paired binding interactions may be reasonably approximated with a strong, single, equilibrium reaction. Overall, our work presents a mathematical model that can serve as a foundational piece for future, extended, studies of signaling reactions involving regulated RAF kinase activity.