Activation of RAF upon growth factor stimulation depends on many factors including dephosphorylation, conformational change, dimerization, membrane recruitment and protein-protein interactions, among others (reviewed in Lavoie and Therien, 2015). In the inactive state, RAF1/CRAF is phosphorylated at S259 and S621 (corresponding to S365 and S729 in BRAF, and S214 and S576 in ARAF). These sites mediate the interaction with YWHAB/14-3-3 proteins (reviewed in Matallanas et al, 2011; Lavoie and Therien, 2015). Dephosphorylation of the S259 site by PP2A and or PP1 (also known as PPP1CC) disrupts interaction with 14-3-3 proteins and promotes a conformational change that exposes the membrane and RAS interacting RBD and CRD, facilitating recruitment of RAF to the plasma membrane (Kubicek et al, 2002; Goetz et al, 2003; Ory et al, 2003). PP1-mediated dephosphorylation occurs in the context of a ternary complex consisting of SHOC2 and the RAS protein family member MRAS in its GTP bound form (Rodriguez-Viciano et al, 2006; Young et al, 2013; reviewed in Simanshu et al, 2017). Consistent with a role of this ternary complex in activating RAF signaling, mutations in SHOC2, MRAS and PP1 are associated with increased RAF pathway activity in Noonan syndrome (Cordeddu et al, 2009; Gripp et al, 2016; Higgin et al, 2017; Young et al 2018).
Although S259 dephosphorylation is shown as occurring before both YWHAB displacement and recruitment of RAF to the plasma membrane, the order of and relationship between these events is not completely clear. In addition, the displacement of 14-3-3 and recruitment of RAF1 to the membrane is also promoted by a direct interaction with cell cycle protein Prohibitin (PHB; Rajalingam et al, 2005; reviewed in Rajalingam and Rudel, 2005; Chowdhury et al, 2014).