PP2A dephosphorylates inactive RAFs

Upon growth factor stimulation, RAF1 S259 (or S365 and S214 in BRAF and ARAF respectively) is dephosphorylated by PP2A and/or PP1, abrogating one of the YWHAB/14-3-3 binding sites (Terai et al, 2005; Ory et al, 2003; Kubicek et al, 2002; Jaumot et al, 2001; Rommel et al, 1996; reviewed in Roskoski, 2010; Matallanas et al, 2011). Release of 14-3-3 binding from the N-terminal site promotes a conformational change that exposes the membrane and RAS interacting RBD and CRD and facilitates recruitment of RAF to the plasma membrane where it binds RAS:GTP (Kubicek et al, 2002; Goetz et al, 2003; Ory et al, 2003). Dephosphorylation of S259/S365/S214 may also promote dimerization of RAF monomers by replacing the intramolecular 14-3-3 binding interaction with an intermolecular one (Rushworth et al, 2006; Weber et al, 2001; Ritt et al, 2010; reviewed in Matallanas et al, 2011). Although the PP2A-mediated 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).

在生长因子刺激下，RAF1的S259位点（或BRAF中的S365和ARAФ中的S214位点）被PP2A和/或PP1去磷酸化，从而取消了YWHAB/14-3-3的结合位点之一（Terai等，2005；Ory等，2003；Kubicek等，2002；Jaumot等，2001；Rommel等，1996；参见Roskoski，2010；Matallanas等，2011）。14-3-3从N端位点的解离促进了构象变化，从而暴露了膜结合的RBD和CRD，并促进了RAF向质膜的募集，在那里它与RAS:GTP结合（Kubicek等，2002；Goetz等，2003；Ory等，2003）。S259/S365/S214的去磷酸化也可能通过将分子内14-3-3结合相互作用替换为分子间相互作用，促进RAF单体二聚化（Rushworth等，2006；Weber等，2001；Ritt等，2010；参见Matallanas等，2011）。尽管PP2A介导的去磷酸化被证明发生在YWHAB的移除和RAF向质膜募集之前，但这些事件之间的顺序及其关系尚不完全清楚。此外，14-3-3的移除和RAF1向膜的结合也受到与细胞周期蛋白Prohibitin（PHB）的直接相互作用的促进（Rajalingam等，2005；参见Rajalingam和Rudel，2005；Chowdhury等，2014）。