"Activator" RAF:YWHAB dimer binds RAS:GTP

Activation of RAS downstream of extracellular signals allows RAS:GTP to recruit BRAF to the plasma membrane, disrupting the pre-existing inactivating interaction between BRAF and the 14-3-3 protein YWHAB (Marais et al, 1997; Yamamori et al, 1995; reviewed in Cseh et al, 2014). BRAF, unique of the three mammalian RAF proteins, is constitutively phosphorylated on the conserved serine residue (446 in BRAF) in the N-terminal acidic motif (NtA). Constitutive negative charge in this region is critical for BRAF to function as an activator of other RAF molecules, allowing signal amplification (Marais et al, 1997; Mason et al, 1999; Wan et al, 2004; Garnett et al, 2005; Hu et al, 2013; reviewed in Cseh et al, 2014). RAS:GTP-bound BRAF heterodimerizes with additional RAF monomers, allowing cis-autophosphorylation in the activation loop of the second RAF protein. Once activated by BRAF in this manner, the 'receiver' kinase monomer is competent to dimerize with and transactivate other monomers in turn (Weber et al, 2001; Garnett et al, 2005; Hu et al, 2013; reviewed in Cseh et al, 2014). Intruigingly, the scaffold protein KSR1 is activated by BRAF in a manner analogous to other RAF monomers and can similarly act as a RAF activator once it is itself activated (Brennan et al, 2011; Ory et al, 2003; reviewed in Raabe and Rapp, 2003; Cseh et al, 2014).<br><br>Although this pathway shows PP2A-mediated dephosphorylation of RAF and transient displacement of 14-3-3 proteins as preceding RAS and plasma membrane binding of RAF proteins, the order and dependency of these events is not clear. Both membrane binding and 14-3-3 displacement also appear to be facilitated by an interaction between RAF and the cell cycle protein Prohibitin (PHB; Rajalingam et al, 2005; reviewed in Rajalingam and Rudel, 2005; Chowdhury et al, 2014).

细胞外信号下游的RAS激活允许RAS:GTP招募BRAF至质膜，中断BRAF与14-3-3蛋白YWHAB（Marais等，1997；Yamamori等，1995；详见Cseh等，2014）之间存在的原有失活相互作用。BRAF作为三种哺乳动物RAF蛋白中独特的一员，其N端酸性基序（BRAF中的446位丝氨酸残基）上呈恒定磷酸化状态。此区域内的恒定负电荷对于BRAF作为其他RAF分子激活剂的功能至关重要，从而实现信号放大（Marais等，1997；Mason等，1999；Wan等，2004；Garnett等，2005；Hu等，2013；详见Cseh等，2014）。与RAS:GTP结合的BRAF与额外的RAF单体形成异源二聚体，允许第二RAF蛋白的激活环中发生cis-自磷酸化。一旦通过BRAF以这种方式激活，'接收者'激酶单体便具备与并依次转活化其他单体二聚的能力（Weber等，2001；Garnett等，2005；Hu等，2013；详见Cseh等，2014）。值得注意的是，支架蛋白KSR1也以类似于其他RAF单体的方式被BRAF激活，并在自身激活后充当RAF激活剂（Brennan等，2011；Ory等，2003；详见Raabe和Rapp，2003；Cseh等，2014）。尽管此途径显示PP2A介导的RAF去磷酸化和14-3-3蛋白的短暂移位先于RAS与质膜结合的RAF蛋白，但这些事件的顺序和依赖性尚不明确。膜结合和14-3-3蛋白的移位似乎也受到RAF与细胞周期蛋白Prohibitin（PHB；Rajalingam等，2005；详见Rajalingam和Rudel，2005；Chowdhury等，2014）之间相互作用的促进。