MAP2Ks and MAPKs bind to the activated RAF complex

RAF kinases have restricted substrate specificity and have as their primary substrates the two MAP2K proteins MAP2K1 and MAP2K2 (also known as MEK1 and 2). MAP2K1 knockout is embryonic lethal in mice, while MAP2K2 knockouts have no apparent abnormalities, suggesting that MAP2K1 can compensate for MAP2K2 in vivo (Giroux et al, 1999; Belanger et al, 2003). MAP2K proteins exist as stable homo- and heterodimers independent of growth factor stimulation and are generally recruited to activated RAF proteins in conjunction with a scaffolding protein and the MAP2K substrates, MAPK1 and 3 (also known as ERK1 and 2) (Ohren et al, 2004; Catalanotti et al, 2009; Catling et al, 1995; reviewed in Matallanas et al, 2011; Roskoski et al, 2012a; Roskoski et al, 2012b). <br><br>Scaffolding proteins promote signaling by providing a docking platform that colocalizes components of the signaling cascade, and provide specificity by controlling the spatial and temporal regulation of the pathway (reviewed in Brown and Sacks, 2009; Matallanas et al, 2011). KSR1 and 2, CNKSR1 and 2, IQGAP1 and the beta arrestins are among the known MAPK scaffold proteins that act at the plasma membrane upon MAPK pathway activation; in addition, paxillin localizes MAPK pathway components to focal adhesion sites in the plasma membrane (Roy et al, 2005; Ren et al, 2007; DeFea et al, 2000; Togho et al, 2003; Ishibe et al, 2003; reviewed in Claperon and Therrien, 2007; Brown and Sacks, 2009; Matallanas et al, 2011). Although this reaction depicts these scaffolding proteins acting equivalently, the details of how they promote pathway activation vary. For instance, KSR1 and 2 are constitutively bound to MAP2K dimers but recruit MAPKs only upon pathway stimulation, while IQGAP1 associates constitutively with both MAP2K and MAPK proteins in unstimulated cells and shows increased interaction with MAP2K1 upon pathway activation by EGF (Stewart et al, 1999; Cacace et al, 2000; Muller et al, 2000; Roy et al, 2004; Roy et al, 2005; reviewed in Brown and Sacks, 2009). Scaffolding complexes may be particularly important for the phosphorylation of cytosolic MAPK targets (reviewed in Casar et al, 2009).

RAF激酶具有特定的底物特异性，其主要底物为两种MAP2K蛋白MAP2K1和MAP2K2（亦称MEK1和2）。小鼠中MAP2K1敲除表现为胚胎致死，而MAP2K2敲除则未观察到明显异常，这表明MAP2K1在体内能够补偿MAP2K2的功能（Giroux等，1999；Belanger等，2003）。MAP2K蛋白在生长因子刺激独立的情况下，以稳定的同源和异源二聚体形式存在，通常与支架蛋白以及MAP2K底物MAPK1和3（亦称ERK1和2）一同被招募至激活的RAF蛋白（Ohren等，2004；Catalanotti等，2009；Catling等，1995；Matallanas等，2011年综述；Roskoski等，2012a；Roskoski等，2012b）。支架蛋白通过提供信号级联成分的锚定位点来促进信号传导，并通过控制途径的空间和时间调节来提供特异性（Brown和Sacks，2009年综述；Matallanas等，2011年综述）。KSR1和2、CNKSR1和2、IQGAP1以及β-阻断剂是已知的MAPK支架蛋白，它们在MAPK途径激活时作用于质膜；此外，paxillin将MAPK途径成分定位于质膜的焦点粘附位点（Roy等，2005；Ren等，2007；DeFea等，2000；Togho等，2003；Ishibe等，2003；Claperon和Therrien，2007年综述；Brown和Sacks，2009年综述；Matallanas等，2011年综述）。尽管这些支架蛋白的作用机制相似，但它们促进途径激活的细节存在差异。例如，KSR1和2在无刺激状态下与MAP2K二聚体呈恒定结合，但仅在途径刺激时招募MAPKs，而IQGAP1在未刺激细胞中与MAP2K和MAPK蛋白呈恒定结合，并在EGF刺激MAPK途径激活后，与MAP2K1的相互作用增强（Stewart等，1999；Cacace等，2000；Muller等，2000；Roy等，2004；Roy等，2005；Brown和Sacks，2009年综述）。支架复合体可能对细胞质中MAPK靶点的磷酸化尤为重要（Casar等，2009年综述）。