Meta-Biol

• Pathways

Downstream of RAF dimerization and allosteric activation, RAF monomers and KSR1 undergo a series of activating phosphorylations in both the activation loop (AL) and, in the case of ARAF, RAF1 and KSR1, in the NtA . Phosphorylation of the activation loop residues (T491, S494 in RAF1, T452, T455 in ARAF and T599, S602 in BRAF) contributes to full kinase activity, although this may be less critical for ARAF, and RAFs in general, than for other kinases due to their regulation by 14-3-3 binding (Zhu et al, 2005; Zhang et al, 2000; Baljuls et al, 2008, reviewed in Matallanas et al, 2011; Udell et al, 2011). AL phosphorylation may occur through cis-autophosphorylation within the RAF dimer, although phosphorylation by other kinases is also possible (Hu et al, 2013; reviewed in Matallanas et al, 2011).

Phosphorylation in the RAF NtA region is required for full kinase activity, for interaction with MAP2K substrates and for the ability of activated RAF to act as an allosteric activator of other RAF monomers (Marais et al, 1995; Diaz et al, 1997; Xiang et al, 2002; Edin et al, 2005; Hu et al, 2013; Mason et al, 1999). Phosphorylation of these residues (S338 and Y441 in RAF1, S299 and Y302 in ARAF and Y602 in KSR1) may be mediated by a kinase of the SRC, JAK or PAK family kinases, by MAP2K kinases, calmodulin kinase CaMKII or through autophosphorylation by RAF itself (Marais et al, 1995; Xia et al, 1996; King et al, 1998; Sun et al, 2000; Tran et al, 2003; Tran et al, 2005; Salzano et al. 2012, Hu et al, 2013; reviewed in Matallanas et al, 2011). The phosphorylated NtA of RAF1 is also the binding site for the negative regulator PEPB1, also known as RKIP. PEBP1 binding to RAF1 prevents phosphorylation of the MAP2K substrates (Park et al, 2006; Rath et al, 2008; reviewed in Shin et al, 2009).