Receptor Tyrosine Kinases

The receptor tyrosine kinases, as their name implies, contain intrinsic tyrosine kinase activity and are generally utilized by growth factors, such as neurotrophic factors, and cytokines. Binding of an agonist initiates receptor dimerization and transphosphorylation of tyrosine residues in its cytoplasmic domain (Patapoutian and Reichardt 2001) (see Figure 1-1). The phosphotyrosine residues of the receptor function as binding sites for recruiting specific cytoplasmic signaling and scaffolding proteins. The ability of multiple effectors to interact with phosphotyrosines is undoubtedly one of the keys to the pleiotropic effects that neurotrophins can exert. These pleiotropic and yet distinct effects of growth factors are mediated by varying degrees of activation of three major signaling pathways: the MAP kinase pathway, the phosphoinositide-3 (pi3) kinase pathway, and the phospholipase C (PLC)-Yl pathway (see Figure 1-9 later in this chapter).

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