Signaling by the platelet-derived growth factor (PDGF) receptor. The unliganded receptor is monomeric and its tyrosine kinase catalytic activity is low (left). On binding to dimeric PDGF, the receptor dimerizes, its catalytic activity increases, and receptors transphosphorylate each other on a number of different sites, represented by pink circles (center). These phosphorylated sites (with one exception) serve to recruit cytosolic effector proteins (gray) that contain phosphotyrosine-specific modular binding domains (right). The exception is the activating phosphorylation, located on the catalytic domain of the receptor adjacent to the active site (red circle). Representative effectors depicted are: Src, Src-family non-receptor tyrosine kinases; PI3K, regulatory subunit of phosphatidylinositol 3-kinase; GAP, RasGAP, a GTPase-activating factor for Ras; PLC, phosphatidylinositol-specific phospholipase C-γ; Shp2, SH2-containing tyrosine phosphatase; Grb2, adaptor protein that recruits the Ras guanine-nucleotide exchange factor Sos.