Binding of the five PDGF isoforms induces different homo- and heterodimeric complexes of PDGFRα and PDGFRβ. The PDGF isoforms are synthesized as precursor molecules with signal sequences (grey), precursor sequences (open) and growth factor domains (red, blue, yellow and green). After dimerization, the isoforms are proteolytically processed (arrows) to their active forms which bind to the receptors. The extracellular parts of the receptors contain 5 Ig-like domains; ligand binding occurs preferentially to domains 2 and 3, and domain 4 stabilizes the dimer by a direct receptor-receptor interaction. The intracellular parts of the receptors contain tyrosine kinase domains split into two parts by an intervening sequence. Ligand-induced dimerization induces autophosphorylation of the receptors, which activates their kinases and create docking sites for SH2-domain-containing signaling molecules, some of which are indicated in the figure. Activation of these signaling pathways promotes cell growth, survival, migration and actin reorganization.