When expressed in PC12 cells, the platelet-derived growth factor  receptor (PDGF-R) mediates cell differentiation. Mutational analysis of the PDGF-R indicated that persistent receptor stimulation of the Ras/Raf/mitogenactivated protein (MAP) kinase pathway alone was insufficient to sustain PC12 cell differentiation. PDGF receptor activation of signal pathways involving p60 c-src or the persistent regulation of phospholipase C␥ was required for PC12 cell differentiation. PDGF-R regulation of phosphatidylinositol 3-kinase, the GTPase-activating protein of Ras, and the tyrosine phosphatase, Syp, was not required for PC12 cell differentiation. In contrast to overexpression of oncoproteins involved in regulating the MAP kinase pathway, growth factor receptor-mediated differentiation of PC12 cells requires the integration of other signals with the Ras/Raf/MAP kinase pathway.The platelet-derived growth factor receptor (PDGF-R) is a transmembrane polypeptide encoding an intrinsic tyrosine kinase in its intracellular domain. Two distinct PDGF-R genes encode either an ␣ (7, 42, 47) or a  (6, 19, 73) subunit. Binding of PDGF (22) induces dimerization (23) and trans phosphorylation of the PDGF-R on specific tyrosine residues (35). The phosphorylated receptor initiates a series of intracellular signals which ultimately lead to cell growth (12), differentiation (20), and chemotaxis (70) depending on the cellular context.A number of tyrosines on the intracellular domain of the PDGF-R are phosphorylated upon activation of the receptor and serve as recognition sites for proteins which contain Src homology 2 (SH2) domains (57). For example, the PDGF-R has been shown to associate with Src family tyrosine kinases p60 c-src , p59 fyn , and p62 c-yes (39) via juxtamembrane tyrosines 579 and 581, which are in vivo phosphorylation sites (50). The SH2 domain-containing proteins p46 and p52 (Shc proteins) bind to the PDGF-R at multiple phosphotyrosines including tyrosine 581 and indirectly via association with other tyrosinephosphorylated proteins (74). Phosphorylation of tyrosines 740 and 751 is critical for association of the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-K) with the PDGF-R (2, 8, 29-31). Phosphorylated tyrosine 751 also binds Nck via its SH2 binding domain (52). The GTPase-activating protein (GAP) of Ras is tyrosine phosphorylated in response to PDGF and binds to the receptor at phosphorylated 33,49). The SH2-containing phosphotyrosine phosphatase, Syp, associates with phosphorylated tyrosine 1009 (34, 41). In addition, there is evidence which suggests that the adaptor protein, Grb2, associates with the PDGF-R via tyrosine 716 (1) and indirectly through Syp (44). Phosphorylated tyrosine 1009 may also influence the binding of phospholipase C␥ (PLC␥) to tyrosine 1021 (28, 59). Mutational analysis of the PDGF-R has demonstrated that phosphorylated tyrosines 740 and 751, which mediate association with PI3-K, and tyrosine 1021, which mediates association with PLC␥, are necessary for the transducti...