The receptor tyrosine kinase RET functions as the signal transducing receptor for the GDNF (for "glial cell-derived neurotrophic factors") family of ligands. Mutations in the RET gene were implicated in Hirschsprung disease (HSCR), multiple endocrine neoplasia type 2 (MEN 2), and thyroid carcinomas. In this report we demonstrate that the docking protein FRS2 is tyrosine phosphorylated by ligand-stimulated and by constitutively activated oncogenic forms of RET. Complex formation between RET and FRS2 is mediated by binding of the phosphotyrosine-binding domain of FRS2 to pY1062, a residue in RET that also functions as a binding site for Shc. However, overexpression of FRS2 but not Shc potentiates mitogen-activated protein (MAP) kinase activation by RET oncoproteins. We demonstrate that oncogenic RET-PTC proteins are associated with FRS2 constitutively, leading to tyrosine phosphorylation of FRS2, MAP kinase stimulation, and cell proliferation. However, loss-of-function HSCR-associated RET mutants exhibit impaired FRS2 binding and reduced MAP kinase activation. These experiments demonstrate that FRS2 couples both ligand-regulated and oncogenic forms of RET, with the MAP kinase signaling cascade as part of the response of RET under normal biological conditions and pathological conditions, such as MEN 2 and papillary thyroid carcinomas.The GDNF family of neurotrophins consists of four members collectively designated GFLs (GDNF family ligands): glial cell-derived neurotrophic factor (GDNF), neurturin, persephin, and artemin. These growth factors play a crucial role in regulating the survival of neurons of the peripheral and central nervous system. The GDNF proteins signal through a multicomponent receptor complex consisting of a ligand-binding GDNF family receptor (GFR), designated the ␣ subunit (GFR␣), and the proto-oncogenic receptor tyrosine kinase, RET, which forms the  subunit. The four GFR␣ receptors, GFR␣1, -2, -3, and -4, are linked to the cell membrane via glycosyl-phosphatidylinositol anchors. It was shown that GFR␣1, -2, -3, and -4 predominantly bind GDNF, neurturin, artemin, and persephin, respectively. RET functions as a common intracellular signal transducing component in conjunction with each of the GFR␣ subunits (1). However, it was recently proposed that the GFR␣ receptors may signal autonomously in the absence of RET (49) and that they target RET to lipid rafts on the cell membrane (47). The GFR␣ and RET receptors are expressed in both distinct and overlapping regions of the developing embryo and the adult mouse. Mice deficient in either GDNF, RET, or GFR␣1 fail to develop the ureteric bud or undergo metanephric development, demonstrating that this ligand-receptor complex plays a role in kidney morphogenesis. Moreover, the mutant mice lack enteric neurons throughout their digestive tracts (39). The striking similarities in the developmental defects observed in the mutant mice is consistent with the notion that GDNF acts primarily through the GFR␣1-RET receptor complex (39).RET has been associated wi...