Adaptor proteins for the various growth factor receptors play a crucial role in signal transduction through tyrosine phosphorylation. Several candidates for adaptor proteins with potential effects on the epidermal growth factor (EGF) receptor-mediated signaling pathway have been identified by recent phosphoproteomic studies. Here, we focus on a novel protein, GAREM (Grb2-associated and regulator of Erk/ MAPK) as a downstream molecule of the EGF receptor. GAREM is phosphorylated at tyrosine 105 and 453 after EGF stimulation. Grb2 was identified as its binding partner, and the proline-rich motifs of GAREM are recognized by the Nand C-terminal SH3 domains of Grb2. In addition, the tyrosine phosphorylations of GAREM are necessary for its binding to Grb2. Because the amino acid sequence surrounding tyrosine 453 is similar to the immunoreceptor tyrosine-based inhibitory motif, Shp2, a positive regulator of Erk, binds to GAREM in this phosphorylation-dependent manner. Consequently, Erk activation in response to EGF stimulation is regulated by the expression of GAREM in COS-7 and HeLa cells, which occurs independent of the presence of other binding proteins, such as Gab1 and SOS, to the activated EGF receptor. Furthermore, the expression of GAREM has an effect on the transformation activity of cultured cells. Together, these findings suggest that GAREM plays a key role in the ligandmediated signaling pathway of the EGF receptor and the tumorigenesis of cells.The interactions between receptor tyrosine kinases and adaptor proteins are crucial for the transduction of intracellular growth signals from the plasma membrane to the nucleus: these signals are propagated by the tyrosine phosphorylation of each molecule (1, 2). Among the numerous adaptor proteins, the complex of Grb2 and the Grb2-associated binder (Gab) 2 family protein can directly bind to several growth factor receptors. This complex can also regulate the activity of downstream protein kinases such as Erk and Akt, which are known regulators of various cellular functions (3-5). These adaptor proteins contain functional domains such as the proline-rich, Src-homology (SH) 2, SH3, phosphotyrosine-binding, or pleckstrin homology (PH) domains (1, 6 -8) required for interaction with their partner proteins. In addition, Gab or insulin receptor substrate family proteins have multiple tyrosine phosphorylation sites and are recognized as substrates by tyrosine kinases. Therefore, Gab or insulin receptor substrate family proteins are targets for interaction with other proteins possessing SH2 domains (9).A great deal of excellent work on the epidermal growth factor (EGF) receptor has established the EGF signaling pathway as a paradigm for growth factor-mediated signal transduction (10). The EGF receptor is known for being involved not only in normal cell proliferation but also in the origin or development of various human cancers (11). Many research groups have applied proteomic techniques, such as mass spectrometry, to identify novel molecules and the post-translational mod...