Repsl, which may coordinate a wide variety of the cellular actions

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Ral proteins constitute a family of small GTPases that can be activated by Ras in cells. In the GTP-bound state, Ral proteins bind to RalBp1, a GTPase-activating protein for CDC42 and Rac GTPases. We have used the two-hybrid system in yeast to clone a cDNA for a novel 〜85-kDa protein that can bind to an additional site on RalBP1. This newly identified protein contains an Eps homology (EH) domain, which was first detected in the epidermal growth factor (EGF) receptor substrate Eps15. The RalBP1 associate Eps-homology domain protein, Reps1, is tyrosine-phosphorylated in response to EGF stimulation of cells. In addition, Reps1 has the capacity to form a complex with the SH3 domains of the adapter proteins Crk and Grb2, which may link Reps1 to an EGF-responsive tyrosine kinase, and with epsin and Eps15, which are involved in endocytosis. Finally we found that EGF receptors, but not βィイD22ィエD2-adrenergic receptors, activated c-Src by a Ral-GTPase-dependent mechanism. Thus, Reps1 may coordinate a wide variety of the cellular actions of activated EGF receptors and Ral-GTPases.