Endocytosis of cell surface proteins is mediated by a complex molecular machinery that assembles on the inner surface of the plasma membrane. Here, we report the identification of two ubiquitously expressed human proteins, stonin 1 and stonin 2, related to components of the endocytic machinery. The human stonins are homologous to the Drosophila melanogaster stoned B protein and exhibit a modular structure consisting of an NH(2)-terminal proline-rich domain, a central region of homology specific to the stonins, and a COOH-terminal region homologous to the mu subunits of adaptor protein (AP) complexes. Stonin 2, but not stonin 1, interacts with the endocytic machinery proteins Eps15, Eps15R, and intersectin 1. These interactions occur via two NPF motifs in the proline-rich domain of stonin 2 and Eps15 homology domains of Eps15, Eps15R, and intersectin 1. Stonin 2 also interacts indirectly with the adaptor protein complex, AP-2. In addition, stonin 2 binds to the C2B domains of synaptotagmins I and II. Overexpression of GFP-stonin 2 interferes with recruitment of AP-2 to the plasma membrane and impairs internalization of the transferrin, epidermal growth factor, and low density lipoprotein receptors. These observations suggest that stonin 2 is a novel component of the general endocytic machinery.
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