We have recently described the purification and characterization of an insulin-degrading enzyme (IDE) from Drosophila melanogaster that can cleave porcine insulin, is highly conserved through evolution and is developmentally regulated. We now report that the IDE is, in fact, an insulin EGF-binding protein (dp100) that we had isolated previously from Drosophila using an antihuman EGF receptor antiserum. This conclusion is based upon the following evidence. (a) dp100, identified by its ability to cross-link to labeled insulin, EGF, and transforming growth factor-alpha (TGF-alpha), and to be immunoprecipitated by anti-EGF receptor antisera, copurifies with the IDE activity. Thus, the purified IDE can be affinity labeled with either 125I-insulin, 125I-EGF, or 125I-TGF-alpha, and this labeling is specifically inhibited with unlabeled insulin, EGF, and the insulin B chain. (b) The antiserum to the human EGF receptor, which recognizes dp100, is able to specifically immunoprecipitate the insulin-degrading activity. (c) The purified IDE preparation contains a single protein of 110 kD which is recognized by both the anti-EGF receptor antiserum and anti-Drosophila IDE antiserum. (d) Polyclonal antiserum to the purified IDE, which specifically recognized only the 110-kD band in Drosophila Kc cells, immunoprecipitates dp100 cross-linked to 125I-TGF-alpha and dp100 cross-linked to 125I-insulin from the purified IDE preparation. (e) EGF, which competes with insulin for binding to dp100, also inhibits the degradation of insulin by the purified IDE. These results raise the possibility that a functional interaction between the insulin and EGF growth factor families can occur which is mediated by the insulin-degrading enzyme.