The interaction between the Drosophila secreted protein Argos and the epidermal growth factor receptor inhibits dimerization of the receptor and binding of secreted Spitz to the receptor

Drosophila Argos (Aos), a secreted protein with an epidermal growth factor (EGF)-like domain, has been shown to inhibit the activation of the Drosophi la EGF receptor (DER), However, it has not been determined whether Aos bind s directly to DER or whether regulation of the DER activation occurs throug h some other mechanism, Using DER-expressing cells (DER/S2) and a recombina nt DER extracellular domain-Fc fusion protein (DER-Fc), we have shown that Aos binds directly to the extracellular domain of DER with its carboxyl-ter minal region, including the EGF-like domain. Furthermore, Aos can block the binding of secreted Spitz (sSpi), a transforming growth factor cr-like lig and of DER, to the extracellular domain of DER, We observed that sSpi stimu lates the dimerization of both the soluble DER extracellular domain (sDER) and the intact DER in the DER/S2 cells and that Aos can block the sSpi-indu ced dimerization of both sDER and intact DER, Moreover, we have shown that, by directly interacting with DER, Aos and SpiAos (a chimeric protein that is composed of the N-terminal region of Spi and the C-terminal region of Ao s) inhibit the dimerization and phosphorylation of DER that are induced by DER's overexpression in the absence of sSpi, These results indicate that Ao s exerts its inhibitory function through dual molecular mechanisms: by bloc king both the receptor dimerization and the binding of activating ligand to the receptor. This is the first description of this novel inhibitory mecha nism for receptor tyrosine kinases.