D-CBL, THE DROSOPHILA HOMOLOG OF THE C-CBL PROTOONCOGENE, INTERACTS WITH THE DROSOPHILA EGF RECEPTOR IN-VIVO, DESPITE LACKING C-TERMINAL ADAPTER BINDING-SITES

The c-Cbl proto-oncogene encodes a multidomain phosphoprotein that has been demonstrated to interact with a wide range of signalling protein s. The biochemical function of c-Cbl in these complexes is, however, u nclear. Recent studies with the C. elegans Cbl homologue, sli-1, have suggested that Cbl proteins may act as negative regulators of EGF rece ptor (EGFR) signalling. As the EGFR and other protein tyrosine kinase receptor signalling pathways are highly conserved between insects and vertebrates, we sought a Drosophila homologue of c-Cbl for a detailed genetic analysis. We report here that Drosophila melanogaster has a si ngle gene, D-cbl, that is homologous to c-cbl. We find that D-cbl enco des a 52 kDa protein that has a high degree of similarity to c-Cbl and SLI-1 across novel phosphotyrosine-binding (PTB) and RING finger doma ins. Surprisingly, however, D-Cbl is C-terminally truncated relative t o c-Cbl and SLI-1 and consequently is unable to bind SH3-domain contai ning adaptor proteins, including the Drosophila Grb2 homologue, Drk. A lthough the D-Cbl protein lacks Drk binding sites it can nevertheless associate with a tyrosine phosphorylated protein, or is itself tyrosin e phosphorylated in an DER dependent manner and associates,vith activa ted Drosophila EGF receptors (DER) in vivo. Consistent with a role for D-Cbl in DER dependent patterning in the embryo and adult, D-Cbl is e xpressed at a high level in early embryos and throughout the imaginal discs in third instar larvae. This study forms the basis for future ge netic analysis of D-Chl, aimed at gaining insights into the role of Cb l proteins in signal transduction.