The carboxyl terminal extension of the Drosophila insulin receptor homologue binds IRS-1 and influences cell survival

The Drosophila insulin receptor (INR) homolog includes an extension of appr oximately 400 amino acids at the carboxyl-terminal end of its beta subunit containing several tyrosine-based motifs known to mediate interactions with signaling proteins. In order to explore the role of this extension in INR function, mammalian expression vectors encoding either the complete INR bet a subunit (beta-Myc) or the INR beta subunit without the carboxyl-terminal extension (beta Delta) were constructed, and the membrane-bound beta subuni ts were expressed in 293 and Madin-Darby canine kidney cells in the absence of the ligand-binding cu subunits, beta-Myc and beta Delta proteins were c onstitutively active tyrosine kinases of 180 and 102 kDa, respectively. INR beta-Myc co-immunoprecipitated a phosphoprotein of 170 kDa identified as i nsulin receptor substrate-1 (IRS-1), whereas INR beta Delta did not, sugges ting that the site of interaction was within the carboxyl-terminal extensio n. IRS-1 was phosphorylated on tyrosine to a much greater extent in cells e xpressing INR beta-Myc than in parental or INR beta Delta cells. Despite th is, a variety of PTB or SH2 domain-containing signaling proteins, including IRS-2, mSos-1, Shc, p85 subunit of phosphatidylinositol 3-kinase, SHP-2, R af-1, and JAK2, were not associated with the INR beta-Myc IRS-1 complex. Ov erexpression of INR beta-Myc and beta Delta kinases conferred an equivalent increase in cell proliferation in both 293 and Madin-Darby canine kidney c ells, indicating that this growth response is independent of the carboxyl-t erminal extension. However, INR beta-Myc-expressing cells exhibited enhance d survival relative to parental and beta Delta cells, suggesting that the c arboxyl-terminal extension, through its interaction with IRS-1, plays a rol e in the regulation of eel death.