INHIBITOR OF APOPTOSIS PROTEINS PHYSICALLY INTERACT WITH AND BLOCK APOPTOSIS INDUCED BY DROSOPHILA PROTEINS HID AND GRIM

Reaper (RPR), HID, and GRIM activate apoptosis in cells programmed to die during Drosophila development. We have previously shown that trans ient overexpression of RPR in the lepidopteran SF-21 cell line induces apoptosis and that members of the inhibitor of apoptosis (IAP) family of antiapoptotic proteins can inhibit RPR-induced apoptosis and physi cally interact with RPR through their BIR motifs (D. Vucic, W. J. Rais er, A. J. Harvey, and L. K. Miller, Proc. Natl. Acad. Sci. USA 94:1018 3-10188, 1997). In this study, we found that transient overexpression of HID and GRIM also induced apoptosis in the SF-21 cell line. Baculov irus and Drosophila IAPs blocked HID- and GRIM-induced apoptosis and a lso physically interacted with them through the BIR motifs of the IAPs . The region of sequence similarity shared by RPR, HID, and GRIM, the N-terminal 14 amino acids of each protein, was required for the induct ion of apoptosis by HID and its binding to IAPs. When stably overexpre ssed by fusion to an unrelated, nonapoptotic polypeptide, the N-termin al 37 amino acids of HID and GRIM were sufficient to induce apoptosis and confer IAP binding activity. However, GRIM was more complex than H ID since the C-terminal 124 amino acids of GRIM retained apoptosis-ind ucing and IAP binding activity, suggesting the presence of two indepen dent apoptotic motifs within GRIM. Coexpression of IAPs with HID stabi lized HID levels and resulted in the accumulation of HID in punctate p erinuclear locations which coincided with IAP localization. The physic al interaction of IAPs with RPR, HID, and GRIM provides a common molec ular mechanism for IAP inhibition of these Drosophila proapoptotic pro teins.