A mutational analysis of the baculovirus inhibitor of apoptosis Op-IAP

A family of antiapoptotic regulators known as inhibitors of apoptosis (IAPs ) was initially identified and functionally described in baculoviruses, and IAP homologues are now known in insects, birds, and mammals. Baculovirus a nd Drosophila IAPs inhibit apoptosis induced by Drosophila proapoptotic pro teins Reaper, HID, and GRIM and physically interact with them through their baculovirus IAP repeat (BIR) region. Here we examined the functional impor tance of BIR and RING finger motifs of Orgyia pseudotsugata nuclear polyhed rosis virus Op-IAP and D-IAP1 in binding to and inhibiting HID. In the abse nce of both the BIR1 and RING motifs, the BIR2 regions of Op-IAP and D-IAP1 were able to associate with HID and block; HID-induced apoptosis. Mutation of conserved amino acid residues within the BIR and RING finger motifs rev ealed that the conserved residues within BIR2 were essential for Op-IAP to inhibit apoptosis. However, most of the conserved residues of the BIR2 were not required for HID binding. A region at the carboxy-proximal end of BIR2 was essential for the association of OP-IAP with HID. Thus binding to HID is necessary but not sufficient to block HID-induced apoptosis: the conserv ed residues within BIR2 must have an additional role in blocking apoptosis. These findings demonstrate that the region encompassing a single BIR of Op -IAP and D-IAP1 can be sufficient for physical interaction with and inhibit ion of apoptosis induced by HID.