The BIR motifs mediate dominant interference and oligomerization of inhibitor of apoptosis Op-IAP

The defining structural motif of the inhibitor of apoptosis (iap) protein f amily is the BIR (baculovirus lap repeat), a highly conserved zinc coordina tion domain of similar to 70 residues. Although the BIR is required for inh ibitor-of-apoptosis (IAP) function, including caspase inhibition, its molec ular role in antiapoptotic activity in vivo is unknown. To define the funct ion of the BIRs, we investigated the activity of these structural motifs wi thin Op-IAP, an efficient, virus derived IAP. We report here that Op-IAP(1- 216), a loss-of-function truncation which contains two BIRs but lacks the C -terminal RING motif, potently interfered with Op-IAP's capacity to block a poptosis induced by diverse stimuli. In contrast, Op-IAP(1-216) had no effe ct on apoptotic suppression by caspase inhibitor P35. Consistent with a mec hanism of dominant inhibition that involves direct interaction between Op-I AP(1-216) and full-length Op-IAP, both proteins formed an immunoprecipitabl e complex in vivo, Op-IAP also self-associated. In contrast, the RING motif -containing truncation Op-IAP(183-268) failed to interact with or interfere with Op-IAP function. Substitution of conserved residues within BIR 2 caus ed loss of dominant inhibition by Op-IAP(1-216) and coincided with loss of interaction with Op-IAP. Thus, residues encompassing the BIRs mediate domin ant inhibition and oligomerization of Op-IAP. Consistent with dominant inte rference by interaction with an endogenous cellular IAP, Op-IAP(1-216) also lowered the survival threshold of cultured insect cells. Taken together, t hese data suggest a new model wherein the antiapoptotic function of IAP req uires homo-oligomerization, which in turn mediates specific interactions wi th cellular apoptotic effecters.