Cellular Roles of the Inhibitor of Apoptosis Protein c-IAP1.

Abstract

Since the discovery that conserved mechanisms of cell death contribute to homeostatic maintenance of multicellular organisms, much effort has been devoted to identifying the effectors and regulatory proteins involved in apoptosis. Originally identified in 1993, inhibitor of apoptosis (iap) genes are an evolutionarily conserved family defined by a unique structural motif, the baculoviral IAP repeat (BIR) domain. Eight mammalian IAPs have been identified, and the functional properties of IAPs have been studied extensively. While X-linked IAP (XIAP) is a potent cell death inhibitor that directly binds and inhibits the enzymatic activities of caspases, cysteine proteases that are the central executioners of apoptosis, some IAPs have much less effective apoptotic inhibitory properties. Relative to XIAP, two related IAP family members, cellular IAP 1 and 2 (c-IAP1 and c-IAP2) have significantly weaker affinity for caspases. Additionally, c-IAP1 and c-IAP2 contain a RING domain with intrinsic E3 ubiquitin ligase activity that can catalyze autoubiquitination and degradation of target proteins. Therefore, it was hypothesized that c-IAPs might inhibit apoptosis by a caspase-independent mechanism, which could be mediated by their ubiquitination status or binding partners. The stability of c-IAPs was evaluated in the presence of adaptor molecules, tumor necrosis factor (TNF) receptor associated factors (TRAFs), since c-IAPs were shown to associate with TRAFs at the type-2 TNF receptor (TNFR2). Surprisingly, c-IAP1 was selectively stabilized by TRAF2. Stabilized c-IAP1 bound to second mitochondrial-derived activator of caspase (Smac), an IAP binding protein released from mitochondria following mitochondrial membrane perturbation. This binding event prevented Smac antagonism of XIAP, and potently inhibited cell death. Since c-IAP1 was stabilized by TRAF2 at TNF receptors, the regulation of c-IAP1 in TNF-mediated signaling was evaluated. TNF receptors, including CD30 and TNFR2, regulate the inflammatory response through activation of the NF-κB pathways. Two distinct mechanisms to target c-IAP1 for degradation were identified. Smac mediates c-IAP1 degradation by a mechanism that utilizes the RING of c-IAP1, while CD30 activation leads to c-IAP1 degradation through a RING independent mechanism. Taken together, these data suggest that c-IAP1 is a molecule critical for apoptotic and non-apoptotic processes that is regulated by multiple distinct mechanisms.