A novel enhancer of the Apaf1 apoptosome involved in cytochrome c-dependent caspase activation and apoptosis

Apaf1/CED4 family members play central roles in apoptosis regulation as act ivators of caspase family cell death proteases, These proteins contain a nu cleotide-binding (NB) self-oligomerization domain and a caspase recruitment domain (CARD). A novel human protein was identified, NAG, that contains an NE domain and CARD. The CARD of NAC interacts selectively with the CARD do main of Apaf1, a caspase-activating protein that couples mitochondria-relea sed cytochrome c (cyt-c) to activation of cytosolic caspases. Cyt-c-mediate d activation of caspases in cytosolic extracts and in cells is enhanced by overexpressing NAC and inhibited by reducing NAC using antisense/DNAzymes. Furthermore, association of NAC with Apaf1 is cyt c inducible, resulting in a mega-complex (>1 MDa) containing both NAC and Apaf1 and correlating with enhanced recruitment and proteolytic processing of pro-caspase-9. NAC also collaborates with Apaf1 in inducing caspase activation and apoptosis in in tact cells, whereas fragments of NAC representing only the CARD or NE domai n suppress Apaf1-dependent apoptosis induction. NAC expression in vivo is a ssociated with terminal differentiation of short Lived cells in epithelia a nd some other tissues. The ability of NAC to enhance Apaf1-apoptosome funct ion reveals a novel paradigm for apoptosis regulation.