Structure of the apoptosome: mechanistic insights into activation of an initiator caspase from Drosophila

• Published in: Genes & development 2015-02, Vol.29 (3), p.277-287
• Main Authors: Pang, Yuxuan, Bai, Xiao-chen, Yan, Chuangye, Hao, Qi, Chen, Zheqin, Wang, Jia-Wei, Scheres, Sjors H W, Shi, Yigong
• Format: Article
• Language: English
• Subjects: Animals; Apoptosomes - chemistry; Apoptosomes - metabolism; Caspases - metabolism; Drosophila - enzymology; Drosophila melanogaster; Drosophila Proteins - metabolism; Enzyme Activation; Models, Molecular; Protein Binding; Protein Structure, Tertiary; Research Paper
• ISSN: 0890-9369; 1549-5477
• DOI: 10.1101/gad.255877.114

Apoptosis is executed by a cascade of caspase activation. The autocatalytic activation of an initiator caspase, exemplified by caspase-9 in mammals or its ortholog, Dronc, in fruit flies, is facilitated by a multimeric adaptor complex known as the apoptosome. The underlying mechanism by which caspase-9 or Dronc is activated by the apoptosome remains unknown. Here we report the electron cryomicroscopic (cryo-EM) structure of the intact apoptosome from Drosophila melanogaster at 4.0 Å resolution. Analysis of the Drosophila apoptosome, which comprises 16 molecules of the Dark protein (Apaf-1 ortholog), reveals molecular determinants that support the assembly of the 2.5-MDa complex. In the absence of dATP or ATP, Dronc zymogen potently induces formation of the Dark apoptosome, within which Dronc is efficiently activated. At 4.1 Å resolution, the cryo-EM structure of the Dark apoptosome bound to the caspase recruitment domain (CARD) of Dronc (Dronc-CARD) reveals two stacked rings of Dronc-CARD that are sandwiched between two octameric rings of the Dark protein. The specific interactions between Dronc-CARD and both the CARD and the WD40 repeats of a nearby Dark protomer are indispensable for Dronc activation. These findings reveal important mechanistic insights into the activation of initiator caspase by the apoptosome.