Three-dimensional structure of a double apoptosome formed by the Drosophila Apaf-1 related killer.

The Drosophila Apaf-1 related killer (Dark) forms an apoptosome that activates Dronc, an apical procaspase in the intrinsic cell death pathway. To study this process, we assembled a large Dark complex in the presence of dATP. Remarkably, we found that cytochrome c was not required for assembly and when added, cytochrome c did not bind to the Dark complex. We then determined a 3D structure of the Dark complex at 18.8A resolution using electron cryo-microscopy and single particle methods. In the structure, eight Dark subunits form a wheel-like particle and two of these rings associate face-to-face. In contrast, Apaf-1 forms a single ring that is comprised of seven subunits and each Apaf-1 binds a molecule of cytochrome c. We then used relevant crystal structures to model the Dark complex. This analysis shows that a single Dark ring and the Apaf-1 apoptosome share many key features. When taken together, the data suggest that a single ring in the Dark complex may represent the Drosophila apoptosome. Thus, our analysis provides a domain model of this complex and gives insights into its function.