| The death of a cell is not always noxious to the organism. Individual cells may be terminated in a planned way to benefit the organism. This concept is known as programmed cell death, where the term apoptosis is used to describe its dominant form.;Apoptosis is carried out by a set of initiator and executioner caspases. Caspases are proteases, which cleave limited recognition sites at their target proteins. The target proteins have important functional or structural roles in the cell and the cleavage of these proteins results in the apoptotic phenotype and cell death. Caspases exist in the cell as inactive zymogens and the activation of initiator procaspases defines a control point in apoptosis. Procaspase-9 is an initiator procaspase and its activation is regulated by the formation of the apoptosome, which is a complex of Apaf-1, cytochrome c and the cofactor dATP/ATP. Once the apoptosome is formed, procaspase-9 binds to the complex and is activated. The bound initiator caspase will in turn activate downstream executioner caspases by proteolytic cleavage and trigger apoptosis.;We have determined the three-dimensional structure of the apoptosome at 27 A resolution, using cryo electron microscopy and image processing techniques. The apoptosome is a seven-fold symmetric, wheel-like particle with dimensions of approximately 270 x 70 A. We have located two WD-40 repeat beta-propellers, the caspase recruitment domain of Apaf-1 and one cytochrome c molecule within the complex, by molecular docking. In a subsequent reconstruction, we determined the structure of the apoptosome with bound procaspase-9. A comparison of the two structures, combined with existing biochemical and structural data, allowed us to propose models for apoptosome assembly, procaspase-9 binding and activation. |
