Apoptosome Formation through Disruption of the K192-D616 Salt Bridge in the Apaf-1 Closed Form.

The molecular mechanism of apoptosome activation through conformational changes of Apaf-1 auto-inhibited form remains largely enigmatic. The crystal structure of Apaf-1 suggests that some ionic bonds, including the bond between K192 and D616, are critical for the preservation of the inactive "closed" form of Apaf-1. Here, a split luciferase complementation assay was used to monitor the effect of disrupting this ionic bond on apoptosome activation and caspase-3 activity in cells. The K192E mutation, predicted to disrupt the ionic interaction with D616, increased apoptosome formation and caspase activity, suggesting that this mutation favors the "open"/active form of Apaf-1. However, mutation of D616 to alanine or lysine had different effects. While both mutants favored apoptosome formation such as K192E, D616K cannot activate caspases and D616A activates caspases poorly, and not as well as wild-type Apaf-1. Thus, our data show that the ionic bond between K192 and D616 is critical for maintaining the closed form of Apaf-1 and that disrupting the interaction enhances apoptosome formation. However, our data also reveal that after apoptosome formation, D616 and K192 play a previously unsuspected role in caspase activation. The molecular explanation for this observation is yet to be elucidated.