Bcl-xL does not inhibit the function of Apaf-1

Abstract

Bcl-2 and its relative, Bcl-x_L, inhibit apoptotic cell death primarily by controlling the activation of caspase proteases. Previous reports have suggested at least two distinct mechanisms: Bcl-2 and Bcl-x_L may inhibit either the formation of the cytochrome c/Apaf-1/caspase-9 apoptosome complex (by preventing cytochrome c release from mitochondria) or the function of this apoptosome (through a direct interaction of Bcl-2 or Bcl-x_L with Apaf-1). To evaluate this latter possibility, we added recombinant Bcl-x_L protein to cell-free apoptotic systems derived from Jurkat cells and Xenopus eggs. At low concentrations (50 nM), Bcl-x_L was able to block the release of cytochrome c from mitochondria. However, although Bcl-x_L did associate with Apaf-1, it was unable to inhibit caspase activation induced by the addition of cytochrome c, even at much higher concentrations (1–5 μM). These observations, together with previous results obtained with Bcl-2, argue that Bcl-x_L and Bcl-2 cannot block the apoptosome-mediated activation of caspase-9. Cell Death and Differentiation (2000) 7, 402–407