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

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 mu M), 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.