Bcl-2 family members do not inhibit apoptosis by binding the caspase activator Apaf-1

The Bcl-2 family of proteins regulates apoptosis, the cell death program tr iggered by activation of certain proteases (caspases), An attractive model for how Bcl-2 and its closest relatives prevent caspase activation is that they bind to and inactivate an adaptor protein required for procaspase proc essing. That model has been supported by reports that mammalian prosurvival Bcl-2 relatives bind the adaptor Apaf-1, which activates procaspase-9, How ever, the itt vivo association studies reported here with both overexpresse d and endogenous Apaf-1 challenge this notion. Apaf-1 could be immunoprecip itated together with procaspase-9, and the Apaf-1 caspase-recruitment domai n was necessary and sufficient for their interaction, Apaf-1 did not bind, however, to any of the six known mammalian prosurvival family members (Bcl- 2, Bcl-x(L), Bcl-w, A1, Mel-1, or Boo), or their viral homologs adenovirus E1B 19K and Epstein-Barr virus BHRF-1. Endogenous Apaf-1 also failed to coi mmunoprecipitate with endogenous Bcl-2 or Bcl-xL, or with two proapoptotic relatives (Bax and Bim), Moreover, apoptotic stimuli did not induce Apaf-1 to bind to these family members. Thus, the prosurvival Bcl-2 homologs do no t appear to act by sequestering Apaf-1 and probably instead constrain its a ctivity indirectly.