P-28 BAP31, A BCL-2 BCL-X-L-ASSOCIATED AND PROCASPASE-8-ASSOCIATED PROTEIN IN THE ENDOPLASMIC-RETICULUM/

We have identified a human Bcl-2-interacting protein, p28 Bap31. It is a 28-kD (p28) polytopic integral protein of the endoplasmic reticulum whose COOH-terminal cytosolic region contains overlapping predicted l eucine zipper and weak death effector homology domains, flanked on eit her side by identical caspase recognition sites. In cotransfected 293T cells, p28 is part of a complex that includes Bcl-2/Bcl-X-L and proca spase-8 (pro-FLICE). Bax, a pro-apoptotic member of the Bcl-2 family, does not associate with the complex; however, it prevents Bcl-2 from d oing so. In the absence (but not presence) of elevated Bcl-2 levels, a poptotic signaling by adenovirus E1A oncoproteins promote cleavage of p28 at the two caspase recognition sites. Purified caspase-8 (FLICE/MA CH/Mch5) and caspase-1(ICE), but not caspase-3 (CPP32/apopain/Yama), e fficiently catalyze this reaction in vitro. The resulting NH2-terminal p20 fragment induces apoptosis when expressed ectopically in otherwis e normal cells. Taken together, the results suggest that p28 Bap31 is part of a complex in the endoplasmic reticulum that mechanically bridg es an apoptosis-initiating caspase, like procaspase-8, with the anti-a poptotic regulator Bcl-2 or Bcl-X-L. This raises the possibility that the p28 complex contributes to the regulation of procaspase-8 or a rel ated caspase in response to E1A, dependent on the status of the Bcl-2 setpoint within the complex.