THE Bcl-2-related protein, Bcl-xL, has been shown to block apoptosis induced by a variety of stimuli1-5 and to be a stronger protector against apoptosis than BcL-2 under certain circumstances2,5. Using site-specific mutagenesis, we show here that the amino-acid residues critical for protection of cells by Bcl-xL, against Sindbis virus-induced apoptosis are clustered within the Bcl-2-homology regions 1 and 2 (BH1 and BH2 regions). The residues necessary for Bcl-xL function are not identical to those required for Bcl-2 function6. Although it has been suggested that heterodimerization between Bcl-xL and Bax is essential for the anti-death activity of Bcl-xL (refs 7,8), our results suggest that the interaction with Bax is not required for Bcl-xL to exert its death-repressing activity. Specific mutations that disrupt the ability of Bcl-xL to interact with Bax or Bax still preserve 70-80% of the anti-death activity of wild-type Bcl-xL.
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