The BH3 Domain of Bcl-x S Is Required for Inhibition of the Antiapoptotic Function of Bcl-x L

Abstract

ABSTRACT bcl-x is a member of the bcl-2 family of genes. The major protein product, Bcl-x L , is a 233-amino-acid protein which has antiapoptotic properties. In contrast, one of the alternatively spliced transcripts of the bcl-x gene codes for the protein Bcl-x S , which lacks 63 amino acids present in Bcl-x L and has proapoptotic activity. Unlike other proapoptotic Bcl-2 family members, such as Bax and Bak, Bcl-x S does not seem to induce cell death in the absence of an additional death signal. However, Bcl-x S does interfere with the ability of Bcl-x L to antagonize Bax-induced death in transiently transfected 293 cells. Mutational analysis of Bcl-x S was conducted to identify the domains necessary to mediate its proapoptotic phenotype. Deletion mutants of Bcl-x S which still contained an intact BH3 domain retained the ability to inhibit survival through antagonism of Bcl-x L . Bcl-x S was able to form heterodimers with Bcl-x L in mammalian cells, and its ability to inhibit survival correlated with the ability to heterodimerize with Bcl-x L . Deletion mutants of Bax and Bcl-2, which lacked BH1 and BH2 domains but contained a BH3 domain, were able to antagonize the survival effect conferred by Bcl-x L . The results suggest that BH3 domains from both pro- and antiapoptotic Bcl-2 family members, while lacking an intrinsic ability to promote programmed cell death, can be potent inhibitors of Bcl-x L survival function.