Bcl-x, a member of the Bcl-2 protein family, inhibits cell death, or apoptosis. Bcl-x is expressed as two isomeric forms, Bcl-xL and Bcl-xS, and is typically present in the cytosol in association with the mitochondrial membrane. Bcl-xL forms heterodimers with various proteins, including Bax, Bak and Bcl-2. It has been found that eterodimerization with Bax does not seem to be required for anti-apoptotic activity. Since Bcl-xL can form an ion channel in synthetic lipid membranes, there is a strong possibility that this property plays a role in heterodimerizationindependent cell survival. The Bcl-xS isoform promotes apoptosis.
