Bcl-x(L) regulates apoptosis by heterodimerization-dependent and -independent mechanisms

A hydrophobic cleft formed by the BH1, BH2 and BH3 domains of Bcl-x(L) is r esponsible for interactions between Bcl-x(L) and BH3-containing death agoni sts, Mutants were constructed which did not bind to Bax but retained anti-a poptotic activity. Since Bcl-x(L) can form an ion channel in synthetic lipi d membranes, the possibility that this property has a role in heterodimeriz ation-independent cell survival was tested by replacing amino acids within the predicted channel-forming domain with the corresponding amino acids fro m Bax. The resulting chimera showed a reduced ability to adopt an open cond uctance state over a wide range of membrane potentials. Although this const ruct retained the ability to heterodimerize with Bax and to inhibit apoptos is, when a mutation was introduced that rendered the chimera incapable of h eterodimerization, the resulting protein failed to prevent both apoptosis i n mammalian cells and Bax-mediated growth defect in yeast. Similar to mamma lian cells undergoing apoptosis, yeast cells expressing Bax exhibited chang es in mitochondrial properties that were inhibited by Bcl-x(L) through hete rodimerization-dependent and -independent mechanisms. These data suggest th at Bcl-x(L) regulates cell survival by at least two distinct mechanisms; on e is associated with heterodimerization and the other with the ability to f orm a sustained ion channel.