Under respiratory growth conditions, Bcl-x(L) and Bcl-2 are unable to overcome yeast cell death triggered by a mutant Bax protein lacking the membrane anchor.

We have reported earlier that cytosolic expression of the full-length human apoptosis inducer Bax-alpha (Bax) in the yeast Saccharomyces cerevisiae suppresses growth and induces mortality in cells containing functional mitochondria. Human Bcl-x(L) overcomes this toxicity. Here we describe that a mutant Bax protein, with a missing membrane anchor region (Bax delta), also inhibits growth and causes cell death in yeast. However, the death inhibitory proteins Bcl-x(L) and Bcl-2 fail to rescue Bax delta-mediated growth inhibition under conditions promoting respiration, although they bind Bax delta in the cell. Results in Jurkat T-cells corroborate that Bcl-x(L) is much less efficient at rescuing mammalian cells from the effect of Bax delta than from full length Bax. We have also inquired if the respiration-dependent toxicity of Bax and Bax delta in yeast is nullified by Bcl-x(L)delta and Bcl-2delta, molecules which lack membrane anchors but bind Bax in the yeast two-hybrid system. It appears that, under conditions which facilitate respiration in yeast, Bcl-x(L)delta and Bcl-2delta are incapable of rescuing both Bax-containing and Bax delta-containing cells. Our results open up the interesting possibility that there might exist proteins, unrelated to the Bcl-2 family, which could negate death induced by a membrane anchor-free form of Bax.