BH4 domain of antiapoptotic Bcl-2 family members closes voltage-dependent anion channel and inhibits apoptotic mitochondrial changes and cell death.

A change of mitochondrial membrane permeability is essential for apoptosis, leading to translocation of apoptogenic cytochrome c and apoptosis-inducing factor into the cytoplasm. We recently showed that the Bcl-2 family of proteins regulate cytochrome c release and the mitochondrial membrane potential (Deltapsi) by directly modulating the activity of the voltage-dependent anion channel (VDAC) through binding. Here we investigated the biochemical role of the conserved N-terminal homology domain (BH4) of Bcl-x(L), which has been shown to be essential for inhibition of apoptosis, with respect to the regulation of mitochondrial membrane permeability and found that BH4 was required for Bcl-x(L) to prevent cytochrome c release and Deltapsi loss. A study using VDAC liposomes revealed that Bcl-x(L), but not Bcl-x(L) lacking the BH4 domain, inhibited VDAC activity. Furthermore, BH4 oligopeptides of Bcl-2 and Bcl-x(L), but not mutant peptides, were able to inhibit both VDAC activity on liposomes even in the presence of Bax and apoptotic Deltapsi loss in isolated mitochondria. It was also shown that the BH4 domain, fused to the protein transduction domain of HIV TAT protein (TAT-BH4), efficiently prevented apoptotic cell death. These results indicate that the BH4 of Bcl-2/Bcl-x(L) is essential and sufficient for inhibiting VDAC activity, which in turn prevents apoptotic mitochondrial changes, and for preventing apoptotic cell death. Finally, the data suggest that the TAT-BH4 peptide is potentially useful as a therapeutic agent for diseases caused by accelerated apoptosis.