Bcl-2 family of proteins plays differential roles in regulation of mitochondria-mediated apoptosis, by either promoting or inhibiting the release of apoptogenic molecules from mitochondria to cytosol. Bcl-2 family proteins modulate the mitochondrial permeability through interaction with adenine nucleotide translocator (ANT), voltage-dependent anion channel (VDAC), ADP/ATP exchange, or oxidative phosphorylation during apoptosis. Although the mitochondrial homeostasis is affected by the relative ratio of pro- and anti-apoptotic Bcl-2 family members, the molecular mechanism underlying the release of mitochondrial intermembrane proteins remains elusive. Here we reported the biochemical evidence that both pro-apoptotic Bax and anti-apoptotic Bcl-X-L might simultaneously contact the putative loop regions of human VDACl, and the existence of VDACl-Bax-Bcl-X-L tertiary complex in vitro suggested that VDACl channel conformation and mitochondrial permeability could be determined by the delicate balance between Bax and Bcl-X-L. (C) 2003 Elsevier Science (USA). All rights reserved.