NMR Structural Investigation of the Mitochondrial Outer Membrane Protein VDAC and Its Interaction with Antiapoptotic Bcl-x_L^†

Bcl-2 family proteins are essential regulators of cell death and exert their primary pro- or antiapoptotic roles at the mitochondrial outer membrane. Previously, pro- and antiapoptotic Bcl-2 proteins have been shown to interact with the voltage-dependent anion channel (VDAC) of the outer mitochondrial membrane. VDAC is a 283-residue integral membrane protein that forms an aqueous pore in the outer mitochondrial membrane, through which metabolites and other small molecules pass between the cytosol and intermembrane space. The essential life-sustaining function of VDAC in metabolite trafficking is believed to be regulated by proteins of the Bcl-2 family. The protective role of antiapoptotic Bcl-x_L may be through its interaction with VDAC. Here, VDAC has been expressed, purified, and refolded into a functional form amenable to NMR studies. Various biophysical experiments indicate that micelle-bound VDAC is in intermediate exchange between monomer and trimer. Using NMR spectroscopy, gel filtration, and chemical cross-linking, we obtained direct evidence for binding of Bcl-x_L to VDAC in a detergent micelle system. The VDAC-interacting region of Bcl-x_L was characterized by NMR with chemical shift perturbation and transferred cross-saturation. The interaction region was mapped to a putative helical hairpin motif of Bcl-x_L that was found to insert into detergent micelles. Our results suggest that Bcl-x_L can bind to one or two VDAC molecules forming heterodimers and heterotrimers. Our characterization of the VDAC/Bcl-x_L complex offers initial structural insight into the role of antiapoptotic Bcl-x_L in regulating apoptotic events in the mitochondrial outer membrane.