The topology of VDAC as probed by biotin modification

The outer membrane of mitochondria contains channels called VDAC (mitochondrial porin), which are formed by a single 30-kDa protein. Cysteine residues introduced by site-directed mutagenesis at sites throughout Neurospora crassa VDAC (naturally devoid of cysteine) were specifically biotinylated prior to reconstitution into planar phospholipid membranes. From previous studies, binding of streptavidin to single biotinylated sites results in one of two effects: reduced single-channel conductance without blockage of voltage gating (type 1) or locking of the channels in a closed conformation (type 2). All sites react with streptavidin only from one side of the membrane. Here, we extend this approach to VDAC molecules containing two cysteines and determine the location of each biotinylated residue with respect to the other within the membrane. When a combination of a type 1 and a type 2 site was used, each site could be observed to react with streptavidin. Two sets of sites located on opposite surfaces of the membrane were identified, thereby establishing the transmembrane topology of VDAC. A revised folding pattern for VDAC, consisting of 1 alpha helix and 13 beta strands, is proposed by combining these results with previously obtained information on which sites are lining the aqueous pore.