Three-dimensional structure of mitochondrial outer-membrane channels from fungus and liver

Abstract

The mitochondrial outer membrane (OM) contains a class of 30-kD polypeptides called VDAC, for the voltage-dependent, anion-selective channel activity which they induce in phospholipid membranes. VDAC occurs in extended periodic arrays on the outer membranes of mitochondria of the fungus Neurospora crassa. The arrays are polymorphic, varying from a parallelogram lattice (Fig. 1a) with six negative-stain-accumulating sites per unit cell (a=12.6 nm, b=11.1 nm, θ=109°) to a rectangular lattice (Fig. 1b) with two densely staining loci per unit cell (a=8.5 nm, b=5.0 nm). We have calculated the three-dimensional structure of the oblique OM array embedded in uranyl acetate. Images were recorded from a crystalline membrane which was tilted relative to the 100keV electron beam from +40° to -40° in 10° intervals. A common crystalline area in each image was Fourier-filtered by a quasi-optical procedure using reflections out to 1/(3.0 nm). Following rotational and translational alignment, the images were used to reconstruct the OM array in three dimensions by a weighted back-projection procedure.