VDAC, the early days

Abstract

VDAC is now universally accepted as the channel in the mitochondrial outer membrane responsible for metabolite flux in and out of mitochondria. Its discovery occurred over two independent lines of investigation in the 1970s and 80s. This retrospective article describes the history of VDAC's discovery and how these lines merged in a collaboration by the authors. The article was written to give the reader a sense of the role played by laboratory environment, personalities, and serendipity in the discovery of the molecular basis for the unusual permeability properties of the mitochondrial outer membrane. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Fig. 1

Early electrophysiological records of VDAC channels from Paramecium. Paramecium membranes were sonicated with a 20 fold mass excess of asolectin (soybean phospholipids), freeze-dried and suspended in hexane prior to layering on the surface of aqueous solutions to form monolayers and then planar bilayer membranes. The amplifier used inverted the current so that downward current is positive, downward transitions are channel openings and upward, channel closings. A. A segment of a current record taken on April 24, 1975, the first experiment performed with VDAC channels. The single-channel fluctuations were recorded under an applied potential of 50 mV in the presence of 0.1 M KCl in the aqueous solution. B. Recording (May 25^th, 1975) of the voltage dependence of VDAC channels in the presence of a CaCl₂ gradient (80 mM CaCl₂ vs 20 mM CaCl₂), the pH maintained at 7.5 with 1 mM Tris buffer. The voltage was changed linearly between +70 and −70 mV. Some of the values are recorded in pencil. There are 6 VDAC channels in the membrane closing around −20 mV. C. The same membrane as in (B) but later in the experiment after more channels had inserted. The voltage was changed linearly between +40 and −40 mV (the voltage values are indicated on top). Channel closure resulted in a remarkable region of negative slope conductance. By extrapolating the current lines back to their intercept, the reversal potential of the voltage-dependent portion of the conductance was obtained. The positive reversal potential on the high Ca²⁺ ion side demonstrated that the channels were highly selective for Cl⁻ over Ca²⁺.

Fig. 2

Visualization by electron microscopy of VDAC pores in the mitochondrial outer membrane. Left: First electron micrograph (recorded in January, 1980) of 2D crystalline arrays of pores in an outer membrane (~ 400 nm across) from N. crassa mitochondria. The specimen was negatively stained with phosphotungstate. Boxes were drawn on the print to guide digitization on a scanning microdensitometer. Upper right: Optical diffraction pattern indicating the presence of two 2D crystal lattices in the center of the collapsed membrane vesicle. (The bright “X” is an optical artifact from the square mask used to delineate the region on the negative illuminated by the laser beam.) Lower right: Density contour map showing the rows of unit cells, each containing six pores, in one of the two overlapped 2D crystals in the membrane vesicle, as revealed by Fourier lattice filtration of the digitized image. This subfield is ~ 40 nm across.