Light-membrane fractions obtained by hypoosmotic lysis of neurospora crassa mitochondria exhibit buoyant densities and marker-enzyme activities characteristic of outer mitochondrial membranes. SDS PAGE of these membrane fractions indicates that a polypeptide of M(r) 31,000 is the main protein component. Under negative-stain electron microscope examination many of the membranes in these fractions appear as large (0.5-1- mum diameter), collapsed vesicles. The surfaces of flattened, open (i.e., ripped) vesicles often exhibit extended two-dimensional arrays of subunits are arranged into hexagons within each parallelogram unit cell, 12.6x11.1 nm (lattice angle = 109 degrees).

The conductance of the voltage-dependent mitochondrial outer membrane channel is modulated by a synthetic anionic polymer. When added to suspensions of membrane crystals of the channel, the polyanion caused disordering of the usual parallelogram array and increased occurrence of a contracted form of the array. Correlation averages obtained from electron microscopic images of the channel crystals indicated a narrowing of the projected channel lumen in the presence of the polyanion and the appearance of new, narrow zones of stain exclusion on the outside of the channel. These effects are interpreted in terms of possible conformational changes induced in the channel by binding of the polyanion.

Patch-clamp studies of the outer mitochondrial membrane indicate a voltage-dependent increase in conductance for potentials positive relative to the exterior of the mitochondrion.The time course of the conductance changes is consistent with an activation of channels. Voltage pulse experiments suggest that the activation phenomenon corresponds to assembly of the channels from subunits with disassembly occurring after recovery of the original conductance.Effects of temperature and concanavalin A on the voltage-induced conductances are also consistent with a channel assembly model Mitochondrial outer membrane; Ion channel; Patch clamp