Lateral segregation of sterol and channel proteins in the mitochondrial outer membrane induced by phospholipase A2: evidence from negative-stain electron microscopy using filipin

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

Purification and characterization of protein H, the major porin of Pasteurella multocida

Protein H (B. Lugtenberg, R. van Boxtel, D. Evenberg, M. de Jong, P. Storm, and J. Frik, Infect. Immun. 52:175-182, 1986) is the major polypeptide of the outer membrane of Pasteurella multocida, a bacterium pathogenic for humans and animals. We have purified this protein to homogeneity by size exclusion chromatography after selective extraction with surfactants and demonstrated its pore-forming ability after reincorporation into planar lipid bilayers. In these experiments, the current through the pores was a linear function of the applied voltage in the range of -50 to +50 mV. Voltages beyond +/- 50 mV tended to partially close the channels, giving rise to apparent negative resistances. These observations suggest that protein H channels are probably not voltage regulated in vivo. With the patch clamp technique, single-channel conductance fluctuations of 0.33 nS were recorded in 1 M KCl. Electrophoretic and circular dichroism analyses showed that protein H forms homotrimers stable in sodium dodecyl sulfate at room temperature, with a high content of beta-sheet secondary structure. Upon boiling, the trimers were fully dissociated into monomers with an increase of alpha helix and irregular structure, at the expense of beta sheets. The apparent molecular mass of fully denatured monomers ranged between 37 and 41.8 kDa, depending on the electrophoretic system used for analysis. The trimeric arrangement of protein H was confirmed by image analysis of negatively stained, two-dimensional crystal arrays. This morphological study revealed, in agreement with electrophoretical data, a trimeric structure with an overall diameter of 7.7 nm. Each monomer appeared to contain a pore with an average diameter of 1 nm. Quantitative comparisons revealed that the amino acid composition (hydropathy index of -0.40) and the N-terminal sequence (determined over 36 residues) of protein H are similar to those of bacterial general porins, notably porin P2 of Haemophilus influenzae. We conclude from this set of structural and functional data that protein H of P. multocida is a pore-forming protein related to the superfamily of the nonspecific bacterial porins.

Cholesterol distribution in rat liver and brain mitochondria as determined by stopped-flow kinetics with filipin

Recently, analysis of protein distribution in rat brain mitochondria suggested the existence of distinct cholesterol domains in the outer membrane (Dorbani et al., 1987, Arch. Biochem. Biophys. 252, 188-196) while such domains were not detected in rat liver mitochondria (Jancsik et al., 1988, Arch. Biochem. Biophys. 264, 295-301). We studied cholesterol distribution in both types of mitochondria by analyzing the kinetics of filipin-cholesterol complex formation, using the stopped-flow technique. In liver mitochondria, the kinetics are characterized by a biphasic curve which presumably corresponds to the two membranes. This was confirmed by the finding that pretreatment with digitonin abolished one of the kinetic components. Sonication of the mitochondria increased the rate of the filipin-cholesterol complex formation and also abolished one of the two components. In the case of brain mitochondria, several distinct cholesterol domains could be revealed: one of them was cholesterol-free and it was directly accessible to filipin. Two other domains were revealed by differences found in the rate of the cholesterol-filipin complex formation. It is noteworthy that only a part of the cholesterol is accessible to filipin. Sonication of mitochondria decreased the proportion of cholesterol molecules accessible to filipin. This suggests specific interactions of cholesterol with other mitochondrial components, which occur only in brain mitochondria.

Structural analysis of mitochondrial pores

Structural information about the channel in the mitochondrial outer membrane, derived from sequence analysis and electron microscopy of two-dimensional crystals, is summarized. A model for the channel is presented, consisting of a cylindrical beta-barrel that is formed by one or two 30-kDa polypeptides, with an alpha-carbon backbone diameter of 3.8 nm. The radial distributions of basic amino acids and lipid-contact regions on the projected cylinder are mapped relative to interchannel bonding sites inferred from channel packing in the arrays. Speculation on the kinds of conformational changes that the channel might undergo is also presented.

Structure of the mitochondrial outer membrane channel derived from electron microscopy of 2D crystals

A structural model for the channel in the mitochondrial outer membrane is presented, derived from electron microscopic studies of two-dimensional crystals and inferences from the primary structure of the 30-kDa polypeptide which forms the channel. The channel is represented as a cylindrical beta-barrel, with a carbon backbone diameter of 3.8 nm. The axial projection of the cylinder is divided radially into four sectors by four interchannel contact points. These sectors are characterized in terms of their interactions with lipid and macromolecular ligands, and in terms of the presence or absence of exposed basic amino acids.

Influence of thyroid status on the membranes of rat liver mitochondria. Unique localization of L-glycerol-3-phosphate dehydrogenase

The effect of thyroid status on the physical properties of rat liver mitochondrial membranes and on the lipid microenvironment of proteins was investigated. The steady-state fluorescence anisotropy of diphenyl-1,3,5-triene and 1-[4-(trimethylaminophenyl)phenyl]-6-phenylhexa-1,3,5-triene revealed an increase of the order of the membranes with the increase of hormone level. Protein arrangement in the inner mitochondrial membrane altered with the thyroid status, which was reflected by digitonin subfractionation of mitochondria. The microenvironment of FAD-linked L-glycerol-3-phosphate dehydrogenase was dramatically influenced by thyroxine.