Comparative X-ray diffraction and electron microscope studies of isolated mitochondrial membranes

Fusion of the mitochondrial outer membrane: use in forming large, two-dimensional crystals of the voltage-dependent, anion-selective channel protein

Phospholipase A2 induces crystallization of the channel protein, VDAC (also called mitochondrial porin), in the outer membrane of Neurospora crassa mitochondria. The channel crystals formed in native membranes typically contain a few hundred unit cells. To increase the size of these membrane crystals for low-contrast electron microscopic imaging and diffraction studies, fusion of the isolated mitochondrial outer membranes was attempted before and after phospholipase treatment. Successful fusion of the untreated membranes was achieved by a procedure involving slow dehydration at acid pH. Single crystals of channels obtained by subsequent action of soluble phospholipase A2 on fused mitochondrial outer membranes may contain several thousand unit cells.

The role of the mitochondrial outer membrane in energy metabolism of tumor cells

The outer mitochondrial membrane contains a pore structure which is composed of a 30,000 Da protein, porin. The pore has an internal diameter of 2 nm and exhibits a molecular-sieving exclusion limit between 3000 and 6000 Da. These pores, therefore, provide the exit/entrance port for metabolites moving between mitochondria and the cytosol. Hexokinase binds to porin on the outer surface of mitochondria. The location of hexokinase has evoked a number of theories in which bound hexokinase is given a central role in regulating glycolysis, and, perhaps, the metabolic communication between oxidative and glycolytic metabolism. This is of particular importance in rapidly growing tumor cells in which the aerobic production of lactate and hexokinase activity are highly induced. In the present paper, we summarize the suggested roles of the outer membrane and bound hexokinase in regulation glycolysis of tumor cells. Experiments attempting to elucidate the role of hexokinase binding in the regulation of tumor cell metabolism are presented.

Structure of the outer mitochondrial membrane: ordered arrays of porelike subunits in outer-membrane fractions from Neurospora crassa mitochondria

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).

Structure of the outer mitochondrial membrane: analysis of X-ray diffraction from the plant membrane

X-ray diffraction from centrifugally oriented specimens of plant outer mitochondrial membranes suggests that these membrane contain prominent in-plane subunits. The short lamellar repeat which these specimens display (as low as 5.1 nm) points to a predominantly internal localization of the protein components of these membranes. The simplest model for the putative in-plane subunit consistent with autocorrelation analysis of the normal-indicence diffraction data consists of two concentric rings of electron density with diameters of (approx.) 2 and 4 nm. These rings could represent the planar projections of concentric cylindrical shells, aligned normal to the membrane surface.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

The lipid composition of plasma membrane and mitochondrial fractions from epididymal adipocytes of cold-acclimated rats

1. The effects of cold acclimation (5 degrees C) on the lipid composition of plasma membrane and mitochondrial fractions from epididymal adipocytes of rats were studied. 2. The adipocyte plasma membrane fraction of the cold-acclimated rats had lower lipid, phospholipid and cholesterol to protein weight ratios, a lower cholesterol to sphingomyelin molar ratio, and a higher linoleic acid content in the phospholipids than controls. 3. The mitochondrial fraction of the cold-acclimated rat adipocyte had lower ratios of cholesterol to protein (weight), to phospholipid and to cardiolipin (molar), and less sphingomyelin content than did controls. 4. These data, discussed in terms of alterations in physical and biochemical properties, indicate cold-induced changes at the membrane level in rat epididymal adipocytes.

Small-angle X-ray scattering from mitochondria

X-ray (CuKalpha) scattering curves of rat liver mitochondria are characterized by continuously decreasing intensity from 0.5 to 5 mrad and a broad maximum centered near 20 mrad. The condensed-to-orthodox morphological transition of the inner membranes of intact mitochondria causes a dramatic decrease in scattering at very small angle and a marked shift of the 20 mrad maximum to smaller angle. A similar small-angle scattering maximum is observed with inner mitochondrial membrane fractions prepared by digitonin treatment and osmotic shock/step gradient centrifugation procedures. However, the small-angle X-ray scattering curves of mitochondria after acetone treatment and osmoticlysis/sonication are essentially continuous. These characteristics of mitochondrial X-ray scattering are discussed in terms of known structural features of the organelle.

X-ray diffraction from oriented outer mitochondrial membranes. Detection of in-plane subunit structure

X-ray diffraction patterns from ultracentrifugally oriented specimens of plant outer mitochondrial membranes show five distinct maxima in the equatorial direction. These diffraction maxima arise from in-plane subunits whose dimensions are consistent with those of the features ("pits") seen in electron micrographs of the membranes in negative stain.

Inorganic pyrophosphatase of rat liver mitochondria. Correlation of latency with catalytic properties and intramitochondrial location

Stimulation of Mg(2+)-dependent inorganic pyrophosphatase activity several fold by disruption of mitochondrial membranes does not appreciably alter the catalytic properties of the enzyme. Stimulation is due to increased accessibility of substrate to the enzyme, which is not solublized on activation. The enzyme is attached to the inside of the inner membrane, and under physiological conditions probably hydrolyzes only intramitochondrially-produced PP(i).

Biophysical contributions to membrane structure

Applications of physical techniques to studies of membrane structure have increased greatly in the past few years and they have begun to provide more precise structural parameters for membranes and also some indication of the physical states of the molecular constituents in the membranes. Direct measurements of membrane features have been made by electron microscopy and by X-ray diffraction methods. Spectroscopic techniques especially infrared absorption, nuclear magnetic resonance absorption and optical rotatory dispersion and circular dichroism measurements have provided additional data relating to the physical states of the molecular components in the membranes.