On the role of anisotropy of membrane constituents in formation of a membrane neck during budding of a multicomponent membrane

The expression for the isotropic membrane bending energy was generalized for the case of a multicomponent membrane where the membrane constituents (single molecules or small complexes of molecules-membrane inclusions) were assumed to be anisotropic. Using this generalized expression for the membrane energy it was shown that the change of intrinsic shape of membrane components may induce first-order-like shape transitions leading to the formation of a membrane neck. The predicted discontinuous membrane shape transition and the concomitant lateral segregation of membrane components were applied to study membrane budding. Based on the results presented we conclude that the budding process might be driven by accumulation of anisotropic membrane components in the necks connecting the bud and the parent membrane, and by accumulation of isotropic (conical) membrane components on the bud. Both processes may strongly depend on the intrinsic shape of membrane components and on the direct interactions between them.     

Role of exercise-induced potassium fluxes underlying muscle fatigue: a brief review

The site of exercise-induced muscle fatigue is suggested to be the muscle membrane, which includes the sarcolemma and T-tubule membrane; the excitability of the membrane is dependent on the membrane potential. Significant potassium flux from the intracellular space of contracting muscle may decrease the membrane potential to half its resting value. This is true for isolated muscle preparations as well as for the whole body exercise in humans. Specific K+ channels have been identified, that may account for the intracellular K+ loss. Calcium-sensitive K+ channels open when intracellular Ca2+ concentrations increase, as during excitation. ATP-sensitive K+ channels may be involved but may open only at ATP concentrations well below those attained at exhaustion. However, ATP may be compartmentalized and only the membrane-bound ATP concentration may be of significance. Ca2+ accumulation and ATP depletion cause cell destruction; these changes induce an increased K+ conductance, which may inactivate the membrane and consequently prevent tension development. It is hypothesized that such a safety mechanism is identical to the fatigue mechanism.     

Membrane associations between subsurface cisternae of endoplasmic reticulum and the plasma membrane of rat Sertoli cells

Close membrane associations between the endoplasmic reticulum and the plasma membrane (ER-PM) occur in specialized regions of the rat Sertoli cell cytoplasm. They are characterized, in freeze fracture replicas, as mesa-like modifications of E membrane fracture faces or as corresponding discoid depressions on P membrane fracture faces. When these structures lie along transitional regions in the membrane fracture plane, they are seen to be complementary, and the space between them to be greatly reduced. These specialized close membrane associations may represent adhesive sites between the endoplasmic reticulum and the plasma membrane. However, their resemblance to vascular endothelial fenestrae which are known to be sites of increased membrane permeability may suggest other functional roles.     

A model for conformational coupling of membrane potential and proton translocation to ATP synthesis and to active transport.

Acceptance of a membrane potential and/or a proton gradient as a possible means of transmitting energy from oxidations to ATP synthesis rests in part on a satisfactory hypothesis for how the potential or proton gradient could drive ATP synthesis. Recognition that energy input may drive ATP synthesis by change in binding of reactants at the catalytic site has led to the suggestions presented in this paper. These are that in oxidative phosphorylation and photophosphorylation, the requisite conformational changes may be coupled to exposure of charged groups to different sides of the membrane. The cycle of charged group exposure or movement may be driven by the membrane potential or, through protonation and deprotonation, may be coupled to proton translocation across the membrane. Effects of proton gradient and membrane potential may be additive. Similar conformational coupling suggestions may explain proton translocation coupled to ATP cleavage and active transport of metabolites coupled to membrane potential, proton gradients of ATP cleavage.     

New aspects of the regulation of glycosphingolipid receptor function

We propose that the fatty acid heterogeneity of glycosphingolipids may compensate for the relative few and simple glycosphingolipid structures found in mammalian cells. Variation in GSL fatty acid composition may mediate aglycone regulation of GSL membrane receptor function by a differential interaction with cholesterol and other membrane components which may be differentially organized within plasma membrane lipid domains. These concepts are specifically illustrated in model membrane studies and in relation to the role of the glycolipid, globotriaosyl ceramide (Gb₃) in verotoxin-induced renal pathology and gp120 binding in HIV infection.     

Mechanism of spontaneous inside-out vesiculation of red cell membranes

In certain conditions, human red cell membranes spontaneously form inside out vesicles within 20 min after hypotonic lysis. Study of the geometry of this process now reveals that, contrary to earlier views of vesiculation by endocytosis or by the mechanical shearing of cytoskeleton-depleted membrane, lysis generates a persistent membrane edge which spontaneously curls, cuts, and splices the membrane surface to form single or concentric vesicles. Analysis of the processes by which proteins may stabilize a free membrane edge led us to formulate a novel zip-type mechanism for membrane cutting-splicing and fusion even in the absence of free edges. Such protein-led membrane fusion represents an alternative to mechanisms of membrane fusion based on phospholipid interactions, and may prove relevant to processes of secretion, endocytosis, phagocytosis, and membrane recycling in many cell types.     

Exocytotic fusion pore stability and topological defects in the membrane with orientational degree of ordering

Regulated exocytosis is a process that strongly depends on the formation and stability of the fusion pore. It was indicated experimentally and theoretically that narrow and highly curved fusion pore may be stabilized by accumulation of anisotropic membrane components possessing orientational ordering. On the other hand, narrow fusion pore may also undergo repetitive opening and closing, disruption in the so called kiss and run process or become completely opened in the process of full fusion of the vesicle with the membrane. In this paper we attempt to elucidate the subtle interplay between the stabilizing and destabilizing processes in the fusion neck. A possible physical mechanism which may lead to disruption of the stable fusion pore or complete fusion of the vesicle with the membrane is discussed. It is indicated that topologically driven defects of the in-plane orientational membrane ordering in the region of the fusion pore may disrupt the fusion. Alternatively, it may facilitate repetitive opening and closing of the fusion pore or induce full fusion of the vesicle with the target membrane.     

The presence of microvilli and other membrane fragments in the non-fat phase of bovine milk

Summary An electron microscopic study of a lipoprotein fraction isolated by ultracentrifugation from skim milk has shown that it is composed of membrane. It is believed that the majority of this membrane arises from shed microvilli. The microvilli may be derived from excess Golgi membrane which accumulates as plasma membrane during the secretion of milk proteins from the mammary cell. These findings indicate that milk may serve as a physiological source of easily isolated membrane. Authorized for publication on July 28, 1971 as paper no. 4025 in the Journal Series of the Pennsylvania Agricultural Experiment Station.     

Influence of the fluidity of the membrane on the response of microorganisms to environmental stresses

The aim of this mini-review is to relate membrane physical properties to the adaptation and resistance of microorganisms to environmental stresses. In the first part, the effects of various stresses on the structure and dynamic properties of phospholipid and biological membranes are presented. The compensation of these effects, i.e., change in membrane fluidity, phase transitions, by the active cellular control of the membrane chemical composition, is then described. In this natural process, the change in membrane fluidity is viewed as the detecting "input" signal that initiates the regulation, activating proteic effectors that in turn may influence the chemical composition of the membrane (feedback). This adaptation system allows the maintenance of the physical characteristics of membranes and, thereby, of their functionality. When environmental stresses are extreme and occur abruptly, the regulation process may not compensate for the changes in the membrane physical characteristics. In such cases, important variations in the membrane fluidity and structure may induce cellular damages and cell death. However, the lethal consequences are not systematically observed because protective effects of changes in the membrane physical state on the resistance to stresses are also reported.     

Isolation of membrane-bound renal kallikrein and kininase.

Fractions highly enriched in plasma membrane, endoplasmic reticulum or brush border were prepared from homogenized rat kidney cortex. Kallikrein was concentrated in the plasma-membrane fraction, but not in the brush border of the proximal tubules. Kininase II or angiotensin I-converting enzyme was localized in the brush-border membrane. It is suggested that kallikrein in the urine may originate from the plasma membrane of the distal tubules and the conversion of angiotensin I and the inactivation of bradykinin may occur on the lumen membrane of the proximal tubular cells.     

Effects of 2,3-diphosphoglyceric acid on the human erythrocyte membrane phosphorylation system

The effect of 2,3-diphosphoglycerate (2,3-P2-glycerate) on the phosphorylation of spectrin in solution by purified membrane cyclic AMP-independent protein kinase and in membrane preparations by the endogenous kinase was investigated. 2,3-P2-Glycerate inhibited spectrin phosphorylation both in solution and in the intact membrane. Kinetic analyses showed that 2,3-P2-glycerate had no effect on the Km for ATP but appeared to lower the Vmax of the reaction. When the effect of 2,3-P2-glycerate was examined in the presence of varying concentrations of spectrin, competitive inhibition kinetics were obtained. Interestingly, low concentrations of 2,3-P2-glycerate were found to effect the release of the membrane kinase from erythrocyte membranes. This release reaction may be related to the ability of 2,3-P2-glycerate to interfere with the interaction between the kinase and spectrin. The data suggest the possibility that the kinase may be bound to spectrin in the erythrocyte membrane. 2,3-P2-glycerate also caused the solubilization of 3-phosphoglyceraldehyde dehydrogenase, but not of cyclic AMP-dependent protein kinase. Taken together, our data indicate that 2,3-P2-glycerate may have a regulatory role in membrane protein phosphorylation and also may regulate the extent of association of the kinase with the membrane.     

表位标签技术在膜蛋白研究中的应用

膜蛋白的研究包括埘膜蛋白在细胞内的运输和定位,膜蛋白的结构和功能,以及膜蛋白和其他蛋白质间的相互作用等方面的研究.在研究过程中,如果能够基于膜蛋白的拓扑学结构预测,选择合适的表位标签,利用基因融合技术在基因水平上对膜蛋白进行改造,可以产生含有表位标签的重组膜蛋白,不仅具有原有膜蛋白的功能活性,还能够被抗体特异性识别,并且结合相关的免疫荧光检测技术,将会极大地促进膜蛋白的结构和功能研究.本文就目前膜蛋白研究中所涉及的表位标签技术及其应用策略和所取得的进展作一简述.     

综述——新型纳米生物材料的研发：医学纳米颗粒对类细胞膜作用的仿真研究进展

细胞膜是包围细胞质、维持细胞内部组分动态平衡的一个半透膜,参与细胞黏附、离子传导、信号传导等分子生物学过程．类细胞膜提供了有效的模型研究这些生物学过程,故而分子层面上研究医学纳米颗粒对类细胞膜的作用有助于评估纳米颗粒的生物安全性以及促进纳米颗粒的生物医学应用．本文初步探讨了医学纳米颗粒对类细胞膜作用的仿真研究进展,并在此基础上结合膜生物物理学的研究热点对后续的研究进行了展望．     

Structural determinants of purple membrane assembly

The purple membrane is a two-dimensional crystalline lattice formed by bacteriorhodopsin and lipid molecules in the cytoplasmic membrane of Halobacterium salinarum. High-resolution structural studies, in conjunction with detailed knowledge of the lipid composition, make the purple membrane one of the best models for elucidating the forces that are responsible for the assembly and stability of integral membrane protein complexes. In this review, recent mutational efforts to identify the structural features of bacteriorhodopsin that determine its assembly in the purple membrane are discussed in the context of structural, calorimetric and reconstitution studies. Quantitative evidence is presented that interactions between transmembrane helices of neighboring bacteriorhodopsin molecules contribute to purple membrane assembly. However, other specific interactions, particularly between bacteriorhodopsin and lipid molecules, may provide the major driving force for assembly. Elucidating the molecular basis of protein-protein and protein-lipid interactions in the purple membrane may provide insights into the formation of integral membrane protein complexes in other systems.     

Freeze-fracture images of exocytosis and endocytosis in anterior pituitary cells of rabbits and mice

Summary Freeze-fracture images of exocytosis and endocytosis were studied in various kinds of secretory cells of the anterior pituitary of mice and rabbits. Exocytotic figures are frequently observed in thin section of the anterior pituitary cells. In freeze-fracture images, small elevated membrane areas without membrane particles are often seen on the PF of the plasma membrane of the secretory cells. There is a secretory granule in the cytoplasm just beneath the particle-free membrane area, and limiting membrane of the granule is also devoid of the membrane particles at the part facing the plasma membrane. The fusion of membranes for exocytosis may occur at this particle-free area.The limiting membrane of the granule which is continuous with the plasma membrane is almost always coated after release of the granule core. This invagination of coated membrane may be an initiation site for the membrane retrieval after exocytosis. In freeze-fracture images, this depressed region with an accumulation of the membrane particles is observed on the PF of the plasma membrane. This particle-rich depressed region is thought to correspond to the coated area of the plasma membrane observed in thin section. It is thought that the membrane retrieval by pinocytosis initiates at the particle-rich depressed region of the plasma membrane.This study was supported by a grant from the Japan Ministry of Education     

Effects of trace elements on membrane fluidity.

According to the Fluid Mosaic Model, a biological membrane is a two-dimensional fluid of oriented proteins and lipids. The lipid bilayer is the basic structure of all cell and organelle membranes. Cell membranes are dynamic, fluid structures, and most of their molecules are able to move in the plane of the membrane. Fluidity is the quality of ease of movement and represents the reciprocal value of membrane viscosity. Fluid properties of biological membranes are essential for numerous cell functions. Even slight changes in membrane fluidity may cause aberrant function and pathological processes. Several evidences suggest that trace elements, e.g., iron, copper, zinc, selenium, chromium, cadmium, mercury and lead may influence membrane fluidity. The interaction of heavy metals with cellular membranes may contribute to explain, at least partially, the toxicity associated with these metals.     

The plasma membrane of young Chara internodal cells revealed by rapid freezing

Young elongating internodal cells of Chara globularis var. capillacea (Thuill.) Zanev. were rapidly frozen and freze-fractured in order to observed transient events occurring within the plasma membrane. Several structures have been observed. Relatively small depressions, varying in depth, are prolific and scattered at random over the plasma membrane. Charasomes and clusters of particle rosettes are common. Arrays of intramembrane particle lines are a characteristic feature of the internodal cell plasma membrane. The charasomes and the arrays of particle lines occupy a considerable proportion of the plasma membrane. In these young cells, substantial movement must take place across this membrane and its basic structure must fluctuate accordingly. The innumerable small depressions may represent pinocytotic and secretory processes. The array of intramembrane particle lines may represent stages in fusion between the membranes of vesicles within the cytoplasm and the plasma membrane. The technique of ultra-rapid freezing allows these events and their intermediate stages to be visualised; some features of the membrane may only be seen by this method.     

The interaction of lymphocyte membrane proteins with the lymphocyte cytoskeletal matrix

Lymphocyte membrane proteins are important in the transduction of signals across the plasma membrane. Visual and biophysical studies have shown that after ligand binding, membrane proteins may become immobile in the plane of the membrane and may cap. In intact cells, binding of cross-linking ligands to surface immunoglobulin converts it to a detergent-insoluble state (77% insoluble). This conversion is positively correlated with the transmission of a mitogenic signal. Class II histocompatibility proteins (Ia) and thy-1 remain predominantly detergent soluble (60 to 97% soluble). Insolubilized membrane proteins may be solubilized by incubating the detergent insoluble cytoskeletons with 0.34 M sucrose, 0.5 mM ATP, 0.5 mM dithiothreitol, 1 mM EDTA, or 3 X 10(-5) M DNAase I, 1 mM EDTA. To determine if the membrane-associated cytoskeleton contains the sufficient components for ligand-induced receptor insolubilization, experiments were done with a crude plasma membrane fraction. The results with whole cells or crude plasma membranes were comparable. These studies support the view that ligand-induced insolubilization of membrane proteins is due to their interaction with cytoskeletal structures.     

The human erythrocyte membrane skeleton may be an ionic gel. III. Micropipette aspiration of unswollen erythrocytes

We have carried out a theoretical analysis of micropipette aspiration of unswollen erythrocytes using the protein-gel-lipid-bilayer membrane model and taking into account that the modulus of area compression of the membrane skeleton may depend on the environmental conditions. Our analysis shows that the aspiration pressure needed to obtain a certain membrane projection length is strongly dependent on the ratio between the membrane skeleton modulus of area compression and the elastic shear modulus. Our analysis therefore predicts that micropipette aspiration of unswollen erythrocytes may be a sensitive method for detection of changes in this ratio. The analysis thus also shows that micropipette aspiration of unswollen erythrocytes can not be used to determine the membrane shear modulus unless something is known about the membrane skeleton modulus of area compression.     

Adsorptive immobilization of erythrocyte membrane

Immobilization of human erythrocyte membrane was carried out by adsorption on Fractosil, a porous form of silica. Acetylcholinesterase (AChE) was chosen as a representative membrane enzyme in this study. Dependency of adsorption on membrane concentration was determined. Positive cooperative interactions that occurred in the process of immobilization increased stability. Presence of hydrophobic ligands on derivatized Fractosil was found to enhance stability of immobilized preparations making them more effective for use in continuous catalytic transformations. It is suggested that adsorptive immobilization of membrane structures such as the human erythrocyte membrane fragments on Fractosil and other inexpensive supports may provide a convenient procedure for utilization of their catalytic potential. Such preparations may be used in diagnostic kits or for construction of biosensors.