Formation of membranes by repeating units

Modular assembly of yeast mitochondrial ATP synthase and cytochrome oxidase

The respiratory pathway of mitochondria is composed of four electron transfer complexes and the ATP synthase. In this article, we review evidence from studies of Saccharomyces cerevisiae that both ATP synthase and cytochrome oxidase (COX) are assembled from independent modules that correspond to structurally and functionally identifiable components of each complex. Biogenesis of the respiratory chain requires a coordinate and balanced expression of gene products that become partner subunits of the same complex, but are encoded in the two physically separated genomes. Current evidence indicates that synthesis of two key mitochondrial encoded subunits of ATP synthase is regulated by the F1 module. Expression of COX1 that codes for a subunit of the COX catalytic core is also regulated by a mechanism that restricts synthesis of this subunit to the availability of a nuclear-encoded translational activator. The respiratory chain must maintain a fixed stoichiometry of the component enzyme complexes during cell growth. We propose that high-molecular-weight complexes composed of Cox6, a subunit of COX, and of the Atp9 subunit of ATP synthase play a key role in establishing the ratio of the two complexes during their assembly.

Update of the 1972 Singer-Nicolson Fluid-Mosaic Model of Membrane Structure

The Fluid-Mosaic Membrane Model of cell membrane structure was based on thermodynamic principals and the available data on component lateral mobility within the membrane plane [Singer SJ, Nicolson GL. The Fluid Mosaic Model of the structure of cell membranes. Science 1972; 175: 720-731]. After more than forty years the model remains relevant for describing the basic nano-scale structures of a variety of biological membranes. More recent information, however, has shown the importance of specialized membrane domains, such as lipid rafts and protein complexes, in describing the macrostructure and dynamics of biological membranes. In addition, membrane-associated cytoskeletal structures and extracellular matrix also play roles in limiting the mobility and range of motion of membrane components and add new layers of complexity and hierarchy to the original model. An updated Fluid-Mosaic Membrane Model is described, where more emphasis has been placed on the mosaic nature of cellular membranes where protein and lipid components are more crowded and limited in their movements in the membrane plane by lipid-lipid, protein-protein and lipid-protein interactions as well as cell-matrix, cell-cell and cytoskeletal interactions. These interactions are important in restraining membrane components and maintaining the unique mosaic organization of cell membranes into functional, dynamic domains.

Crystallization of intrinsic membrane proteins

After many years of discouraging failures, it is now possible to crystallize the intrinsic membrane proteins and to obtain structural information from diffraction studies on the crystals. The strategy for the crystallization consists of depletion of boundary phospholipids from the protein and complex formation with specific ligands.

Crystallization of cytochrome bc1 complex

Complex III (cytochrome bc1 particle; ubiquinol:ferricytochrome c oxidoreductase, EC 1.10.2.2) was purified from beef heart mitochondria by a combination of hydrophobic interaction and affinity chromatography. By washing the complex with detergent on the hydrophobic interaction column, phospholipids were effectively depleted; 7 mol of cardiolipin per mol of cytochrome c1 was retained in the final sample. NaDodSO4 gel electrophoresis showed nine polypeptide subunits in the sample. The molecular weight of the complex was estimated to be approximately equal to 225,000 from the specific heme c1 content and the subunit composition. The purified complex was crystallized by slow removal of the detergent in which the complex was dispersed. Electron micrographs and electron diffraction patterns showed that the crystal is hexagonal with unit cell dimensions a = b = 113 A, c = 132 A, and with angles alpha = beta = 90 degrees, gamma = 120 degrees. The role of bound cardiolipin in the structural integrity of the complex was discussed.

[Role of lipids in the function of microsomal electron transport chains of potato]

Microsomal membranes from potato tubers were extracted by acetone solutions of increasing concentrations (5, 10, 15, 20, 30, 40, 50, 70 and 90 p. cent). Microsomal lipids were progressively extracted: acetone concentrations exceeding 30 p. cent extracted large amounts of membraneous phospholipids (figure). Lipid extraction reduced NADH-cytochrome c reductase activity but did not affect NADH-ferricyanide reductase and NADPH-cytochrome c reductase activities. This was confirmed by experiments using increasing concentrations of sodium deoxycholate. After lipid extraction with acetone (or solubilization by triton X100), NADH-cytochrome c reductase activity of microsomal membranes could not be recovered by adding back lipids under various experimental conditions. These results strongly suggest that, in potato microsomes, lipids are undispensable components of the electron transport chain starting from NADH especially in the portion involving cytochrome b5. On the contrary, the second microsomal electron transport chain, starting from NADPH, is not regulated by lipids. However, plant microsomal membranes would be much more disturbed by liped extraction than animal microsomes and suitable relipidation conditions remain to be found to prove definitely the lipid dependence of plant microsomal electron transport.

Membrane reconstitution in chl-r mutants of Escherichia coli K 12. IX. Part played by phospholipids in the complementation process

The supernatant extracts of the chl A and chl B mutants of Escherichia coli K 12, the phospholipids of which are labeled by growth in 32 P or [2- 3H]glycerol media, contain 20 times more radioactivity than the supernatant extract of the wild-type strain grown under the same conditions. We have observed that, after complementation, 80% of the radioactivity previously contained by Extracts A and B is incorporated into reconstituted particles. The chromatography of 3H-labeled Extract B on DEAE-cellulose and followed by gel filtration of radioactive fractions on Sephadex G-200 has shown that the phospholipids of Extract B are only bound to soluble proteins and not to fragments of membranes; it can be assumed that they have been solubilized in the form of a lipid-protein complex by cell breakage. When Extracts A and B are treated by phospholipase C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3) before being mixed together, an inhibition of the reconstitution of nitrate reductase activity which is proportional to the phospholipase C concentration and the length of treatment is observed. The analysis of lipids and phospholipids of particles (Peak I, Peak II and Peak III) formed during complementation and reconstituted nitrate reductase shows that their phospholipid contents (phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol and phosphatidylserine) and especially that of Peak II (d equals 1.18) are closely related to that of native particles from the wild-type strain. These results allow one to propose a hypothesis explaining the mechanism involved in complementation.

Exposure of an arginine-rich protein at surface of cells in S, G2, and M phases of the cell cycle

Phenylglyoxal (PG) is shown to be a cell surface probe specific for arginine moieties in protein: (1) It does not enter the cell as evidenced by lack of PG in the cytoplasm. (2) It does not cause excessive cell leakage as measured by release of 51Cr. (3) It reacts with positively-charged groups in proteins at the cell surface but not with those of phospholipids at the surface; since pronase removes PG from the surface, but phospholipase C does not. (4) Under the conditions used in these experiments, it reacts virtually exclusively with arginine moieties in protein (Freedman et al., '68; Takahashi, '68; Werber and Sokolovsky, '72). Synchronized cells were exposed to radioactive PG to assess quantity of arginine moieties in protein at the surface. There is a sharp decrease in arginine at the cell surface at entry into G1 phase from M and a 24-fold increase upon entry into S phase. There is a slight drop in exposed arginine in late S phase followed by an increase to 26 times the G1 level immediately prior to mitosis. Lactoperoxidase-catalyzed iodination of tyrosine moieties in protein at the surface of synchronized cells shows a very gradual increase in protein as the cells move through the cycle and increase in size. Since the increase in arginine moieties in protein at the surface does not reflect a similar increase in total protein at the surface, an arginine-rich protein appears to be exposed at the cell surface during the division-related phases of the cell cycle.

Lipid-polypeptide interactions in bilayer lipid membranes

The modifications of the electrical properties of bilayer lipid membranes (BLM) composed of cholesterol and an ionic surfactant upon interaction with charged polypeptides were studied. The addition of 10(-8) M polylysine (Ps(+)) to one side of anionic cholesterol dodecylphosphate BLM increases the specific membrane conductance over 1000-fold (from 10(-8) to 10(-5) mho/cm(2)) and develops a cationic transmembrane potential larger than 50 mV. This potential is reverted by addition of polyanions such as RNA, polyglutamic or polyadenilic acid to the same side on which Ps(+) is present, by addition of Ps(+) to the opposite side, or by addition of trypsin to either side. Both conductance and potential changes are hindered by increasing the ionic strength or by raising the pH of the bathing medium, disappearing above pH 11.5 where it is known that Ps(+) folds into an α-helix. The interaction of polyglutamic acid (PGA) with a cationic cholesterol-hexadecyltrimethylammonium bromide BLM results in increased membrane conductance and development of an anionic transmembrane potential which is reverted by addition of polycations to the same aqueous phase where PGA is present. Addition of either Ps(+) or PGA to one or both sides of a neutral BLM composed of 7-dehydrocholesterol induces no significant change. The observations suggest the formation of a lipid polymer membrane resultant from the interaction, predominantly electrostatic, of the isolated components. The implications of these results are discussed in terms of the current models of membrane structure.

Physical aggregation and functional reconstitution of solubilized membranes of Bacillus stearothermophilus

Isolated membranes of Bacillus stearothermophilus 2184D can be disrupted by treatment with sodium dodecyl sulfate (SDS). This disruption is attended by a decreased turbidity of membrane suspensions and a differential loss of activities of the electron transport system. Reduced methyl viologen (MVH)-nitrate reductase activity is insensitive to SDS treatment, whereas reduced nicotinamide adenine dinucleotide (NADH)-nitrate reductase and cyanide-sensitive NADH oxidase activities are decreased by 80% at an SDS concentration of 0.5 mg/mg of membrane protein. NADH-menadione reductase activity is unaffected at this SDS concentration, but at higher detergent levels it also decreases in activity. The abilities of NADH to reduce and nitrate to oxidize the cytochrome components of the membrane were also decreased after SDS treatment. Dilution of solubilized membrane in buffer containing divalent cation results in formation of an aggregate with an increased turbidity and reconstituted NADH-nitrate reductase and cyanide-sensitive NADH oxidase activities. Of several cations tested, magnesium was the most effective, and the reconstitution process was pH-dependent with an optimum at pH 7.4. Intact and aggregated membranes had similar densities and cytochrome contents, and the sensitivity of NADH-nitrate reductase to several inhibitors was similar in intact and reconstituted membranes.

Paracrystalline sheets reaggregated from solubilized exosporium of Bacillus cereus

Fragments of exosporium, isolated from dormant spores of Bacillus cereus, were disintegrated by treatment with sodium dodecyl sulfate (SDS) or with phenol and acetic acid. After centrifugation of each preparation, proteins in the supernatant fractions were resolved by disc gel electrophoresis into either two or eight bands, respectively. The SDS-solubilized fraction contained spheroidal particles 11 to 44 nm in diameter. When centrifuged until clear, this fraction after dialysis still gave rise to crystal-like sheets which had the same lattice symmetry and major chemical components (protein, lipid, and carbohydrate) as fragments of the native exosporium.

Binding of dodecyl sulfate to proteins at high binding ratios. Possible implications for the state of proteins in biological membranes

A wide variety of proteins have been shown to bind identical amounts of an amphiphile, sodium dodecyl sulfate, on a gram per gram basis at monomer equilibrium concentrations above 0.5 mM. The binding is independent of ionic strength and primarily hydrophobic in nature. Only the monomeric form of the amphiphile binds to proteins, not the micellar form. The application of these results to models for biological membranes and to gel electrophoresis in sodium dodecyl sulfate is discussed.