Lipid and protein segregation in Escherichia coli membrane: morphological and structural study of different cytoplasmic membrane fractions

Low membrane fluidity triggers lipid phase separation and protein segregation in living bacteria

All living organisms adapt their membrane lipid composition in response to changes in their environment or diet. These conserved membrane-adaptive processes have been studied extensively. However, key concepts of membrane biology linked to regulation of lipid composition including homeoviscous adaptation maintaining stable levels of membrane fluidity, and gel-fluid phase separation resulting in domain formation, heavily rely upon in vitro studies with model membranes or lipid extracts. Using the bacterial model organisms Escherichia coli and Bacillus subtilis, we now show that inadequate in vivo membrane fluidity interferes with essential complex cellular processes including cytokinesis, envelope expansion, chromosome replication/segregation and maintenance of membrane potential. Furthermore, we demonstrate that very low membrane fluidity is indeed capable of triggering large-scale lipid phase separation and protein segregation in intact, protein-crowded membranes of living cells; a process that coincides with the minimal level of fluidity capable of supporting growth. Importantly, the in vivo lipid phase separation is not associated with a breakdown of the membrane diffusion barrier function, thus explaining why the phase separation process induced by low fluidity is biologically reversible.

Massive formation of intracellular membrane vesicles in Escherichia coli by a monotopic membrane-bound lipid glycosyltransferase

The morphology and curvature of biological bilayers are determined by the packing shapes and interactions of their participant molecules. Bacteria, except photosynthetic groups, usually lack intracellular membrane organelles. Strong overexpression in Escherichia coli of a foreign monotopic glycosyltransferase (named monoglycosyldiacylglycerol synthase), synthesizing a nonbilayer-prone glucolipid, induced massive formation of membrane vesicles in the cytoplasm. Vesicle assemblies were visualized in cytoplasmic zones by fluorescence microscopy. These have a very low buoyant density, substantially different from inner membranes, with a lipid content of > or = 60% (w/w). Cryo-transmission electron microscopy revealed cells to be filled with membrane vesicles of various sizes and shapes, which when released were mostly spherical (diameter approximately 100 nm). The protein repertoire was similar in vesicle and inner membranes and dominated by the glycosyltransferase. Membrane polar lipid composition was similar too, including the foreign glucolipid. A related glycosyltransferase and an inactive monoglycosyldiacylglycerol synthase mutant also yielded membrane vesicles, but without glucolipid synthesis, strongly indicating that vesiculation is induced by the protein itself. The high capacity for membrane vesicle formation seems inherent in the glycosyltransferase structure, and it depends on the following: (i) lateral expansion of the inner monolayer by interface binding of many molecules; (ii) membrane expansion through stimulation of phospholipid synthesis, by electrostatic binding and sequestration of anionic lipids; (iii) bilayer bending by the packing shape of excess nonbilayer-prone phospholipid or glucolipid; and (iv) potentially also the shape or penetration profile of the glycosyltransferase binding surface. These features seem to apply to several other proteins able to achieve an analogous membrane expansion.

Genetic basis of evolutionary adaptation by Escherichia coli to stressful cycles of freezing, thawing and growth

Microbial evolution experiments offer a powerful approach for coupling changes in complex phenotypes, including fitness and its components, with specific mutations. Here we investigate mutations substituted in 15 lines of Escherichia coli that evolved for 1000 generations under freeze-thaw-growth (FTG) conditions. To investigate the genetic basis of their improvements, we screened many of the lines for mutations involving insertion sequence (IS) elements and identified two genes where multiple lines had similar mutations. Three lines had IS150 insertions in cls, which encodes cardiolipin synthase, and 8 lines had IS150 insertions in the uspA-uspB intergenic region, encoding two universal stress proteins. Another line had an 11-bp deletion mutation in the cls gene. Strain reconstructions and competitions demonstrated that this deletion is beneficial under the FTG regime in its evolved genetic background. Further experiments showed that this cls mutation helps maintain membrane fluidity after freezing and thawing and improves freeze-thaw (FT) survival. Reconstruction of isogenic strains also showed that the IS150 insertions in uspA/B are beneficial under the FTG regime. The evolved insertions reduce uspB transcription and increase both FT survival and recovery, but the physiological mechanism for this fitness improvement remains unknown.

TP0453, a concealed outer membrane protein of Treponema pallidum, enhances membrane permeability

The outer membrane of Treponema pallidum, the non-cultivable agent of venereal syphilis, contains a paucity of protein(s) which has yet to be definitively identified. In contrast, the outer membranes of gram-negative bacteria contain abundant immunogenic membrane-spanning beta-barrel proteins mainly involved in nutrient transport. The absence of orthologs of gram-negative porins and outer membrane nutrient-specific transporters in the T. pallidum genome predicts that nutrient transport across the outer membrane must differ fundamentally in T. pallidum and gram-negative bacteria. Here we describe a T. pallidum outer membrane protein (TP0453) that, in contrast to all integral outer membrane proteins of known structure, lacks extensive beta-sheet structure and does not traverse the outer membrane to become surface exposed. TP0453 is a lipoprotein with an amphiphilic polypeptide containing multiple membrane-inserting, amphipathic alpha-helices. Insertion of the recombinant, non-lipidated protein into artificial membranes results in bilayer destabilization and enhanced permeability. Our findings lead us to hypothesize that TP0453 is a novel type of bacterial outer membrane protein which may render the T. pallidum outer membrane permeable to nutrients while remaining inaccessible to antibody.

Phenotypic changes in the fluidity of the tonoplast membrane of crassulacean-acid-metabolism plants in response to temperature and salinity stress

Electron paramagnetic resonance-spectroscopic studies on spin-labeled purified tonoplast membranes showed that in the obligate crassulacean-acid-metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. the fluidity of the tonoplast decreased during acclimation to higher temperatures. This phenotypic change in tonoplast fluidity was paralleled by a decrease in the mobilization of malic acid from the vacuoles during CAM in the light. The shift from the C3 to the CAM mode of photosynthesis in the facultative CAM plant Mesembryanthemum crystallinum L. also led to a decrease in the fluidity of the tonoplast membrane. The results are consistent with the hypothesis that the ability to store malic acid during CAM in the vacuoles depends largely on the actual fluidity of the tonoplast membrane.

Outer membrane ultrastructure explains the limited antigenicity of virulent Treponema pallidum

Freeze fracture and deep etching were used to investigate the ultrastructural basis for the observation that anti-treponemal antibodies bind poorly to the surface of virulent Treponema pallidum. Fractures of T. pallidum outer membranes contained scarce, uniformly sized intramembranous particles (IMPs). IMPs on the convex faces often appeared to form linear arrays that wound in spirals about the organism. In contrast to the outer membrane, IMPs of the cytoplasmic membrane were randomly distributed, numerous, and heterogeneous in size. In Escherichia coli and T. pallidum cofractures, IMPs of the E. coli outer membranes were densely packed within the concave fracture faces, while the T. pallidum fractures were identical to the experiments lacking the E. coli internal controls. Outer membranes of two representative nonpathogenic treponemes, Treponema phagedenis biotype Reiter and Treponema denticola, contained numerous IMPs, which segregated preferentially with the concave halves. Examination of apposed replicas and deep-etched specimens indicated that at least some of the IMPs extend through the T. pallidum outer membrane and are exposed on the surface of the organism. The outer membrane of intact T. pallidum appears to contain a paucity of integral membrane proteins that can serve as targets for specific antibodies. These findings appear to represent an unusual parasitic strategy for evasion of host humoral defenses.

Unequal distribution of penicillin-binding proteins among inner membrane vesicles of Escherichia coli

Escherichia coli penicillin-binding proteins (PBPs) were associated only with inner membrane vesicles when separated on 30 to 65% or 19 to 49% (wt/wt) sucrose gradients. Fractionation of vesicles through the low-density gradient revealed at least two classes of PBP-inner membrane associations. The first class consisted of PBPs 1 through 4, and the second class consisted of PBPs 5 through 8. These classes were distinguished by the density of vesicles with which they were associated; class 1 PBPs migrated with vesicles of higher density than did class 2 PBPs. Such combinations suggest that PBPs are nonrandomly distributed within the inner membrane, implying potential functional relationships among the PBPs themselves and with particular membrane domains. In addition, in cell lysates and in vesicle fractions, a 60,000-dalton aztreonam-insensitive PBP or protein fragment was observed which could potentially be confused with PBP3.

Identification, immunochemical characterization, and purification of a major lipoprotein antigen associated with the inner (cytoplasmic) membrane of Escherichia coli

A major antigenic constituent of the inner membrane of Escherichia coli ML308-225 was identified as a 28.5-kilodalton lipoprotein containing covalently bound glycerol and palmitate. This lipoprotein corresponded to antigen 47 in the crossed immunoelectrophoresis profile of membrane vesicles (P. Owen and H.R. Kaback, Proc. Natl. Acad. Sci. USA 75:3148-3152, 1978) and to new lipoprotein 4 described for E. coli B by Ichihara et al. (S. Ichihara, H. Hussain, and S. Mizushima, J. Biol. Chem. 256:3125-3129, 1980). Experiments involving isopycnic centrifugation of spheroplast envelopes indicated that antigen 47 was enriched in cytoplasmic membrane subfractions of low density. The protein did not manifest an obvious association with peptidoglycan of the types displayed by the bound form of the Braun (Lpp) lipoprotein, the 21-kilodalton peptidoglycan-associated lipoprotein, or the ompF/C gene products. Antibodies specific for antigen 47 were used to demonstrate that the molecule was immunologically distinct from both the Braun lipoprotein and the peptidoglycan-associated lipoprotein of E. coli. Antigens of similar molecular mass to and cross-reacting with antigen 47 were present in the envelopes of eight type species of the Enterobacteriaceae. A protocol for the purification of antigen 47, based upon its solubility in a chloroform-methanol-water mixture, was developed.

X-ray analysis of the kinetics of Escherichia coli lipid and membrane structural transitions

Synchrotron radiation was used to follow the time course of the transitions, induced by temperature jump, in Escherichia coli membranes and their lipid extracts isolated from a fatty acid auxotroph grown with different fatty acids. We measured the relaxation times associated with the phase transitions as well as with the conformational transition of the hydrocarbon chains and observed different behavior as a function of chemical composition. Relaxation times of about 1-2 s were found at a hexagonal to lamellar phase transition and within a lamellar phase whose parameters display important variations with temperature when the conformational transition takes place. On the other hand, no delay was observed for a phase transition where large lipid or water diffusion was not needed. We have shown that phase transitions and conformational transitions are, to a large extent, uncoupled and that the relaxation times corresponding to the latter transition could be related to the size of the ordered domains. In all cases, the order to disorder conformational transition is more rapid than the disorder to order transition. Finally, the relaxation times of the disorder to order transition observed with the membranes and with their lipid extracts were found to be strongly correlated, indicating that the proteins do not play a role in this transition.

Proteins of ribosome-bearing and free-membrane domains in Bacillus subtilis

In lysates of Bacillus subtilis a free-membrane fraction without ribosomes can be separated from the denser membrane-ribosome complexes. As determined by one-dimensional sodium dodecyl sulfate gel electrophoresis, these two fractions differ markedly in protein composition; at least six major bands (molecular weights, 130,000, 92,000, 68,000, 64,000, 45,000, and 31,000) are essentially unique to the complexed-membrane fraction (CM proteins), and two are unique to the free-membrane fraction. After growth was slowed, the proportion of the free-membrane fraction increased, but the composition of this fraction was the same, whereas after puromycin treatment, which abruptly increased the proportion of the free-membrane fraction, this fraction contained CM proteins. Thus, it appears that the two fractions recovered from growing cells represent topographically and functionally distinct domains. In addition, the effect of growth rate suggests that formation of the complexed domain is regulated at least roughly in parallel with the formation of ribosomes. The separation of these membrane fractions should facilitate the study of protein secretion, membrane topography, and morphogenesis in bacteria.

A biophysical study of protein-lipid interactions in membranes of Escherichia coli. Fluoromyristic acid as a probe

Fluorine-19 nuclear magentic resonance spectroscopy and transport assays have been used to investigate and compare the membrane properties of unsaturated fatty acid auxotrophs of two strains of Escherichia coli, K1060B5 and ML 308-225-UFA-8. A fluorinated analog of myristic acid, 8, 8-difluoromyristic acid, can be incorporated into the membrane phospholipids by substitution for oleate in the growth medium. Growth for one generation on 8, 8-difluoromyristate results in a 20% content of fluorinated fatty acid in the membranes, changes in the protein to lipid ratio, and altered transport of methyl beta-D-thiogalactopyranoside. The differences in membrane composition and transport behavior seen in oleate supplemented E. coli K1060B5 relative to ML 308-225-UFA-8 are enhanced by the incorporation of 8, 8-difluoromyristate. The phase transition behavior becomes distinctly different and some differences in lipid organization persist above the transition temperature. Concomitantly, the rate and extent of concentration of methyl beta-D-thiogalactopyranoside are reduced two-fold more in E. coli K1060B5 compared to ML 308-225-UFA-8. Such behavior suggests that these fluorinated fatty acid supplemented strains of E. coli are useful to study subtle differences in protein-lipid interactions and their effects on the function of membrane-bound enzymes.

Physical principles of membrane organization

Membranes are the most common cellular structures in both plants and animals. They are now recognized as being involved in almost all aspects of cellular activity ranging from motility and food entrapment in simple unicellular organisms, to energy transduction, immunorecognition, nerve conduction and biosynthesis in plants and higher organisms. This functional diversity is reflected in the wide variety of lipids and particularly of proteins that compose different membranes. An understanding of the physical principles that govern the molecular organization of membranes is essential for an understanding of their physiological roles sincestructureandfunctionare much more interdependent in membranes than in, say, simple chemical reactions in solution. We must recognize, however, that the word ‘understanding’ means different things in different disciplines, and nowhere is this more apparent than in this multidisciplinary area where biology, chemistry and physics meet.

Freeze-fracture evidence for the presence of cholesterol in particle-free patches of basal disks and the plasma membrane of retinal rod outer segments of mice and frogs

The freeze-fracture technique was used to examine the membranes of the photoreceptors of mice and frogs. Particle-free patches were found in the plasma membrane and basal disk membranes of the outer segments of both mice and frogs housed at room temperature, but not in frogs kept in a cold room. These patches were shown not to be artifacts of cryoprotection or fixation, and they persisted when fresh isolated outer segments were frozen by an ultrarapid method. They were also found to persist in mouse rods when retinas were incubated and subsequently fixed at temperatures up to 80 degrees C. Cholesterol was implicated as a significant component of the patches by the observation that, in the outer segments, pits, induced by treatment with the sterol-specific polyene antibiotic filipin, were present in and confined to the particle-free patches. That these lesions are not inherently limited to particle-free membrane areas was evident in the apical plasma membrane of the photoreceptor inner segments, where particles and pits were intermixed. Treatment with saponin, a surface-active agent which specifically complexes cholesterol, resulted in the disappearance of the particle-free patches. Patches were found in basal disks of both mouse and frog rods but not in older disks nearer the pigment epithelium, which indicates that changes occur in the composition of disk membranes and/or in the molecular ordering of their protein and lipid components during the early phase of their transit from the base towards the apex of the outer segment.

Functional lac carrier protein in cytoplasmic membrane vesicles isolated from Escherichia coli: temperature and pH dependence of dansyl-galactoside binding

6'-(N-Dansyl)aminohexyl-1-thio-beta-D-galactopyranoside binds specifically to the lac carrier protein in cytoplasmic membrane vesicles isolated from Escherichia coli. Binding can be induced by substrate oxidation (generation of an electrochemical gradient of protons), by potassium efflux in the presence of valinomycin (generation of a potassium diffusion potential), and by passive, carrier-mediated lactose efflux. We show that in all three cases the number of binding sites is temperature dependent. Binding is maximal and constant above 20 degrees ; it decreases between 20 degrees and 10 degrees . Oxidation of substrate (D-lactate) leads to the development of an electrochemical gradient of protons across the membrane (interior negative and alkaline), which is composed of interconvertible electrical and chemical gradients. We show that both the electrical potential across the membrane and the chemical difference in proton concentrations across the membrane are independent of temperature between 5 degrees and 25 degrees . We show that the number of binding sites induced by D-lactate oxidation depends on pH. At both 25 degrees and 5 degrees , the number of binding sites increases from pH 5 to pH 6.5, remains constant between pH 6.5 and 7, and decreases from pH 7 to pH 8. In contrast, the number of binding sites induced by passive, carrier-mediated lactose efflux is independent of pH between pH 5.5 and pH 8. From these findings, we conclude that the pH- and temperature-dependent effects on the number of 6'-(N-dansyl)aminohexyl-1-beta-thio-D-galactopyranoside binding sites have different origins. The pH dependence of binding is energy linked and reflects in part the pH dependence of the electrochemical gradient of protons across the membrane generated by substrate oxidation. The temperature dependence is not an energy-linked phenomenon. The decrease of the number of binding sites at low temperature probably reflects the aggregation of the lac carrier protein with other membrane proteins. This aggregation takes place as a consequence of the conformational disorder-to-order transition of the membrane lipids and the concomitant preferential segregation of the lac carrier protein in the membrane domains containing the disordered lipids.

Involvement of membrane lipids in radiation damage to potassium ion permeability of Escherichia coli

Radiation damage to K+ permeability of an unsaturated fatty acid auxotroph of E. coli grown with oleate or linolenate was investigated at different temperatures. A remarkable effect of radiation was observed at 0 degrees C with cells that had been grown with linolenate at 42 degrees C. This indicates that, besides protein, membrane lipids at least are involved in the radiation damage. The damage also seems to be affected by the fluidity of membrane lipids.