Essential cell division protein FtsZ assembles into one monomer-thick ribbons under conditions resembling the crowded intracellular environment

Experimental conditions that simulate the crowded bacterial cytoplasmic environment have been used to study the assembly of the essential cell division protein FtsZ from Escherichia coli. In solutions containing a suitable concentration of physiological osmolytes, macromolecular crowding promotes the GTP-dependent assembly of FtsZ into dynamic two-dimensional polymers that disassemble upon GTP depletion. Atomic force microscopy reveals that these FtsZ polymers adopt the shape of ribbons that are one subunit thick. When compared with the FtsZ filaments observed in vitro in the absence of crowding, the ribbons show a lag in the GTPase activity and a decrease in the GTPase rate and in the rate of GTP exchange within the polymer. We propose that, in the crowded bacterial cytoplasm under assembly-promoting conditions, the FtsZ filaments tend to align forming dynamic ribbon polymers. In vivo these ribbons would fit into the Z-ring even in the absence of other interactions. Therefore, the presence of mechanisms to prevent the spontaneous assembly of the Z-ring in non-dividing cells must be invoked.

Rapid Transbilayer Movement of Ceramides in Phospholipid Vesicles and inHumanErythrocytes

The transbilayer diffusion of unlabeled ceramides with different acyl chains (C6-Cer, C10-Cer, and C16-Cer) was investigated in giant unilamellar vesicles (GUVs) and in human erythrocytes. Incorporation of a very small percentage of ceramides (approximately 0.1% of total lipids) to the external leaflet of egg phosphatidylcholine GUVs suffices to trigger a shape change from prolate to pear shape vesicle. By observing the reversibility of this shape change the transmembrane diffusion of lipids was inferred. We found a half-time for unlabeled ceramide flip-flop below 1 min at 37 degrees C. The rapid diffusion of ceramides in a phosphatidylcholine bilayer was confirmed by flip-flop experiments with a spin-labeled ceramide analogue incorporated into large unilamellar vesicles. Shape change experiments were also carried out with human erythrocytes to determine the trans-membrane diffusion of unlabeled ceramides into a biological membrane. Addition of exogenous ceramides to the external leaflet of human erythrocytes did not trigger echinocyte formation immediately as one would anticipate from an asymmetrical accumulation of new amphiphiles in the outer leaflet but only after approximately 15 min of incubation at 20 degrees C in the presence of an excess of ceramide. We interpret these data as being indicative of a rapid ceramide equilibration between both erythrocyte leaflets as indicated also by electron spin resonance spectroscopy with a spin-labeled ceramide. The late appearance of echinocytes could reveal a progressive trapping of a fraction of the ceramide molecules in the outer erythrocytes leaflet. Thus, we cannot exclude the trapping of ceramides into plasma membrane domains.

Strong FtsZ is with the force: mechanisms to constrict bacteria

FtsZ, the best-known prokaryotic division protein, assembles at midcell with other proteins forming a ring during septation. Widely conserved in bacteria, FtsZ represents the ancestor of tubulin. In the presence of GTP it forms polymers able to associate into multi-stranded flexible structures. FtsZ research is aimed at determining the role of the Z-ring in division, describing the polymerization and potential force-generating mechanisms and evaluating the roles of nucleotide exchange and hydrolysis. Systems to reconstruct the FtsZ ring in vitro have been described and some of its mechanical properties have been reproduced using in silico modeling. We discuss current research in FtsZ, some of the controversies, and finally propose further research needed to complete a model of FtsZ action that reconciles its in vitro properties with its role in division.

Visualization of single Escherichia coli FtsZ filament dynamics with atomic force microscopy

FtsZ, the prokaryotic homologue of tubulin, is an essential cell division protein. In the cell, it localizes at the center, forming a ring that constricts during division. In vitro, it binds and hydrolyzes GTP and polymerizes in a GTP-dependent manner. We have used atomic force microscopy to study the structure and dynamics of FtsZ polymer assembly on a mica surface under buffer solution. The polymers were highly dynamic and flexible, and they continuously rearranged over the surface. End-to-end joining of filaments and depolymerization from internal zones were observed, suggesting that fragmentation and reannealing may contribute significantly to the dynamics of FtsZ assembly. The shape evolution of the restructured polymers manifested a strong inherent tendency to curve. Polymers formed in the presence of non-hydrolyzable nucleotide analogues or in the presence of GDP and AlF(3) were structurally similar but showed a slower dynamic behavior. These results provide experimental evidence supporting the model of single-strand polymerization plus cyclization recently proposed to explain the hydrodynamic behavior of the polymers in solution.

Cooperative behavior of Escherichia coli cell-division protein FtsZ assembly involves the preferential cyclization of long single-stranded fibrils

A mechanism of noncooperative (isodesmic) assembly coupled with preferential cyclization of long polymers is proposed to explain the previously posed question of how a single-stranded filament of the bacterial cell-division protein FtsZ can assemble in an apparently cooperative manner. This proposal is based on results of GTP-mediated assembly of FtsZ from Escherichia coli that was studied under physiologically relevant steady-state solution conditions by a combination of methods including measurement of sedimentation velocity, atomic force and electron microscopy, and precipitation assays. Sedimentation-velocity experiments carried out at multiple protein concentrations reveal an essentially bimodal distribution of slowly sedimenting species and a relatively narrow distribution of rapidly sedimenting species that appears only above an apparent "critical concentration" of protein. In a precipitation assay, the amount of protein that pellets, which correlates with the fraction of rapidly sedimenting species observed in sedimentation-velocity experiments, increases linearly with the total concentration of protein in excess of the critical concentration. Sedimentation coefficients of the rapidly sedimenting fraction are qualitatively consistent with the presence of single-stranded cyclic oligomers with a size range of approximately 50-150 protomers, similar to polymeric single-stranded rings observed in atomic force and electron micrographs. The proposed model is in accord with the results obtained from our experimental observations.

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.

The bacteriophage phi29 head-tail connector imaged at high resolution with the atomic force microscope in buffer solution

The surfaces of two- and three-dimensional phi29 connector crystals were imaged in buffer solution by atomic force microscopy (AFM). Both topographies show a rectangular unit cell with dimensions of 16.5 nm x 16.5 nm. High resolution images of connectors from the two-dimensional crystal surface show two connectors per unit cell confirming the p42(1)2 symmetry. The height of the connector was estimated to be at least 7.6 nm, a value close to that found in previous studies using different techniques. The 12 subunits of the wide connector domain were clearly resolved and showed a right-handed vorticity. The channel running along the connector had a diameter of 3.7 nm in the wide domain, while it was 1.7 nm in the narrow domain end, thus suggesting a tronco-conical channel shape. Moreover, the narrow connector end appears to be rather flexible. When the force applied to the stylus was between 50 and 100 pN, the connector end was fully extended. At forces of approximately 150 pN, these ends were pushed towards the crystal surface. The complementation of the AFM data with the three-dimensional reconstruction obtained from electron microscopy not only confirmed the model proposed, but also offers new insights that may help to explain the role of the connector in DNA packing.

Effect of pulmonary surfactant protein SP-B on the micro- and nanostructure of phospholipid films

Monolayers of dipalmitoylphosphatidylcholine (DPPC) and DPPC/dipalmitoylphosphatidylglycerol (DPPG) (7:3, w/w) in the absence or in the presence of 2, 5, 10, or 20 weight percent of porcine surfactant protein SP-B were spread at the air-liquid interface of a surface balance, compressed up to surface pressures in the liquid-expanded/liquid-condensed (LE-LC) plateau of the isotherm, transferred onto mica supports, and analyzed by scanning force microscopy. In the absence of protein, the films showed micrometer-sized condensed domains with morphology and size that were analogous to those observed in situ at the air-liquid interface by epifluorescence microscopy. Scanning force microscopy permits examination of the coexisting phases at a higher resolution than previously achieved with fluorescent microscopy. Both LE and LC regions of DPPC films were heterogeneous in nature. LC microdomains contained numerous expanded-like islands whereas regions apparently liquid-expanded were covered by a condensed-like framework of interconnected nanodomains. Presence of increasing amounts of pulmonary surfactant protein SP-B affected the distribution of the LE and LC regions of DPPC and DPPC/DPPG films both at the microscopic and the nanoscopic level. The condensed microdomains became more numerous but their size decreased, resulting in an overall reduction of the amount of total LC phase in both DPPC and DPPC/DPPG films. At the nanoscopic level, SP-B also caused a marked reduction of the size of the condensed-like nanodomains in the LE phase and an increase in the length of the LE/LC interface. SP-B promotes a fine nanoscopic framework of lipid and lipid-protein nanodomains that is associated with a substantial mechanical resistance to film deformation and rupture as observed during film transference and manipulation. The effect of SP-B on the nanoscopic structure of the lipid films was greater in DPPC/DPPG than in pure DPPC films, indicating additional contributions of electrostatic lipid-protein interactions. The alterations of the nanoscopic structures of phospholipid films by SP-B provide the structural framework for the protein simultaneously sustaining structural stability as well as dynamical flexibility in surfactant films at the extreme conditions imposed by the respiratory mechanics. SP-B also formed segregated two-dimensional clusters that were associated with the boundaries between LC microdomains and the LE regions of DPPC and DPPC/DPPG films. The presence of these clusters at protein-to-lipid proportions above 2% by weight suggests that the concentration of SP-B in the surfactant lipid-protein complexes may be close to the solubility limit of the protein in the lipid films.

Predicting forage indigestible NDF from lignin concentration

We used chemical composition and in vitro digestibility data from temperate and tropical forages to develop relationships between indices of lignification and forage indigestible NDF. Neutral detergent fiber indigestibility increased nonlinearly as the lignin concentration of the NDF increased. Differences in estimated indigestible NDF using equations developed for a specific forage class (C3 and C4 grasses and legumes) were small and are probably not biologically significant when compared to those estimated from a common equation. Selected equations were compared with the Cornell Net Carbohydrate and Protein System (CNCPS) for the prediction of ADG. The linear equation (2.4 times NDF lignin content) used by the CNCPS and the Beef NRC had some of the largest errors due to mean bias. A log-log model [4.37 x (lignin/NDF)(.84)] provided the best combination of low total prediction error, low mean bias, and minimal error due to regression bias when permanganate lignin was used. A similar equation based on sulfuric acid lignin [6.17 x (lignin/NDF)(.77)] also met the above criteria. These equations then were evaluated with the CNCPS model against animal growth data from diets ranging in forage quality. Regardless of the equation used for predicting unavailable fiber, the CNCPS underpredicted daily gain, with mean biases ranging from -.10 to -.22 kg/d. Regression bias ranged from .13 to .14 kg/d and the coefficients differed from unity (P = .0001). The new equations gave numerically lower energy allowable ADG by steers compared to the linear equation currently used by the CNCPS model. The estimates were lower due to a higher predicted indigestible NDF, which resulted in a lower estimated forage energy value.

Surface tension induced by sphingomyelin to ceramide conversion in lipid membranes

We have investigated the effect of sphingomyelin (SM) to ceramide enzymatic conversion on lipid bilayers using Giant Unilamellar Vesicles (GUVs). Sphingomyelinase was added externally to GUVs containing various proportions of SM. In situ asymmetrical SM conversion to ceramide reduced the area of one leaflet. In the absence of equilibration of all the lipids between the two leaflets, a mismatch between the two monolayers was generated. The tension generated by this mismatch was sufficient to trigger the formation of membrane defects and total vesicle collapse at relatively low percentage of SM ( approximately 5% mol). The formation of nanometric size defects was visualised by AFM in supported bilayers. Vesicle rupture was prevented in two circumstances: (a) in GUVs containing a mixture of l(d) and l(o) domains and (b) in GUVs containing 5% lyso-phosphatidylcholine. In both cases, the accumulation of enough ceramide (at initial SM concentration of 10%) allowed the formation of ceramide-rich domains. The coupling between the two asymmetrical monolayers and the condensing effect produced by the newly formed ceramide generated a tension that could underlie the mechanism through which ceramide formation induces membrane modifications observed during the late stages of apoptosis.

Langevin computer simulations of bacterial protein filaments and the force-generating mechanism during cell division

FtsZ is a bacterial protein that forms filaments that play an essential role in midcell constriction during the process of cell division. The shape of individual filaments of different lengths imaged with atomic force microscopy was modeled considering the protein monomers as beads in a chain and a few parameters to represent their effective interactions. The flexural rigidity and persistence length of the filaments were estimated. This latter value was comparable to the filament length, implying that these biological polymers are halfway between the perfectly stiff linear aggregate whose shapes are fully controlled by the angle between the monomers and highly flexible polymers whose shapes follow a random walk model. The lateral interactions between adjacent filaments, also estimated in the modeling, were found to play an essential role in determining the final shape and kinetics of the coiled structures found in longer polymers. The estimated parameters were used to model the behavior of the polymers also on a cylindrical surface. This analysis points to the formation of helical structures that suggest a mechanism for force generation and amplification through the development of FtsZ spirals at the midcell division point.

Oriented immobilization of a membrane-bound hydrogenase onto an electrode for direct electron transfer

The interaction of redox enzymes with electrodes is of great interest for studying the catalytic mechanisms of redox enzymes and for bioelectronic applications. Efficient electron transport between the biocatalysts and the electrodes has achieved more success with soluble enzymes than with membrane enzymes because of the higher structural complexity and instability of the latter proteins. In this work, we report a strategy for immobilizing a membrane-bound enzyme onto gold electrodes with a controlled orientation in its fully active conformation. The immobilized redox enzyme is the Ni-Fe-Se hydrogenase from Desulfovibrio vulgaris Hildenborough, which catalyzes H(2)-oxidation reversibly and is associated with the cytoplasmic membrane by a lipidic tail. Gold surfaces modified with this enzyme and phospholipids have been studied by atomic force microscopy (AFM) and electrochemical methods. The combined study indicates that by a two-step immobilization procedure the hydrogenase can be inserted via its lipidic tail onto a phospholipidic bilayer formed over the gold surface, allowing only mediated electron transfer between the enzyme and electrode. However, a one-step immobilization procedure favors the formation of a hydrogenase monolayer over the gold surface with its lipidic tail inserted into a phospholipid bilayer formed on top of the hydrogenase molecules. This latter method has allowed for the first time efficient electron transfer between a membrane-bound enzyme in its native conformation and an electrode.

Becker's naevus: a comparative study between erbium: YAG and Q-switched neodymium:YAG; clinical and histopathological findings

BACKGROUND

Becker's naevus (BN) may represent a distressing cosmetic handicap and a challenging issue regarding treatment.

OBJECTIVES

To compare clinical and histopathological findings of patients with BN treated with two different lasers: an erbium:yttrium-aluminium-garnet (Er:YAG) system vs. a neodymium:YAG (Nd:YAG) laser.

PATIENTS AND METHODS

In this prospective and comparative study we present the clinical and histopathological evolution during a 2-year follow-up of 22 patients with BN treated with only one pass of the Er:YAG laser (n = 11) or three treatment sessions with the Q-switched Nd:YAG system (n = 11).

RESULTS

Clinical evaluation 2 years after treatment with the Er:YAG laser showed complete clearance (100%) in 54% of the patients (n = 6) and clearance of > 50% in 100% of the subjects. In relation to Nd:YAG laser treatments our results echo those of other authors. Numerous sessions are necessary to get an acceptable clinical clearance rate. Only one patient showed marked clearance (51-99%) after three treatment sessions. Moderate (26-50%) and mild (1-25%) clearance was observed in 45.5% (n = 5) and 27.3% (n = 3) of the patients.

CONCLUSIONS

Both Er:YAG and Nd:YAG are safe tools to treat BN. However, in terms of pigment removal, one pass with Er:YAG is a superior technique to three treatment sessions with the Nd:YAG.

Influence of a fluorescent probe on the nanostructure of phospholipid membranes: dipalmitoylphosphatidylcholine interfacial monolayers

Monolayers of dipalmitoylphosphatidylcholine (DPPC), both in the absence and in the presence of 1% (mol/mol) of a fluorescent phospholipid probe, have been spread at the air-liquid interface of a surface balance, compressed up to pressures in the liquid-expanded/liquid-condensed plateau of the isotherm, transferred onto mica supports, and analyzed by scanning force microscopy (SFM). Supported DPPC films showed micrometer-sized condensed domains with morphology and size that were entirely analogous to those observed in situ at the air-liquid interface by epifluorescence microscopy. The analysis by SFM, however, allowed the study and comparison of monolayers in the absence and in the presence of the fluorescent marker. This analysis revealed that the presence of dye reduced by 10-20% the total amount of the liquid-condensed phase in the DPPC films. The presence of the dye also decreased the mechanical stability of the film and increased the time required for the monolayer to equilibrate. The resolution of SFM permitted the determination that the structures of both the liquid-expanded and the liquid-condensed regions of DPPC films were heterogeneous at the nanometer scale. Liquid-condensed DPPC microdomains contained nanoholes covering 4-8% of their area whereas 60-80% of the surface detected as liquid-expanded by fluorescence microscopy consisted of a condensed-like framework of nanodomains. The total area, the shape of the nanodomains, and their interconnectivity were affected by the presence of the probe, suggesting that care must be taken when studying the structure, especially at the nanometer scale, and properties of model lipid films in the presence of extrinsic probes.

FtsZ polymers bound to lipid bilayers through ZipA form dynamic two dimensional networks

Bacteria divide by forming a contractile ring around their midcell region. FtsZ, a cytoskeletal soluble protein structurally related to tubulin, is the main component of this division machinery. It forms filaments that bundle at the inner side of the cytoplasmic membrane. These FtsZ bundles do not attach to bare lipid surfaces. In Escherichia coli they remain near the membrane surface by attaching to the membrane protein ZipA and FtsA. In order to study the structure and dynamics of the ZipA-FtsZ bundles formed on a lipid surface, we have oriented a soluble form of ZipA (sZipA), with its transmembrane domain substituted by a histidine tag, on supported lipid membranes. Atomic force microscopy has been used to visualize the polymers formed on top of this biomimetic surface. In the presence of GTP, when sZipA is present, FtsZ polymers restructure forming higher order structures. The lipid composition of the underlying membrane affects the aggregation kinetics and the shape of the structures formed. On the negatively charged E. coli lipid membranes, filaments condense from initially disperse material to form a network that is more dynamic and flexible than the one formed on phosphatidyl choline bilayers. These FtsZ-ZipA filament bundles are interconnected, retain their capacity to dynamically restructure, to fragment, to anneal and to condense laterally.

H2 -Fueled ATP Synthesis on an Electrode: Mimicking Cellular Respiration

ATP, the molecule used by living organisms to supply energy to many different metabolic processes, is synthesized mostly by the ATPase synthase using a proton or sodium gradient generated across a lipid membrane. We present evidence that a modified electrode surface integrating a NiFeSe hydrogenase and a F₁F₀‐ATPase in a lipid membrane can couple the electrochemical oxidation of H₂ to the synthesis of ATP. This electrode‐assisted conversion of H₂ gas into ATP could serve to generate this biochemical fuel locally when required in biomedical devices or enzymatic synthesis of valuable products.

Long pulse Nd:YAG laser for treatment of leg veins in 40 patients with assessments at 6 and 12 months

BACKGROUND AND OBJECTIVES

This study assessed subjectively and objectively the efficacy of a long-pulsed Nd:YAG laser system in clearing dermal leg veins, successful treatment of which remains problematic.

STUDY DESIGN/PATIENTS AND METHODS

Forty female patients (24-58 years old, skin types II-IV) with leg veins were treated with synchronized micropulses from a long-pulsed 1,064 nm Nd:YAG laser, 6 mm diameter spot size, 130 and 140 J/cm2. One to three treatments were given at 6-week intervals, with post-treatment assessments at 6 and 12 months. Patients assessed improvement subjectively with a satisfaction index (SI). Objective assessment was based on the clinical photography, and in addition on computer-generated data from a Canny operator-based edge-detection program.

RESULTS

The overall patient satisfaction rates and objective assessments at the 6 and 12 month assessments were 42.5 and 57.5%, and 75 and 82.5%, respectively.

CONCLUSIONS

The long-pulsed Nd:YAG laser offered efficient treatment of leg veins. Side effects were minimal and transient. The edge-detection program may help patients appreciate better the actual results of the treatment.

Simple modeling of FtsZ polymers on flat and curved surfaces: correlation with experimental in vitro observations

FtsZ is a GTPase that assembles at midcell into a dynamic ring that constricts the membrane to induce cell division in the majority of bacteria, in many archea and several organelles. In vitro, FtsZ polymerizes in a GTP-dependent manner forming a variety of filamentous flexible structures. Based on data derived from the measurement of the in vitro polymerization of Escherichia coli FtsZ cell division protein we have formulated a model in which the fine balance between curvature, flexibility and lateral interactions accounts for structural and dynamic properties of the FtsZ polymers observed with AFM. The experimental results have been used by the model to calibrate the interaction energies and the values obtained indicate that the filaments are very plastic. The extension of the model to explore filament behavior on a cylindrical surface has shown that the FtsZ condensates promoted by lateral interactions can easily form ring structures through minor modulations of either filament curvature or longitudinal bond energies. The condensation of short, monomer exchanging filaments into rings is shown to produce enough force to induce membrane deformations.PACS codes: 87.15.ak, 87.16.ka, 87.17.Ee.