Search

It is widely acknowledged that quantum entities with minimal mass cannot undergo spontaneous symmetry breaking due to strong quantum fluctuations. Here, we report the discovery of a positionally settled single electric dipole that can be manipulated and electrically polarized in a monolayer CoCl 2 -graphite heterostructure, which demonstrates an unprecedented example of spontaneous lattice-translational-symmetry breaking. Scanning tunneling microscopy and atomic force microscopy show that the solitons are intrinsic paraelectric dipoles driven by synchronous charge-lattice distortion around individual CoCl 6 octahedrons. Both the dipole moment and lateral position of the soliton can be manipulated by the electric field exerted from the tip, which offers polarity-switchable and layout-designable electrostatic potential landscapes that determine the band bending configuration. This study exemplifies a brand-new type of local charge-lattice order, appealing for further research on the mechanism underlying the soliton robustness, and the electrically and positionally controllable single dipole supports the feasibility of band tailoring in device applications.

The K2 Summit camera was initially the only commercially available direct electron detection camera that was optimized for high-speed counting of primary electrons and was also the only one that implemented centroiding so that the resolution of the camera can be extended beyond the Nyquist limit set by the physical pixel size. In this study, we used well-characterized two-dimensional crystals of the membrane protein aquaporin-0 to characterize the performance of the camera below and beyond the physical Nyquist limit and to measure the influence of electron dose rate on image amplitudes and phases.

The results of theoretical modeling from the first principles of atomic structure and properties of promising low-dimensional structures from semiconductors, performed for the past five years at the Center of Nanoelectronics and Novel Materials of Belarusian state university of informatics and radioelectronics, are summarized. The discovered principal new properties of two-dimensional structures from dichalcogenides of refractory metals and semiconductor silicides, one-dimensional structures of silicon, А3В5 semiconductors and semiconductor metal oxides, and zero-dimensional structures of carbon - nanodiamonds - are presented.

Two-dimensional (2D) crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions. To achieve their superior properties, it is of critical importance to fully understand the growth processes and mechanisms for tailored design and controlled growth of 2D crystals. Due to the high spatiotemporal resolution and the capability to mimic the realistic growth conditions, in situ transmission electron microscopy (TEM) becomes an effective way to monitor the growth process in real-time at the atomic scale, which is expected to provide atomic-scale insights into the nucleation and growth of 2D crystals. Here we review the recent in situ TEM works on the formation of 2D crystals under electron irradiation, thermal excitation as well as voltage bias. The underlying mechanisms are also elucidated in detail, providing key insights into the nucleation and formation of 2D crystals. [ABSTRACT FROM AUTHOR]

Optical encryption is receiving much attention with the rapid growth of information technology. Conventional optical encryption usually relies on specific configurations, such as metasurface-based holograms and structure colors, not meeting the requirements of increasing dynamic and programmable encryption. Here, we report a programmable optical encryption approach using WS 2 /SiO 2 /Au metal-oxide-semiconductor (MOS) devices, which is based on the electrical-field-controlled exciton-trion transitions in monolayer WS 2 . The modulation depth of the MOS device reflection amplitude up to 25% related to the excitons ensures the fidelity of information, and the decryption based on the near excitonic resonance assures security. With such devices, we successfully demonstrate their applications in real-time encryption of ASCII codes and visual images. For the latter, it can be implemented at the pixel level. The strategy shows significant potential for low-cost, low-energy-consumption, easily integrated, and high-security programmable optical encryptions.

Crystalline monolayers prevalent in nature and technology possess elusive elastic properties with important implications in fundamental physics, biology, and nanotechnology. Leveraging the recently discovered shape transitions of oil-in-water emulsion droplets, upon which these droplets adopt cylindrical shapes and elongate, we investigate the elastic characteristics of the crystalline monolayers covering their interfaces. To unravel the conditions governing Euler buckling and Brazier kink formation in these cylindrical tubular interfacial crystals, we strain the elongating cylindrical droplets within confining microfluidic wells. Our experiments unveil a nonclassical relation between the Young's modulus and the bending modulus of these crystals. Intriguingly, this relation varies with the radius of the cylindrical crystal, presenting a nonclassical mechanism for tuning of elasticity in nanotechnology applications.

Manipulating the valley degree of freedom as an information carrier has been a focused topic for both fundamental and applied research. Here, using first-principles calculations, we report the identification of monolayer CrX _2 (X = S, Se) as a novel two-dimensional valleytronic crystal. It shows large valley spin splitting in the valence band, attractive for the integration of valleytronics and spintronics. More importantly, through proximity coupling with monolayer CrCl _3 , the valley polarization in monolayer CrX _2 is achieved, which can be further engineered by stacking patterns. Also, the valley polarization in monolayer CrX _2 can be obtained via magnetically doping V and Mn. Specially for V-doped monolayer CrSe _2 , there are no impurity states in the band gap, beneficial for its practical applications. Our works thus provide not only exceptional two-dimensional valleytronic crystals but also promising ways for realizing valley polarizations in them.

A model for ab initio calculation of the phonon properties of three-component solid solutions of refractory-metal dichalcogenides was developed based on the assumption that displacements of the same type of chalcogen atoms and decoupled displacements of the metal atoms were identical. The calculated phonon frequencies at the Γ-point for monomolecular layers of MoSSe and MoSTe agreed with existing experimental Raman spectra. [ABSTRACT FROM AUTHOR]

The phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) is a structurally and functionally complex system that mediates sugar uptake in bacteria. Besides several soluble subunits, the glucose-specific PTS includes the integral membrane protein IICB that couples the transmembrane transport of glucose to its phosphorylation. Here, we used electron crystallography of sugar-embedded tubular crystals of the glucose-specific IIC transport domain from Escherichia coli (ecIIC glc ) to visualize the structure of the transporter in the presence and absence of its substrate. Using an in vivo transport assay and binding competition experiments, we first established that, while it transports d -glucose, ecIIC glc does not bind l -glucose. We then determined the projection structure of ecIIC glc from tubular crystals embedded in d - and l -glucose and found a subtle conformational change. From comparison of the ecIIC glc projection maps with crystal structures of other IIC transporters, we can deduce that the transporter adopts an inward-facing conformation, and that the maps in the presence and absence of the substrate reflect the transporter before and after release of the transported glucose into the cytoplasm. The transition associated with substrate release appears to require a subtle structural rearrangement in the region that includes hairpin 1. [ABSTRACT FROM AUTHOR]

The results from calculation of the energy spectra and densities of states of phonons in bulk and two-dimensional crystals of monomolecular thickness from molybdenum disulfide and ditelluride are presented. They demonstrate the dominant role of the interaction of atoms within a single monomolecular layer in the phonon characteristics of these materials. [ABSTRACT FROM AUTHOR]

Strongly bound excitons confined in two-dimensional (2D) semiconductors are dipoles with a perfect in-plane orientation. In a vertical stack of semiconducting 2D crystals, such in-plane excitonic dipoles are expected to efficiently couple across van der Waals gap due to strong interlayer Coulomb interaction and exchange their energy. However, previous studies on heterobilayers of group 6 transition metal dichalcogenides (TMDs) found that the exciton decay dynamics is dominated by interlayer charge transfer (CT) processes. Here, we report an experimental observation of fast interlayer energy transfer (ET) in MoSe2/WS2 heterostructures using photoluminescence excitation (PLE) spectroscopy. The temperature dependence of the transfer rates suggests that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. The estimated ET time of the order of 1 ps is among the fastest compared to those reported for other nanostructure hybrid systems such as carbon nanotube bundles. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering. [ABSTRACT FROM AUTHOR]

Mitochondrial cytochrome c oxidase utilizes electrons provided by cytochrome c for the active vectorial transport of protons across the inner mitochondrial membrane through the reduction of molecular oxygen to water. Direct structural evidence on the transient cytochrome c oxidase-cytochrome c complex thus far, however, remains elusive and its physiological relevant oligomeric form is unclear. Here, we report on the 2D crystallization of monomeric bovine cytochrome c oxidase with tightly bound cytochrome c at a molar ratio of 1:1 in reconstituted lipid membranes at the basic pH of 8.5 and low ionic strength. [ABSTRACT FROM AUTHOR]

Innexins are invertebrate-specific gap junction proteins with four transmembrane helices. These proteins oligomerize to constitute intercellular channels that allow for the passage of small signaling molecules associated with neural and muscular electrical activity. In contrast to the large number of structural and functional studies of connexin gap junction channels, few structural studies of recombinant innexin channels are reported. Here we show the three-dimensional structure of two-dimensionally crystallized Caenorhabditis elegans innexin-6 (INX-6) gap junction channels. The N-terminal deleted INX-6 proteins are crystallized in lipid bilayers. The three-dimensional reconstruction determined by cryo-electron crystallography reveals that a single INX-6 gap junction channel comprises 16 subunits, a hexadecamer, in contrast to chordate connexin channels, which comprise 12 subunits. The channel pore diameters at the cytoplasmic entrance and extracellular gap region are larger than those of connexin26. Two bulb densities are observed in each hemichannel, one in the pore and the other at the cytoplasmic side of the hemichannel in the channel pore pathway. These findings imply a structural diversity of gap junction channels among multicellular organisms. [ABSTRACT FROM AUTHOR]

To date, a number of two-dimensional (2D) topological insulators (TIs) have been realized in Group 14 elemental honeycomb lattices, but all are inversionsymmetric. Here, based on first-principles calculations, we predict a new family of 2D inversion-asymmetric TIs with sizeable bulk gaps from 105 meV to 284 meV, in X2-GeSn (X = H, F, Cl, Br, I) monolayers, making them in principle suitable for room-temperature applications. The nontrivial topological characteristics of inverted band orders are identified in pristine X2-GeSn with X = (F, Cl, Br, I), whereas H2-GeSn undergoes a nontrivial band inversion at 8% lattice expansion. Topologically protected edge states are identified in X2-GeSn with X = (F, Cl, Br, I), as well as in strained H2-GeSn. More importantly, the edges of these systems, which exhibit single-Dirac-cone characteristics located exactly in the middle of their bulk band gaps, are ideal for dissipationless transport. Thus, Group 14 elemental honeycomb lattices provide a fascinating playground for the manipulation of quantum states. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

The bacterial phosphoenolpyruvate: sugar phosphotransferase system serves the combined uptake and phosphorylation of carbohydrates. This structurally and functionally complex system is composed of several conserved functional units that, through a cascade of phosphorylated intermediates, catalyze the transfer of the phosphate moiety from phosphoenolpyruvate to the substrate, which is bound to the integral membrane domain IIC. The wild-type glucose-specific IIC domain (wt-IIC glc ) of Escherichia coli was cloned, overexpressed and purified for biochemical and functional characterization. Size-exclusion chromatography and scintillation-proximity binding assays showed that purified wt-IIC glc was homogenous and able to bind glucose. Crystallization was pursued following two different approaches: (i) reconstitution of wt-IIC glc into a lipid bilayer by detergent removal through dialysis, which yielded tubular 2D crystals, and (ii) vapor-diffusion crystallization of detergent-solubilized wt-IIC glc , which yielded rhombohedral 3D crystals. Analysis of the 2D crystals by cryo-electron microscopy and the 3D crystals by X-ray diffraction indicated resolutions of better than 6 Å and 4 Å, respectively. Furthermore, a complete X-ray diffraction data set could be collected and processed to 3.93 Å resolution. These 2D and 3D crystals of wt-IIC glc lay the foundation for the determination of the first structure of a bacterial glucose-specific IIC domain. [ABSTRACT FROM AUTHOR]

Most organisms are able to synthesize vitamin C whereas humans are not. In order to contribute to the elucidation of the molecular working mechanism of vitamin C transport through biological membranes, we cloned, overexpressed, purified, functionally characterized, and 2D- and 3D-crystallized a bacterial protein (UraDp) with 29% of amino acid sequence identity to the human sodium-dependent vitamin C transporter 1 (SVCT1). Ligand-binding experiments by scintillation proximity assay revealed that uracil is a substrate preferably bound to UraDp. For structural analysis, we report on the production of tubular 2D crystals and present a first projection structure of UraDp from negatively stained tubes. On the other hand the successful growth of UraDp 3D crystals and their crystallographic analysis is described. These 3D crystals, which diffract X-rays to 4.2 Å resolution, pave the way towards the high-resolution crystal structure of a bacterial homologue with high amino acid sequence identity to human SVCT1. [ABSTRACT FROM AUTHOR]

This paper reviews the recent studies on the laser patterning of two-dimensional planar crystals on the glass surface. The laser-induced crystallization method is applied to pattern two-dimensional planar crystals such as β-BaB2O4 and β'-(Sm, Gd)2(MoO4)3 on the glass surface. In the case of β-BaB2O4, by scanning Yb:YVO4 fiber lasers (wavelength: 1080 nm) continuously with a small step (0.5 fim) between laser irradiated areas, homogeneous planar β-BaB2O4 crystals are patterned successfully, and a preferential growth orientation of β-BaB2O4 crystals is confirmed from linearly polarized micro-Raman scattering spectrum and second harmonic intensity measurements. It is found that the crystal growth direction is perpendicular to the laser scanning direction. This relation, i.e., the perpendicular relation, is different from the behavior in discrete crystal line patterning, where the crystal growth direction is consistent with the laser scanning direction. A similar behavior is observed in the patterning of two-dimensional planar β''-(Sm,Gd)2(MoO4)3 crystals. [ABSTRACT FROM AUTHOR]

Abstract: We show that the “two-dimensional” graphene is stable due to transverse short-range displacements of carbon atoms, which may be described in a framework of Ising model with competing interactions. When temperature decreases, two transitions, high temperature disorder into order and order into low-temperature glass, arise. The graphene looks like a microscopic “washboard” with the wavelength of about 2–4Å. Due to up–down asymmetry of the lattice distortions in graphene on substrate, a mini-bandgap arises. This leads to many new phenomena: a rectification of AC current induced by microwave or infrared radiation, the existence of self-trapping and a new type of fermionic mini-exciton-polaritons. [Copyright &y& Elsevier]

Abstract: The glucose transporter IICB of the Escherichia coli phosphotransferase system (PTS) consists of a polytopic membrane domain (IIC) responsible for substrate transport and a hydrophilic C-terminal domain (IIB) responsible for substrate phosphorylation. We have overexpressed and purified a triple mutant of IIC (mut-IIC), which had recently been shown to be suitable for crystallization purposes. Mut-IIC was homodimeric as determined by blue native-PAGE and gel-filtration, and had an eyeglasses-like structure as shown by negative-stain transmission electron microscopy (TEM) and single particle analysis. Glucose binding and transport by mut-IIC, mut-IICB and wildtype-IICB were compared with scintillation proximity and in vivo transport assays. Binding was reduced and transport was impaired by the triple mutation. The scintillation proximity assay allowed determination of substrate binding, affinity and specificity of wildtype-IICB by a direct method. 2D crystallization of mut-IIC yielded highly-ordered tubular crystals and made possible the calculation of a projection structure at 12Å resolution by negative-stain TEM. Immunogold labeling TEM revealed the sidedness of the tubular crystals, and high-resolution atomic force microscopy the surface structure of mut-IIC. This work presents the structure of a glucose PTS transporter at the highest resolution achieved so far and sets the basis for future structural studies. [Copyright &y& Elsevier]

Bacteria of the Bacillus cereus family form highly resistant spores, which in the case of the pathogen B. anthracis act as the agents of infection. The outermost layer, the exosporium, enveloping spores of the B. cereus family as well as a number of Clostridia, plays roles in spore adhesion, dissemination, targeting, and germination control. We have analyzed two naturally crystalline layers associated with the exosporium, one representing the "basal" layer to which the outermost spore layer ("hairy nap") is attached, and the other likely representing a subsurface ("parasporal") layer. We have used electron cryomicroscopy at a resolution of 0.8-0.6 nm and circular dichroism spectroscopic measurements to reveal a highly it-helical structure for both layers. The helices are assembled into 2D arrays of "cups" or "crowns." High-resolution atomic force microscopy of the outermost layer showed that the open ends of these cups face the external environment and the highly immunogenic collagen-like fibrils of the hairy nap (BcIA) are attached to this surface. Based on our findings, we present a molecular model forthe spore surface and propose how this surface can act as a semipermeable barrier and a matrix for binding of molecules involved in defense, germination control, and other interactions of the spore with the environment. [ABSTRACT FROM AUTHOR]

Abstract: The calculation of projection structures (PSs) from Protein Data Bank (PDB)-coordinate files of membrane proteins is not well-established. Reports on such attempts exist but are rare. In addition, the different procedures are barely described and thus difficult if not impossible to reproduce. Here we present a simple, fast and well-documented method for the calculation and visualization of PSs from PDB-coordinate files of membrane proteins: the projection structure visualization (PSV)-method. The PSV-method was successfully validated using the PS of aquaporin-1 (AQP1) from 2D crystals and cryo-transmission electron microscopy, and the PDB-coordinate file of AQP1 determined from 3D crystals and X-ray crystallography. Besides AQP1, which is a relatively rigid protein, we also studied a flexible membrane transport protein, i.e. the l-arginine/agmatine antiporter AdiC. Comparison of PSs calculated from the existing PDB-coordinate files of substrate-free and l-arginine-bound AdiC indicated that conformational changes are detected in projection. Importantly, structural differences were found between the PSV-method calculated PSs of the detergent-solubilized AdiC proteins and the PS from cryo-TEM of membrane-embedded AdiC. These differences are particularly exciting since they may reflect a different conformation of AdiC induced by the lateral pressure in the lipid bilayer. [ABSTRACT FROM AUTHOR]

TRIM5α proteins are restriction factors that protect mammalian cells from retroviral infections by binding incoming viral capsids, accelerating their dissociation, and preventing reverse transcription of the viral genome. Individual TRIM5 isoforms can often protect cells against a broad range of retroviruses. as exemplified by rhesus monkey TRIM5α and its variant. TRIM5-21R. which recognize HIV-1 as well as several distantly related retroviruses. Although capsid recognition is not yet fully understood, previous work has shown that the C-terminal SPRY/B30.2 domain of dimeric TRIM5α binds directly to viral capsids. and that higher-order TRIM5α oligomerization appears to contribute to the efficiency of capsid recognition. Here, we report that recombinant TRIM5-21R spontaneously assembled into two-dimensional paracrystalline hexagonal lattices comprising open, six-sided rings. TRIM5-21R assembly did not require the C-terminal SPRY domain, but did require both protein dimerization and a B-box 2 residue (Argl2l) previously implicated in TRIM5a restriction and higher-order assembly. Furthermore, TRIM5-21R assembly was promoted by binding to hexagonal arrays of the HIV-1 CA protein that mimic the surface of the viral capsid. We therefore propose that TRIM5a proteins have evolved torestrict a range of different retroviruses by assembling a deformable hexagonal scaffold that positions the capsid-binding domains to match the symmetry and spacing of the capsid surface lattice. Capsid recognition therefore involves a synergistic combination of direct binding interactions, avidity effects, templated assembly, and lattice complementarity. [ABSTRACT FROM AUTHOR]

High-resolution microscopy techniques provide a plethora of information on biological structures from the cellular level down to the molecular level. In this review, we present the unique capabilities of transmission electron and atomic force microscopy to assess the structure, oligomeric state, function and dynamics of channel and transport proteins in their native environment, the lipid bilayer. Most importantly, membrane proteins can be visualized in the frozen-hydrated state and in buffer solution by cryo-transmission electron and atomic force microscopy, respectively. We also illustrate the potential of the scintillation proximity assay to study substrate binding of detergent-solubilized transporters prior to crystallization and structural characterization. [ABSTRACT FROM AUTHOR]

Abstract: Membrane proteins fulfill many important roles in the cell and represent the target for a large number of therapeutic drugs. Although structure determination of membrane proteins has become a major priority, it has proven to be technically challenging. Electron microscopy of two-dimensional (2D) crystals has the advantage of visualizing membrane proteins in their natural lipidic environment, but has been underutilized in recent structural genomics efforts. To improve the general applicability of electron crystallography, high-throughput methods are needed for screening large numbers of conditions for 2D crystallization, thereby increasing the chances of obtaining well ordered crystals and thus achieving atomic resolution. Previous reports describe devices for growing 2D crystals on a 96-well format. The current report describes a system for automated imaging of these screens with an electron microscope. Samples are inserted with a two-part robot: a SCARA robot for loading samples into the microscope holder, and a Cartesian robot for placing the holder into the electron microscope. A standard JEOL 1230 electron microscope was used, though a new tip was designed for the holder and a toggle switch controlling the airlock was rewired to allow robot control. A computer program for controlling the robots was integrated with the Leginon program, which provides a module for automated imaging of individual samples. The resulting images are uploaded into the Sesame laboratory information management system database where they are associated with other data relevant to the crystallization screen. [Copyright &y& Elsevier]

Abstract: Gastric H+,K+-ATPase is responsible for gastric acid secretion. In order to characterize the phosphorylation events on H+,K+-ATPase, the properties of fluorinated phosphate analogs [XFs, e.g. aluminum fluoride (AlF), beryllium fluoride (BeF) and magnesium fluoride (MgF)], and the structural differences induced by XFs were investigated. The addition of divalent cations to the XF-inhibited H+,K+-ATPase restores the activity of the AlF- or MgF-inhibited, but not of the BeF-inhibited enzyme, although limited trypsin digestion reveals that they assume the same E2P-like state. To clarify the conformational differences induced by XFs, the structure of BeF-bound H+,K+-ATPase was analyzed at 8Å resolution. The structure is almost identical to the previously reported AlF-bound E2P structure, unlike the distinctive X-ray structure of BeF-bound SERCA, in which the luminal gate was observed to be widely opened. Since the analyzed structure of the H+,K+-ATPase revealed that both AlF and BeF-bound to the P domain were not exposed to the solvent, the dissociation of XFs induced by divalent cations could be interpreted in terms of stability against thermal fluctuations. Furthermore, the conformational differences found between the cytoplasmic domains of H+,K+-ATPase and SERCA provide a framework to understand the characteristic mechanism, by which divalent cations reactivate the XF-inhibited H+,K+-ATPase. [Copyright &y& Elsevier]

Abstract: The membrane domain of human erythrocyte anion exchanger 1 (AE1) works as a Cl−/HCO3 − antiporter. This exchange is a key step for CO2/O2 circulation in the blood. In spite of their importance, structural information about AE1 and the AE (anion exchanger) family are still very limited. We used electron microscopy to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking, at 7.5-Å resolution. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of the ClC chloride channel, a Cl−/H+ antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into the membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of the ClC transporter (helices B+C and J+K), are well fitted to those AE1 densities after simple domain movement. [Copyright &y& Elsevier]

Abstract: Cellular uptake of di- and tripeptides has been characterized in numerous organisms, and various transporters have been identified. In contrast, structural information on peptide transporters is very sparse. Here, we have cloned, overexpressed, purified, and biochemically characterized DtpD (YbgH) from Escherichia coli, a prokaryotic member of the peptide transporter family. Its homologues in mammals, PEPT1 (SLC15A1) and PEPT2 (SLC15A2), not only transport peptides but also are of relevance for uptake of drugs as they accept a large spectrum of peptidomimetics such as β-lactam antibiotics, antivirals, peptidase inhibitors, and others as substrates. Uptake experiments indicated that DtpD functions as a canonical peptide transporter and is, therefore, a valid model for structural studies of this family of proteins. Blue native polyacrylamide gel electrophoresis, gel filtration, and transmission electron microscopy of single-DtpD particles suggest that the transporter exists in a monomeric form when solubilized in detergent. Two-dimensional crystallization of DtpD yielded first tubular crystals that allowed the determination of a projection structure at better than 19 Å resolution. This structure of DtpD represents the first structural view of a member of the peptide transporter family. [Copyright &y& Elsevier]

Abstract: We have used high-speed atomic force microscopy to study the dynamics of bacteriorhodopsin (bR) molecules at the free interface of the crystalline phase that occurs naturally in purple membrane. Our results reveal temporal fluctuations at the crystal edges arising from the association and dissociation of bR molecules, most predominantly pre-formed trimers. Analysis of the dissociation kinetics yields an estimate of the inter–trimer single-bond energy of −0.9kcal/mol. Rotational motion of individual bound trimers indicates that the inter–trimer bond involves W10–W12 tryptophan residues. [Copyright &y& Elsevier]

The gastric H+,K+-ATPase is an ATP-driven proton pump responsible for generating a million-fold proton gradient across the gastric membrane. We present the structure of gastric H+,K+-ATPase at 6.5 Å resolution as determined by electron crystallography of two-dimensional crystals. The structure shows the catalytic α-subunit and the non-catalytic β-subunit in a pseudo-E2P conformation. Different from Na+,K+-ATPase, the N-terminal tail of the β-subunit is in direct contact with the phosphorylation domain of the α-subunit. This interaction may hold the phosphorylation domain in place, thus stabilizing the enzyme conformation and preventing the reverse reaction of the transport cycle. Indeed, truncation of the β-subunit N-terminus allowed the reverse reaction to occur. These results suggest that the β-subunit N-terminus prevents the reverse reaction from E2P to E1P, which is likely to be relevant for the generation of a large H+ gradient in vivo situation. [ABSTRACT FROM AUTHOR]

The native environment of integral membrane proteins is a lipid bilayer. The structure of a membrane protein is thus ideally studied in a lipidic environment. In the first part of this review we describe some membrane protein structures that revealed the surrounding lipids and provide a brief overview of the techniques that can be used to study membrane proteins in a lipidic environment. in the second part of this review we focus on electron crystallography of two-dimensional crystals as potentially the most suitable technique for such studies. We describe the individual steps involved in the electron crystallographic determination of a membrane protein structure and discuss current challenges that need to be overcome to transform electron crystallography into a technique that can be routinely used to analyze the structure of membrane proteins embedded in a lipid bilayer. [ABSTRACT FROM AUTHOR]

Abstract: Members of the tetraspanin superfamily function as transmembrane scaffold proteins that mediate the assembly of membrane proteins into specific signaling complexes. Tetraspanins also interact with each other and concentrate membrane proteins into tetraspanin-enriched microdomains (TEMs). Here we report that lens-specific tetraspanin MP20 can form multiple types of higher-order assemblies and we present crystalline arrays of MP20. When isolated in the absence of divalent cations, MP20 is solubilized predominantly in tetrameric form, whereas the presence of divalent cations during solubilization promotes the association of MP20 tetramers into higher-order species. This effect only occurs when divalent cations are present during solubilization but not when divalent cations are added to solubilized tetrameric MP20, suggesting that other factors may also be involved. When purified MP20 tetramers are reconstituted with native lens lipids in the presence of magnesium, MP20 forms two-dimensional (2D) crystals. A projection map at 18 Å resolution calculated from negatively stained 2D crystals showed that the building block of the crystal is an octamer consisting of two tetramers related to each other by 2-fold symmetry. In addition to 2D crystals, reconstitution of MP20 with native lipids also produced a variety of large protein–lipid complexes, and we present three-dimensional (3D) reconstructions of the four most abundant of these complexes in negative stain. The various complexes formed by MP20 most likely reflect the many ways in which tetraspanins can interact with each other to allow formation of TEMs. [Copyright &y& Elsevier]

ATP synthesis by V-ATPase from the thermophilic bacterium Thermus thermophilus driven by the acid-base transition was investigated. The rate of ATP synthesis increased in parallel with the increase in proton motive force (PMF) >110 mV, which is composed of a difference in proton concentration (+pH) and the electrical potential differences (ΔΨ) across membranes. The optimum rate of synthesis reached 85 s+, and the H+/ATP ratio of 4.0 ± 0.1 was obtained. ATP was synthesized at a considerable rate solely by ΔpH, indicating ΔΨ was not absolutely required for synthesis. Consistent with the H+/ATP ratio, cryoelectron micrograph images of 2D crystals of the membrane-bound rotor ring of the V-ATPase at 7.0-Å resolution showed the presence of 12 Vo-c subunits, each composed of two transmembrane helices. These results indicate that symmetry mismatch between the rotor and catalytic domains is not obligatory for rotary ATPases/synthases. [ABSTRACT FROM AUTHOR]

Abstract: SecA is a multifunctional protein involved in protein translocation in bacteria. The structure of SecA on membrane is dramatically altered compared with that in solution, accompanying with functional changes. We previously reported the formation of a novel ring-like structure of SecA on lipid layers, which may constitute part of the preprotein translocation channel. In the present work, two-dimensional crystallization of Escherichia coli SecA on lipid monolayers was performed to reveal the structural details of SecA on lipid layers and to investigate its function. The 2D crystals composed of ring-like structures were obtained by specific interaction between SecA and negatively charged lipid. The 2D projection map and 3D reconstruction from negative stained 2D crystals exhibited a distinct open channel-like structure of SecA, with an outer diameter of 7nm and an inner diameter of 2nm, providing the structural evidence for SecA importance in forming the part of the translocation channel. This pore structure is altered after transferring crystals to the SecB solution, indicating that the lipid-specific SecA structure has the SecB binding activity. The strategy developed here provides a promising technique for studying structure of SecA complex with its ligand on membrane. [Copyright &y& Elsevier]

Abstract: Aquaporin-9, an aquaglyceroporin present in diverse tissues, is unique among aquaporins because it is not only permeable to water, urea and glycerol, but also allows passage of larger uncharged solutes. Single particle analysis of negatively stained recombinant rat aquaporin-9 revealed a particle size characteristic of the tetrameric organization of all members of the aquaporin family. Reconstitution of aquaporin-9 into two-dimensional crystals enabled us to calculate a projection map at 7 Å resolution. The projection structure indicates a tetrameric structure, similar to GlpF, with each square-like monomer forming a pore. A comparison of the pore-lining residues between the crystal structure of GlpF and a homology model of aquaporin-9 locates substitutions in these residues predominantly to the hydrophobic edge of the tripathic pore of GlpF, providing first insights into the structural basis for the broader substrate specificity of aquaporin-9. [Copyright &y& Elsevier]

Abstract: Melibiose permease (MelB) of Escherichia coli is a secondary transporter that couples the uptake of melibiose and various other galactosides to symport of cations that can be Na+, Li+ or H+. MelB belongs to the glycoside-pentoside-hexuronide: cation symporter family of porters and is suggested to have 12 transmembrane helices. We have determined the three-dimensional structure of MelB at 10Å resolution in the membrane plane with cryo-electron microscopy from two-dimensional crystals. The three-dimensional map shows a heart-shaped molecule composed of two domains with a large central cavity between them. The structure is constricted at one side of the membrane while it is open to the other. The overall molecular shape resembles those of lactose permease and glycerol-3-phosphate transporter. However, organization of helices in MelB seems less symmetrical than in these two members of the major facilitator superfamily. [Copyright &y& Elsevier]

Quantum interference (QI) correction to dynamical conductivity is calculated with the diagrammatic technique in a weakly-disordered two-dimensional (2D) square lattice around half filling, in which the pointlike nonmagnetic impurities are assumed to be substituted randomly for the host atoms. It is found that the conductivity correction is inversely proportional to the frequency for the case of perfectly-nested Fermi surface, resulting from the contribution of diffusive π modes to the QI effect. Such an antilocalization effect is strikingly different from the logarithmic weak-localization correction predicted for 2D free electron systems. [ABSTRACT FROM AUTHOR]

Image shift due to beam-induced specimen charging has become the most severe problem in electron microscopy for imaging two-dimensional (2D) crystals of biological macromolecules, especially in the case of highly tilted specimens. Image shift causes diffraction spots perpendicular to the tilt axis to disappear even at medium or low resolution. The yield of good images from tilted specimens prepared on a single layer of continuous carbon support film is therefore very low. In this paper, we have used 2D crystals of aquaporin-4 to investigate the effect of a carbon sandwich preparation method on specimen charging. We find that a larger number of images show sharp diffraction spots perpendicular to the tilt axis if crystals are placed in between two sheets of carbon film as compared to images taken from specimens prepared by the conventional single carbon support film technique. Our results demonstrate that the reproducible carbon sandwich preparation technique overcomes the severe specimen charging problem and thus has the potential to significantly speed up structure analysis by electron crystallography. [Copyright &y& Elsevier]

A molecular dynamics computer simulation was carried out for two-dimensional anharmonic mass-spring model crystals. Instead of the square lattice crystals in our previous study, hexagonal lattice crystals were chosen. Interactions between the nearest neighbor and the next nearest neighbor atoms were taken into account. Mechanical excitations produced in the lattice by an input pulse displacement applied on the edge of the crystal were studied. Phonons and solitons were, respectively, produced when the input was small and not small. Our interest was focused on anticipating probable laboratory experiment of soliton production in real crystals. [Copyright &y& Elsevier]

Two-dimensional (2D) crystals of Na+/K+-ATPase were vitrified and observed with an electron cryo-microscope that allowed specimen observation at liquid helium temperature. Images showing 19 Å spots on the optical diffraction patterns were digitized and processed by a crystallographic method. After optimization of parameters for the image processing, the locally averaged images revealed that each 2D crystal contained heterogeneous molecules. Single panicle analysis was therefore applied to the 2D crystal images to select homogeneous molecule images, and averaging the selected images significantly improved map quality. Five major peak densities were clearly resolved in the new projection map, while two were found previously. [ABSTRACT FROM PUBLISHER]

Fourier transform and an image filtering technique were used for structural analysis of the pellicular strip in Euglena gracilis. Freeze-fracture images of a two-dimensional crystalline structure in the plasma membrane were taken from elongated and rounded cells. Their lattice constants were precisely determined from Fourier transform patterns, and masked filter images were generated. Although differences in their lattice constants could not be detected, the Fourier transform patterns showed a significant difference between these two stages of cell shape. In elongated cells, the pseudo-crystal lattice was generated in the half periodicity of the minor striation. In contrast, it disappeared when the cells became rounded. The filtered images confirmed the crystallographic disagreement between these two stages in cell shape, probably reflecting morphological changes of integrated plasma membrane particles. Four particles detected in a unit structure showed quite similar shapes across the pseudo-lattice in elongated cells, though a unit structure in rounded cells consisted of four heterogeneous particles with opaque boundaries. A 39 kDa integral plasma membrane protein (IP39) is known as a major component in the plasma membrane of E. gracilis. The compositions of monomeric and oligomeric forms of IP39 were determined in both elongated and rounded cells by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, which showed the possibility that an oligomerization of IP39 proceeds during rounding-up movement of the cell. These morphological and stoichiometrical changes of the integrated plasma membrane proteins may suggest an active involvement in cell shape change. [ABSTRACT FROM AUTHOR]

Layered 2D transition metal dichalcogenides (TMDCs) exhibited fascinating nonlinear optical (NLO) properties for constructing varied promising optoelectronics. However, exploring the desired 2D materials with both superior nonlinear absorption and ultrafast response in broadband spectra remain the key challenges to harvest their greatest potential. Here, based on synthesizing 2D PdSe 2 films with the controlled layer number, the authors systematically demonstrated the broadband giant NLO performance and ultrafast excited carrier dynamics of this emerging material under femtosecond visible-to-near-infrared laser-pulse excitation (400-1550 nm). Layer-dependent and wavelength-dependent evolution of optical bandgap, nonlinear absorption, and photocarrier dynamics in the obtained 2D PdSe 2 are clearly revealed. Specially, the transition from semiconducting to semimetallic PdSe 2 induced dramatic changes of their interband absorption-relaxation process. This work makes 2D PdSe 2 more competitive for future ultrafast photonics and also opens up a new avenue for the optical performance optimization of various 2D materials by rational design of these materials., (© 2021 Wiley-VCH GmbH.)