西野 有里

Chloroplast-actin (cp-actin) filaments are crucial for light-induced chloroplast movement, and appear in the front region of moving chloroplasts when visualized using GFP-mouse Talin. They are short and thick, exist between a chloroplast and the plasma membrane, and move actively and rapidly compared to cytoplasmic long actin filaments that run through a cell. The average period during which a cp-actin filament was observed at the same position was less than 0.5 s. The average lengths of the cp-actin filaments calculated from those at the front region of the moving chloroplast and those around the chloroplast periphery after stopping the movement were almost the same, approximately 0.8 µm. Each cp-actin filament is shown as a dotted line consisting of 4-5 dots. The vector sum of cp-actin filaments in a moving chloroplast is parallel to the moving direction of the chloroplast, suggesting that the direction of chloroplast movement is regulated by the vector sum of cp-actin filaments. However, once the chloroplasts stopped moving, the vector sum of the cp-actin filaments around the chloroplast periphery was close to zero, indicating that the direction of movement was undecided. To determine the precise structure of cp-actin filaments under electron microscopy, Arabidopsis leaves and fern Adiantum capillus-veneris gametophytes were frozen using a high-pressure freezer, and observed under electron microscopy. However, no bundled microfilaments were found, suggesting that the cp-actin filaments were unstable even under high-pressure freezing.

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that play an important role in signal transduction at the neuromuscular junction (NMJ). Movement of the nAChR extracellular domain following agonist binding induces conformational changes in the extracellular domain, which in turn affects the transmembrane domain and opens the ion channel. It is known that the surrounding environment, such as the presence of specific lipids and proteins, affects nAChR function. Diffracted X-ray tracking (DXT) facilitates measurement of the intermolecular motions of receptors on the cell membranes of living cells, including all the components involved in receptor function. In this study, the intramolecular motion of the extracellular domain of native nAChR proteins in living myotube cells was analyzed using DXT for the first time. We revealed that the motion of the extracellular domain in the presence of an agonist (e.g., carbamylcholine, CCh) was restricted by an antagonist (i.e., alpha-bungarotoxin, BGT).

It is important to understand and control the fine structure of the fuel cell catalyst layer in order to improve the battery characteristics of the fuel cell. A major challenge in observing the microstructure of the catalyst layer by electron microscopy is the visualization of ionomers, which have low contrast and are susceptible to damage by electron beam irradiation. Previous papers have reported transmission electron microscopy (TEM) observations of ionomers neutralized with cesium (Cs) ions. However, this approach involves chemical reactions and an indirect visualization of ionomers. In contrast, we have previously revealed the microstructure of ionomers in frozen catalyst inks by cryogenic scanning electron microscopy (cryo-SEM) and cryo-TEM. In general, ionomers are basically used under high temperature and humid conditions while the fuel cell is operating. Therefore, in this study, ultrathin sections prepared from the fuel cell catalyst layer (membrane electrode assemblies) were incubated in the chamber at high temperature and humid conditions and then rapidly frozen for observation by cryo-TEM. As a result, we succeeded in observing the pore structure of the catalyst layer in the swollen state of the ionomer. The swollen ionomer surrounded and enclosed the Pt/C aggregates and bridged over the pores in the catalyst layer.

Phospholipids in the membrane consist of diverse pairs of fatty acids bound to a glycerol backbone. The biological significance of the diversity, however, remains mostly unclear. Part of this diversity is due to lysophospholipid acyltransferases (LPLATs), which introduce a fatty acid into lysophospholipids. The human genome has 14 LPLATs and most of them are highly conserved in vertebrates. Here, we analyzed the function of one of these enzymes, lysophosphatidylglycerol acyltransferase 1 (Lpgat1), in zebrafish. We found that the reproduction of heterozygous (lpgat1+/-) male mutants was abnormal. Crosses between heterozygous males and wild-type females produced many eggs with no obvious cleavage, whereas eggs produced by crosses between heterozygous females and wild-type males cleaved normally. Consistent with this, spermatozoa from heterozygous males had reduced motility and abnormal morphology. We also found that the occurrence of lpgat1 homozygous (lpgat1-/-) mutants was far lower than expected. In addition, downregulation of lpgat1 by morpholino antisense oligonucleotides resulted in severe developmental defects. Lipidomic analysis revealed that selective phospholipid species with stearic acid and docosahexaenoic acid were reduced in homozygous larvae and spermatozoa from heterozygotes. These results suggest that the specific phospholipid molecular species produced by Lpgat1 have an essential role in sperm fertilization and in embryonic development.

An emulsion, a type of soft matter, is complexed with at least two materials in the liquid state (e.g. water and oil). Emulsions are classified into two types: water in oil (W/O) and oil in water (O/W), depending on the strength of the emulsifier. The properties and behavior of emulsions are directly correlated with the size, number, localization and structure of the dispersed phases in the continuous phase. Therefore, an understanding of the microstructure comprising liquid-state emulsions is essential for producing and evaluating these emulsions. Generally, it is impossible for conventional electron microscopy to examine liquid specimens, such as emulsion. Recent advances in cryo-scanning electron microscopy (cryo-SEM) could allow us to visualize the microstructure of the emulsions in a frozen state. Immersion freezing in slush nitrogen has often been used for preparing the frozen samples of soft matters. This preparation could generate ice crystals, and they would deform the microstructure of specimens. High-pressure freezing contributes to the inhibition of ice-crystal formation and is commonly used for preparing frozen biological samples with high moisture content. In this study, we compared the microstructures of immersion-frozen and high-pressure frozen emulsions (O/W and W/O types, respectively). The cryo-SEM observations suggested that high-pressure freezing is more suitable for preserving the microstructure of emulsions than immersion freezing.

The physical properties of butter are influenced by its microstructure; therefore, observation techniques should be employed to improve this structure. Current observation methods are coupled with the pretreatment process, which destroys the microstructure. In this study, therefore, we provide a novel approach without the pretreatment process to observe fat crystal platelets—aggregated to provide a network structure—by using atomic force microscopy (AFM) while the sample was immersed in water at a temperature of 10 °C; in particular, we used Young's modulus mode for the observation. To confirm the structure, the measurements of milk fat-crystalline nanoplatelets (MF-CNPs) were observed with the transmission electron microscopy (TEM) and the small-angle X-ray scattering (SAXS). Thus, the milk fat particles were found to measure 796 nm × 223 nm, and the thickness of MF-CNPs was found to be 20.1 nm by SAXS. These measurements corresponded to the images of fat crystals observed by AFM. Using this observation method, the relationship between the physical properties of butter and its fat crystal network can be discerned; therefore, the bread and confectionery industries can improve their butter processing conditions.

The effects of sucrose ester of fatty acid (SEF) on the nanostructure and the physical properties of water-in-oil (W/O)-type emulsified semisolid fats were investigated. Model emulsions including palm-based semisolid fats and fully hydrogenated rapeseed oils with 0.5% SEF or fractionated lecithin, were prepared by rapidly cooling crystallization using 0.5% monoacylglycerol as an emulsifier. The SEFs used in this study were functionalized with various fatty acids, namely, lauric, palmitic, stearic, oleic, and erucic acids. Cryogenic transmission electron microscopy (cryo-TEM) was used to observe the sizes of the solventextracted nanoplatelets. The solid fat content (SFC), oil migration value, and storage elastic modulus were also determined. The average crystal size, which was measured in length, of the fat blends with SEFs containing saturated fatty acids (namely, palmitic and stearic acids) was smaller than that of the SEFs containing unsaturated fatty acids (namely, oleic and erucic acids). The effects exerted by these fatty acid moieties on the spherulite size in the corresponding bulk fat blends were observed via polarized microscopy (PLM). The results suggest that nanostructure formation upon the addition of SEF ultimately influenced these aggregated microstructures. Generally, smaller platelets resulted in higher SFC in the fat phase, and a high correlation between the SFC and the G’ values in W/O emulsion fats was observed (R = 0.884) at 30°C. In contrast, the correlation was low at 10°C. Furthermore, samples with larger nanocrystals had a higher propensity for oil migration. Thus, the addition of SEF regulated the fat crystal nanostructure during nucleation and crystal growth, which could ultimately influence the physical properties of commercially manufactured fat products such as margarine. 2

The emergence of drug-resistant influenza type A viruses (IAVs) necessitates the development of novel anti-IAV agents. Here, we target the IAV hemagglutinin (HA) protein using multivalent peptide library screens and identify PVF-tet, a peptide-based HA inhibitor. PVF-tet inhibits IAV cytopathicity and propagation in cells by binding to newly synthesized HA, rather than to the HA of the parental virus, thus inducing the accumulation of HA within a unique structure, the inducible amphisome, whose production from the autophagosome is accelerated by PVF-tet. The amphisome is also produced in response to IAV infection in the absence of PVF-tet by cells overexpressing ABC transporter subfamily A3, which plays an essential role in the maturation of multivesicular endosomes into the lamellar body, a lipid-sorting organelle. Our results show that the inducible amphisomes can function as a type of organelle-based anti-viral machinery by sequestering HA. PVF-tet efficiently rescues mice from the lethality of IAV infection.

To elucidate the effect of fat crystals and liquid fat on the bubble interface in whipped cream, we observed structural changes in fat globules and bubble interfaces caused by whipping using cryo-transmission electron microscopy. The results suggested that fat crystals formed lamellae at the fat globule interface, together with liquid fat at the center of the fat globules, which were dissociated from the fat globule under whipping. As whipping progressed, fat crystals fragmented and were adsorbed on the bubble interface, such that the area occupied by the fat crystals in the bubbles gradually increased. This behavior was presumed to contribute to the stability of whipped cream bubbles. Furthermore, liquid fat released from the fat globules was inferred to occupy gaps between the fat globules.

Shiga toxin (Stx), a major virulence factor of enterohemorrhagic Escherichia coli (EHEC), is classified into two subgroups, Stx1 and Stx2. Clinical data clearly indicate that Stx2 is associated with more severe toxicity than Stx1, but the molecular mechanism underlying this difference is not fully understood. Here, we found that after being incorporated into target cells, Stx2, can be transported by recycling endosomes, as well as via the regular retrograde transport pathway. However, transport via recycling endosome did not occur with Stx1. We also found that Stx2 is actively released from cells in a receptor-recognizing B-subunit dependent manner. Part of the released Stx2 is associated with microvesicles, including exosome markers (referred to as exo-Stx2), whose origin is in the multivesicular bodies that formed from late/recycling endosomes. Finally, intravenous administration of exo-Stx2 to mice causes more lethality and tissue damage, especially severe renal dysfunction and tubular epithelial cell damage, compared to a free form of Stx2. Thus, the formation of exo-Stx2 might contribute to the severity of Stx2 in vivo, suggesting new therapeutic strategies against EHEC infections.

Casein micelles are present in bovine milk as colloidal particles with diameters of 20-600 nm, which are complex macromolecular assemblies composed of four distinct types of casein and colloidal calcium phosphate (CCP). Multiple structural models of casein micelles have been proposed based on their biochemical or physical properties and observed using electron microscopy. However, the CCP distribution and crosslinking structure between CCP and casein remain unclear. Therefore, the internal structure of casein micelles in raw milk was observed using cryo-electron microscopy of vitreous sections (CEMOVIS) with high precision at high resolution. The results confirmed that the average casein micelle diameter was about 140 nm, and that the CCP diameter in casein micelles was about 2-3 nm, with an average diameter of 2.3 nm. The distribution of CCP in casein micelles was not uniform, with an average interval between CCPs of about 5.4 nm. Areas containing no black particles (attributed to CCP) were present, with an average size of about 19.1 nm. Considering previous reports, these areas possibly correspond to pores or cavities filled with water. Based on differences in the density of structures in casein micelles, we estimated that some of the casein aggregates were able to connect with CCP in a string.

The effect of nanostructured fat crystals on oil migration properties in water-in-oil-type emulsified semisolid fats was investigated. Model emulsions containing 4 different semisolid fats (palm oil, partially hydrogenated palm oil, partially hydrogenated soybean oil, and milk fat) and 1 bulk fat blend were prepared with rapidly cooling crystallization. The length of the nanoplatelets was observed by cryo transmission electron microscopy, the crystal thickness was calculated by small-angle X-ray diffraction, and the solid fat content (SFC) was determined. Although the interfacial surface of the dispersed water droplets did not influence nanoplatelet size, oil migration in the emulsified samples was lower than in the bulk fat. The crystal sizes in samples with partially hydrogenated soybean oil involving elaidic acid were larger, in contrast to that of milk fat, involving low to medium chain length fatty acids, which had smaller crystal sizes and showed wide length distribution. The length of the platelets and SFC were related to the oil migration value. These results suggest that the oil binding ability of fat products, such as margarine, is influenced by the nanostructure, which is related to fatty acid composition and interfacial structure.

In order to improve the electricity generation performance of fuel cell electric vehicles, it is necessary to optimize the microstructure of the catalyst layer of a polymer electrolyte fuel cell. The catalyst layer is formed by a wet coating process using catalyst inks. Therefore, it is very important to observe the microstructure of the catalyst ink. In this study, the morphology of carbon-supported platinum (Pt/C) particles in catalyst inks with a different solvent composition was investigated by cryogenic scanning electron microscopy (cryo-SEM). In addition, the morphology of the ionomer, which presumably influences the formation of agglomerated Pt/C particles in a catalyst ink, was investigated by cryogenic transmission electron microscopy (cryo-TEM). The results of a cryo-SEM observation revealed that the agglomerated Pt/C particles tended to become coarser with a higher 1-propanol (NPA) weight fraction. The results of a cryo-TEM observation indicated that the actual ionomer dispersion in a catalyst ink formed a network structure different from that of the ionomer in the solvent.

© The Electrochemical Society. The formation of a catalyst-layer structure induced by solvent evaporation of a catalyst ink was investigated by cryogenic scanning electron microscopy (cryo-SEM) and cryogenic transmission electron microscopy (cryo-TEM). Cryo-SEM and cryo-TEM reveal that the catalyst ink contains agglomerates of Pt/C catalyst particles that are several hundreds of nanometers to micrometers in size, and rod-like Nafion® particles that are around 3 nm in diameter. During the drying process, the Pt/C agglomerates are mutually connected in a network structure that resembles a spider web and that is induced by solvent evaporation, - although this network structure is no longer observed after complete drying.

Optimizing the catalyst layer structure is one of the key issues for improving performance despite lower platinum loading. The catalyst ink, consisting of platinum-loaded carbon particles and ionomer dispersed in an aqueous solvent, is a key factor for controlling the structure of the catalyst layer because the catalyst layer is prepared in a wet coating process. For that purpose, we visualized the nanostructure of the ionomer in the catalyst ink by cryogenic electron microscopy, especially cryogenic transmission electron microscopy (cryo-TEM). By cryo-TEM, it was revealed that ionomer molecules formed rod-like aggregates macro-homogeneously in the solvent, and a similar morphology was observed in a carbon-particle containing solvent. In contrast, ionomer aggregates in the catalyst ink containing platinum nanoparticles loaded on carbon particles were denser in the vicinity of the platinum-loaded carbon particles. That can be attributed to strong interaction between platinum nanoparticles and sulfonic acid groups in the ionomer. It also implies that a good understanding of ionomer morphology in the catalyst ink.can play an important role in controlling the catalyst layer microstructure for reducing platinum loading. (C) 2016 Elsevier Ltd. All rights reserved.

In order to analyse the internal structures of multi-component fluid materials such as emulsions (including the inter-particle spacing) by cryo-electron microscopy, it is necessary to observe their smooth cross-sectional surfaces over wide areas. We have developed a system that involves the following steps: preservation of the structure of an emulsion adhesive using freeze fixation in its normal (moist) state and during the drying process after being coated, preparation of cross sections of the internal structure using a cryo-ultramicrotome and then transferral of the cross sections into a cryo-scanning electron microscope for observation via a cryo-transfer system. This system allows the direct observation of the cross sections of emulsions and of several fluid materials.

The major light-harvesting pigment protein complex (fucoxanthin-chlorophyll-binding protein complex; FCP) was purified from a marine centric diatom, Chaetoceros gracilis, by mild solubilization followed by sucrose density gradient centrifugation, and then characterized. The dynamic light scattering measurement showed unimodality, indicating that the complex was highly purified. The amount of chlorophyll a (Chl a) bound to the purified FCP accounted for more than 60 % of total cellular Chl a. The complex was composed of three abundant polypeptides, although there are nearly 30 FCP-related genes. The two major components were identified as Fcp3 (Lhcf3)- and Fcp4 (Lhcf4)-equivalent proteins based on their internal amino acid sequences and a two-dimensional isoelectric focusing electrophoresis analysis developed in this work. Compared with the thylakoids, the FCP complex showed higher contents of fucoxanthin and chlorophyll c but lower contents of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Fluorescence excitation spectra analyses indicated that light harvesting, rather than photosystem protection, is the major function of the purified FCP complex, which is associated with more than 60 % of total cellular Chl a. These findings suggest that the huge amount of Chl bound to the FCP complex composed of Lhcf3, Lhcf4, and an unidentified minor protein has a light-harvesting function to allow efficient photosynthesis under the dim-light conditions in the ocean.

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems. Insect GluCls show alternative splicing, and, to determine its impact on channel function and pharmacology, we isolated GluCl cDNAs from larvae of the silkworm (Bombyx mori). We show that six B. mori glutamate-gated chloride channel variants are generated by splicing in exons 3 and 9 and that exons 3b and 3c are common in the brain and third thoracic ganglion. When expressed in Xenopus laevis oocytes, the three functional exon 3 variants (3a, b, c) all had similar EC50 values for l-glutamate and ivermectin (IVM); however, Imax (the maximum l-glutamate- and IVM-induced response of the channels at saturating concentrations) differed strikingly between variants, with the 3c variant showing the largest l-glutamate- and IVM-induced responses. By contrast, a partial deletion detected in exon 9 had a much smaller impact on l-glutamate and IVM actions. Binding assays using [(3)H]IVM indicate that diversity in IVM responses among the GluCl variants is mainly due to the impact on channel assembly, altering receptor cell surface numbers. GluCl variants expressed in HEK293 cells show that structural differences influenced Bmax but not Kd values of [(3)H]IVM. Domain swapping and site-directed mutagenesis identified four amino acids in exon 3c as hot spots determining the highest amplitude of the l-glutamate and IVM responses. Modeling the GluCl 3a and 3c variants suggested that three of the four amino acids contribute to intersubunit contacts, whereas the other interacts with the TM2-TM3 linker, influencing the receptor response.

We observed the dynamic three-dimensional (3D) single molecule behaviour of acetylcholine-binding protein (AChBP) and nicotinic acetylcholine receptor (nAChR) using a single molecule tracking technique, diffracted X-ray tracking (DXT) with atomic scale and 100 μs time resolution. We found that the combined tilting and twisting motions of the proteins were enhanced upon acetylcholine (ACh) binding. We present the internal motion maps of AChBP and nAChR in the presence of either ACh or α-bungarotoxin (αBtx), with views from two rotational axes. Our findings indicate that specific motion patterns represented as biaxial angular motion maps are associated with channel function in real time and on an atomic scale.

Virus particles are promising vehicles and templates for vaccination, drug delivery and material sciences. Although infectious picornaviruses can be synthesized from genomic or synthetic RNA by cell-free protein expression systems derived from mammalian cell extract, there has been no direct evidence that authentic viral particles are indeed synthesized in the absence of living cells. We purified encephalomyocarditis virus (EMCV) synthesized by a HeLa cell extract-derived, cell-free protein expression system, and visualized the viral particles by transmission electron-microscopy. The in vitro-synthesized EMCV particles were indistinguishable from the in vivo-synthesized particles. Our results validate the use of the cell-free technique for the synthesis of EMCV particles.

A cadmium-binding, genetically encoded protein tag, consisting of three repeats of metallothionein (3MT), can be used in electron microscopy for the visualization of multimeric- but not monomeric-tagged proteins due to insufficient electron density in monomeric proteins. Here, we present a technique for detecting monomeric 3MT-tagged green fluorescent protein (GFP-3MT) using a platinum compound to intensify the electron density. Substitution of cadmium by platinum as a result of incubating purified cadmium-binding 3MT-tagged GFP (GFP-Cd-3MT) with cis-diamminedichloroplatinum(II) (cisDDP) was assessed by a UV absorption band centered at 284 nm thereby indicating platinum-sulfhydryl bonds. The incubation time and the concentration of cisDDP to reach maximal absorption were 2 h and 36-fold molar equivalent of cisDDP, respectively. GFP-Pt-3MT isolated by gel filtration chromatography contained 29 platinum atoms per single GFP-3MT molecule. Electron-dense particles were observed in a GFP-Pt-3MT sample by electron microscopy without negative staining. Further image processing and image analysis demonstrated that particles with higher density relative to their surroundings were detectable in both GFP-Cd-3MT and GFP-Pt-3MT samples. These results demonstrate that replacement of cadmium with platinum, together with proper image analyses, improve detection efficiency and enable the visualization of 3MT-tagged monomeric protein by electron microscopy.

The eukaryotic cytosolic chaperonin CCT (chaperonin-containing TCP-1) assists folding of newly synthesized polypeptides. The fully functional CCT is built from two identical rings, each composed of single copies of eight distinct subunits. To study the structure and function of the CCT complex and the role of each subunit, a rapid and efficient method for preparing a recombinant CCT complex is needed. In this work, we established an efficient expression and purification method to obtain human recombinant CCT. BHK-21 cells were infected with a vaccinia virus expressing T7 RNA polymerase and transfected with eight plasmids, each encoding any one of the eight CCT subunits in the T7 RNA polymerase promoter/terminator unit. The CCT1 subunit was engineered to carry a hexa-histidine tag or FLAG tag in the internal loop region. Three clays later, cells were harvested for purification of the CCT complex through tag-dependent affinity chromatography and gel filtration. The purified recombinant CCT complexes were indistinguishable from the endogenous CCT purified from HeLa cells in terms of morphology and function. In conclusion, the co-expression system established in this study should be a simple and powerful tool for reconstitution of a large multi-subunit complex. (C) 2011 Elsevier Inc. All rights reserved.

Antibodies against acetylcholine receptors (AChRs) cause pathogenicity in myasthenia gravis (MG) patients through complement pathway-mediated destruction of postsynaptic membranes at neuromuscular junctions (NMJs). However, antibodies against muscle-specific kinase (MuSK), which constitute a major subclass of antibodies found in MG patients, do not activate the complement pathway. To investigate the pathophysiology of MUSK-MG and establish an experimental autoimmune MG (EAMG) model, we injected MuSK protein into mice deficient in complement component five (C5). MuSK-injected mice simultaneously developed severe muscle weakness, accompanied by an electromyographic pattern such as is typically observed in MG patients. In addition, we observed morphological and functional defects in the NMJs of EAMG mice, demonstrating that complement activation is not necessary for the onset of MuSK-MG. Furthermore, MUSK-injected mice exhibited acetylcholinesterase (AChE) inhibitor-evoked cholinergic hypersensitivity, as is observed in MuSK-MG patients, and a decrease in both AChE and the AChE-anchoring protein collagen Q at postsynaptic membranes. These findings suggest that MuSK is indispensable for the maintenance of NMJ structure and function, and that disruption of MuSK activity by autoantibodies causes MG. This mouse model of EAMG could be used to develop appropriate medications for the treatment of MuSK-MG in humans. (Am J Pathol 2012, 180:798-810; DOI: 10.1016/j.ajpath.2011.10.031)

The subcellular localization of biomolecules at high resolution has traditionally been investigated by combining transmission electron microscopy (TEM) and chemical staining with heavy metals or immuno-based labeling with gold-conjugated antibodies. Here, we employ genetically encoded tags to examine the localization of proteins in transfected cultured cells by TEM. We purified a fusion protein of postsynaptic density-95 (PSD-95) coupled to three tandem repeats of metallothionein (MT) (PDS-953MT) from COS7 cells grown in the presence of Cd2+. PSD-953MT was detected as black particles by TEM. To visualize the subcellular localization of PSD-953MT, expression constructs encoding this fusion protein were transfected into primary hippocampal neurons cultured in medium containing Cd2+. The subcellular accumulation of PSD-953MT and Cd2+ provided excellent contrast in TEM micrographs. To address if genetically encoded tags affect the function of the target proteins, we found that the conjugation of 3MT to PSD-95 did not alter its association with known binding partners. These results demonstrate that 3MT coordinating Cd2+ is a valuable genetically encoded tag to study the localization of proteins by TEM.

The observation of biological materials by transmission electron microscopy (TEM) frequently requires the use of negative staining and/or immuno-gold labeling to visualize or identify the specimen, but these techniques are limited. In contrast, genetic labeling with green fluorescent protein (GFP) and its homologs have led to rapid advances in the observation of proteins of interest in cells by light microscopy (LM). These fluorescent tags allow for the visualization of dynamic processes in live cells without the use of secondary reagents. Here, we report the development of an artificial metalloprotein fusion protein expressed in Escherichia coli grown in Cd2+-containing medium that allows for efficient protein detection by TEM without additional staining steps. We linked the subunits of the bacterial 14-mer protein GroEL with three repeats of metallothionein (3MT). The 3MT-fused GroEL (GroEL-14(3MT)) was successfully expressed in E. coli, and the purified protein included 250 cadmium atoms per molecule on average. Cd2+-bound GroEL-14(3MT) was detected by TEM in the absence of negative staining on a carbon grid, and the particle densities of GroEL-14(3MT) were much greater than those of untagged GroEL in vitreous ice. Taken together, our data indicate that the 3MT tag provides a promising means of allowing the identification of oligomeric proteins isolated from cells in the absence of other detection techniques. © The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved.

Pombe Cdc15 homology (PCH) proteins play an important role in a variety of actin-based processes, including clathrin-mediated endocytosis (CME). The defining feature of the PCH proteins is an evolutionarily conserved EFC/F-BAR domain for membrane association and tubulation. In the present study, we solved the crystal structures of the EFC domains of human FBP17 and CIP4. The structures revealed a gently curved helical-bundle dimer of similar to 220 angstrom in length, which forms filaments through end-to-end interactions in the crystals. The curved EFC dimer fits a tubular membrane with an similar to 600 angstrom diameter. We subsequently proposed a model in which the curved EFC filament drives tubulation. In fact, striation of tubular membranes was observed by phase-contrast cryo-transmission electron microscopy, and mutations that impaired filament formation also impaired membrane tubulation and cell membrane invagination. Furthermore, FBP17 is recruited to clathrin-coated pits in the late stage of CME, indicating its physiological role.