Identification of a 521-kilodalton protein (Gli521) involved in force generation or force transmission for Mycoplasma mobile gliding

Several mycoplasma species are known to glide on solid surfaces such as glass in the direction of the membrane protrusion, but the mechanism underlying this movement is unknown. To identify a novel protein involved in gliding, we raised monoclonal antibodies against a detergent-insoluble protein fraction of Mycoplasma mobile, the fastest glider, and screened the antibodies for inhibitory effects on gliding. Five monoclonal antibodies stopped the movement of gliding mycoplasmas, keeping them on the glass surface, and all of them recognized a large protein in immunoblotting. This protein, named Gli521, is composed of 4,738 amino acids, has a predicted molecular mass of 520,559 Da, and is coded downstream of a gene for another gliding protein, Gli349, which is known to be responsible for glass binding during gliding. Edman degradation analysis indicated that the N-terminal region is processed at the peptide bond between the amino acid residues at positions 43 and 44. Analysis of gliding mutants isolated previously revealed that the Gli521 protein is missing in a nonbinding mutant, m9, where the gli521 gene is truncated by a nonsense mutation at the codon for the amino acid at position 1170. Immunofluorescence and immunoelectron microscopy indicated that Gli521 localizes all around the base of the membrane protrusion, at the "neck," as previously observed for Gli349. Analysis of the inhibitory effects of the anti-Gli521 antibody on gliding motility revealed that this protein is responsible for force generation or force transmission, a role distinct from that of Gli349, and also suggested conformational changes of Gli349 and Gli521 during gliding.     

Identification of a 349-kilodalton protein (Gli349) responsible for cytadherence and glass binding during gliding of Mycoplasma mobile

Several mycoplasma species are known to glide in the direction of the membrane protrusion (head-like structure), but the mechanism underlying this movement is entirely unknown. To identify proteins involved in the gliding mechanism, protein fractions of Mycoplasma mobile were analyzed for 10 gliding mutants isolated previously. One large protein (Gli349) was observed to be missing in a mutant m13 deficient in hemadsorption and glass binding. The predicted amino acid sequence indicated a 348,758-Da protein that was truncated at amino acid residue 1257 in the mutant. Immunofluorescence microscopy with a monoclonal antibody showed that Gli349 is localized at the head-like protrusion's base, which we designated the cell neck, and immunoelectron microscopy established that the Gli349 molecules are distributed all around this neck. The number of Gli349 molecules on a cell was estimated by immunoblot analysis to be 450 +/- 200. The antibody inhibited both the hemadsorption and glass binding of M. mobile. When the antibody was used to treat gliding mycoplasmas, the gliding speed and the extent of glass binding were inhibited to similar extents depending on the concentration of the antibody. This suggested that the Gli349 molecule is involved not only in glass binding for gliding but also in movement. To explain the present results, a model for the mechanical cycle of gliding is discussed.     

Molecular shape and binding force of Mycoplasma mobile’s leg protein Gli349 revealed by an AFM study

Recent studies of the gliding bacteria Mycoplasma mobile have identified a family of proteins called the Gli family which was considered to be involved in this novel and yet fairly unknown motility system. The 349 kDa protein called Gli349 was successfully isolated and purified from the bacteria, and electron microscopy imaging and antibody experiments led to the hypothesis that it acts as the “leg” of M. mobile, responsible for attachment to the substrate as well as for gliding motility. However, more precise evidence of the molecular shape and function of this protein was required to asses this theory any further. In this study, an atomic force microscope (AFM) was used both as an imaging and a force measurement device to provide new information about Gli349 and its role in gliding motility. AFM images of the protein were obtained revealing a complex structure with both rigid and flexible parts, consistent with previous electron micrographs of the protein. Single-molecular force spectroscopy experiments were also performed, revealing that Gli349 is able to specifically bind to sialyllactose molecules and withstand unbinding forces around 70 pN. These findings strongly support the idea that Gli349 is the “leg” protein of M. mobile, responsible for binding and also most probably force generation during gliding motility.     

Triskelion Structure of the Gli521 Protein,Involved in the Gliding Mechanism of Mycoplasma mobile

Mycoplasma mobile binds to solid surfaces and glides smoothly and continuously by a unique mechanism. A huge protein, Gli521 (521 kDa), is involved in the gliding machinery, and it is localized in the cell neck, the base of the membrane protrusion. This protein is thought to have the role of force transmission. In this study, the Gli521 protein was purified from M. mobile cells, and its molecular shape was studied. Gel filtration analysis showed that the isolated Gli521 protein forms mainly a monomer in Tween 80-containing buffer and oligomers in Triton X-100-containing buffer. Rotary shadowing electron microscopy showed that the Gli521 monomer consisted of three parts: an oval, a rod, and a hook. The oval was 15 nm long by 11 nm wide, and the filamentous part composed of the rod and the hook was 106 nm long and 3 nm in diameter. The Gli521 molecules form a trimer, producing a “triskelion” reminiscent of eukaryotic clathrin, through association at the hook end. Image averaging of the central part of the triskelion suggested that there are stable and rigid structures. The binding site of a previously isolated monoclonal antibody on Gli521 images showed that the hook end and oval correspond to the C- and N-terminal regions, respectively. Partial digestion of Gli521 showed that the molecule could be divided into three domains, which we assigned to the oval, rod, and hook of the molecular image. The Gli521 molecule''s role in the gliding mechanism is discussed.Mycoplasmas are commensal and occasionally parasitic bacteria with small genomes that lack a peptidoglycan layer (31). Several mycoplasma species form membrane protrusions, such as the headlike structure in Mycoplasma mobile and the attachment organelle in Mycoplasma pneumoniae (15, 19, 21, 22, 25, 33, 34, 36). On solid surfaces, these species exhibit gliding motility in the direction of the protrusion; this motility is believed to be involved in the pathogenicity of mycoplasmas (12, 13, 16, 20, 21). Interestingly, mycoplasmas have no surface flagella or pili, and their genomes contain no genes related to other known bacterial motility systems. In addition, no homologs of motor proteins that are common in eukaryotic motility have been found (11).M. mobile, which was isolated from the gills of a freshwater fish in the early 1980s, is a fast gliding mycoplasma (14). It glides smoothly and continuously on glass at an average speed of 2.0 to 4.5 μm/s, or three to seven times the length of the cell per second, exerting a force of up to 27 pN (8, 9, 24, 25, 32). Previously, we identified huge proteins involved in this gliding mechanism that are localized at the so-called cell neck, the base of the membrane protrusion (17, 26, 30, 35, 37, 39); we also visualized the putative machinery and the binding protein (1, 18, 23) and identified both the direct energy source used and the direct binding target (10, 27, 38). The force generated by the gliding machinery may be supported from inside the cell by a cytoskeletal “jellyfish” structure (28, 29). On the basis of these results, we proposed a working model, called the centipede or power stroke model, where cells are propelled by “legs” composed of Gli349 that repeatedly catch and release sialic acids fixed on the glass surface (5, 19, 21). These legs are driven by the force exerted by P42 through Gli521 molecules, which is supported by the jellyfish structure, based on energy from ATP hydrolysis.The Gli521 protein, which has an unusually high molecular mass (521 kDa), is suggested to have the role of force transmission, because a monoclonal antibody against this protein stops gliding, keeping the cells on a solid surface (35). About 450 molecules are estimated to be clustered in the gliding machinery with other component proteins, although their alignment has not been clarified (35, 37, 39). In this study, we isolated the Gli521 protein and studied its molecular shape using electron microscopy (EM) and biochemical analyses in order to understand the gliding mechanism.     

A mitochondrial protein essential for the formation of the cytochrome c1-c complex. Isolation, purification, and properties

A new mitochondrial protein was isolated to pure form. This protein was indispensable for the formation of the cytochrome c1-c complex; hence, it was provisionally named the hinge protein for formation of the cytochrome c1-c complex, or for simplicity, merely called the hinge protein. The simplest method for the preparation of the pure protein involved essentially pH 5.5 treatment of high purity of "two-band" cytochrome c1 prepared from an improved method. The use of two band cytochrome c1 prepared by an improved method was preferred because the improved method apparently yielded less tight bonding between the heme-containing and colorless protein entities than that from the original methods (King, T. E. (1978) Methods Enzymol. 53, 181-191). The c1-c complex comprised 1 molar equivalent each of the hinge protein, "one-band" cytochrome c1 and cytochrome c. It was demonstrated by gel filtration chromatography that in the absence of the hinge protein, there was no complex formation between cytochromes c and one-band c1. In titration of the complex formed between one-band cytochrome c1 and cytochrome c with the hinge protein present by using the increase of the Soret-Cotton effect as a criterion (Chiang, Y. L., Kaminsky, L. S., and King, T. E. (1976) J. Biol. Chem. 251, 29-36), a sharp break was observed which showed the three species to be present in equivalent amounts. The hinge protein showed low extinction in the 280 nm region and exhibited poor color value and diffuse character of the band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis after staining with Coomassie brilliant blue. The molecular weight was found to be (i) 9,800 from sedimentation equilibrium, (ii) 11,000 from sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and (iii) 23,000 with a Stokes radius of 22.4 A from gel filtration chromatography estimated from a standard curve with proteins of known molecular parameters. The disparities in these data from the actual value of 9,175 from calculations based on amino acid sequence, as previously reported (Wakabayashi, S., Takeda, H., Matsubara, H., Kim, C. H., and King, T. E. (1982) J. Biochem. (Tokyo) 91, 2077-2085), have been discussed.     

Recruitment of the Arp2/3 complex to vinculin: coupling membrane protrusion to matrix adhesion

Cell migration involves many steps, including membrane protrusion and the development of new adhesions. Here we have investigated whether there is a link between actin polymerization and integrin engagement. In response to signals that trigger membrane protrusion, the actin-related protein (Arp)2/3 complex transiently binds to vinculin, an integrin-associated protein. The interaction is regulated, requiring phosphatidylinositol-4,5-bisphosphate and Rac1 activation, and is sufficient to recruit the Arp2/3 complex to new sites of integrin aggregation. Binding of the Arp2/3 complex to vinculin is direct and does not depend on the ability of vinculin to associate with actin. We have mapped the binding site for the Arp2/3 complex to the hinge region of vinculin, and a point mutation in this region selectively blocks binding to the Arp2/3 complex. Compared with WT vinculin, expression of this mutant in vinculin-null cells results in diminished lamellipodial protrusion and spreading on fibronectin. The recruitment of the Arp2/3 complex to vinculin may be one mechanism through which actin polymerization and membrane protrusion are coupled to integrin-mediated adhesion.     

Cytoskeleton of mollicutes

Mollicutes are a class of bacteria that lack a peptidoglycan layer but have various cell shapes. They perform chromosome segregation and binary fission in a well-organized manner. Especially, species with polarized cell morphology duplicate their membrane protrusion at a position adjacent to the original one and move the new protrusion laterally to the opposite end pole before cell division. The featured various cell shapes of Mollicutes are supported by cytoskeletal structures composed of proteins. Recent progress in the study of cytoskeletons of walled bacteria and genome sequencing has revealed that the cytoskeletons of Mollicutes are not common with those of other bacteria. Mollicutes have special cytoskeletal proteins and structures that are sometimes not shared even by other mollicute species.     

Melting of myosin rod as revealed by electron microscopy. II. Effects of temperature and pH on length and stability of myosin rod and its fragments

Effects of temperature and pH on intact rabbit and chicken myosin, isolated myosin rods, rabbit subfragment-2 (61 kDa, 53 kDa, and 34 kDa) and chicken light meromyosin (LMM) fragments were tested to induce a phase transition from alpha-helix to coil conformation, within the hinge region. The influence of temperature and pH were studied directly with length determination by electron microscopy. An increase of temperature to 50 degrees C yielded a shortening of 16 nm, 8 to 9 nm and 7 to 11 nm for intact myosin, isolated rods and long S-2 fragments, respectively. The length of the 34 kDa short S-2 and LMM fragments were unchanged. An increase of pH from neutral to pH 8.0 yielded values that were somewhat smaller, e.g. 12 nm, 6 nm and 6 to 8 nm for intact myosin, isolated rods and long S-2 fragments, respectively, whereas the 34 kDa short S-2 LMM fragments were also unaffected. Thus, melting and subsequent shortening is confined to the region between LMM and short S-2 segment, that is the hinge region. Alteration of temperature had a stronger shortening effect than alteration of pH, and shortening of long S-2 was more pronounced under physiological salt conditions as compared with high (0.3 M) salt. The shortening of rods in intact myosin amounted to twice the value observed with isolated rods. The amount of contraction was somewhat smaller in rods than in the 61 kDa and 53 kDa long S-2 fragments.     

Molecular structure of isolated MvspI, a variable surface protein of the fish pathogen Mycoplasma mobile

Mycoplasma mobile is a parasitic bacterium that causes necrosis in the gills of freshwater fishes. This study examines the molecular structure of its variable surface protein, MvspI, whose open reading frame encodes 2,002 amino acids. MvspI was isolated from mycoplasma cells by a biochemical procedure to 92% homogeneity. Gel filtration and analytical ultracentrifugation suggested that this protein is a cylinder-shaped monomer with axes of 66 and 2.7 nm. Rotary shadowing transmission electron microscopy of MvspI showed that the molecule is composed of two rods 30 and 45 nm long; the latter rod occasionally features a bulge. Immuno-electron microscopy and epitope mapping showed that the bulge end of the molecular image corresponds to the C terminus of the amino acid sequence. Partial digestion by various proteases suggested that the N-terminal part, comprised of 697 amino acids, is flexible. Analysis of the predicted amino acid sequence showed that the molecule features a lipoprotein and 16 repeats of about 90 residues; 15 positions exist between residues 88 and 1479, and the other position is between residues 1725 and 1807. The amino acid sequence of MvspI was mapped onto a molecular image obtained by electron microscopy. The present study is the first to elucidate the molecular shape of a variable surface protein of mycoplasma.     

Functional Compartmentalization of the Plasma Membrane of Neurons by a Unique Acyl Chain Composition of Phospholipids

In neurons, the plasma membrane is functionally separated into several distinct segments. Neurons form these domains by delivering selected components to and by confining them within each segment of the membrane. Although some mechanisms of the delivery are elucidated, that of the confinement is unclear. We show here that 1-oleoyl-2-palmitoyl-phosphatidylcholine (OPPC), a unique molecular species of phospholipids, is concentrated at the protrusion tips of several neuronal culture cells and the presynaptic area of neuronal synapses of the mouse brain. In PC12 cells, NGF-stimulated neuronal differentiation induces a phospholipase A1 activity at the protrusion tips, which co-localizes with the OPPC domain. Inhibition of the phospholipase A1 activity leads to suppression of phospholipid remodeling in the tip membrane and results in disappearance of the OPPC at the tips. In these cells, confinement of dopamine transporter and Gα_o proteins to the tip was also disrupted. These findings link the lateral distribution of the molecular species of phospholipids to the formation of functional segments in the plasma membrane of neurons and to the mechanism of protein confinement at the synapse.     

Bending of smooth muscle myosin rod

Electron microscopy of mammalian smooth muscle myosin rods showed them to be 153 +/- 7 nm (SD) long, and to bend sharply (greater than 90 degrees) but infrequently, and pH independently (range 6.5-9.5), at a single site 45 +/- 4 nm from one end of the molecule. Light meromyosin (LMM) preparations were 99 +/- 10 nm long, and showed no bends. Intrinsic viscosity vs temperature plots for rods and LMM indicated that neither fragment changed in flexibility in the range 4-40 degrees C. Peptide mapping in the presence and absence of SDS established that the proteolytic susceptibility of the hinge at the N terminus of LMM reflects the presence of locally different structure, and not simply a clustering of susceptible residues. The isolated smooth muscle myosin rod thus contains only a single hinge, having significant stiffness, and lacks the second bend seen under certain conditions in the intact molecule.     

The Symmetrical Structure of Structural Maintenance of Chromosomes (SMC) and MukB Proteins: Long,Antiparallel Coiled Coils,Folded at a Flexible Hinge

Structural maintenance of chromosomes (SMC) proteins function in chromosome condensation and several other aspects of DNA processing. They are large proteins characterized by an NH₂-terminal nucleotide triphosphate (NTP)-binding domain, two long segments of coiled coil separated by a hinge, and a COOH-terminal domain. Here, we have visualized by EM the SMC protein from Bacillus subtilis (BsSMC) and MukB from Escherichia coli, which we argue is a divergent SMC protein. Both BsSMC and MukB show two thin rods with globular domains at the ends emerging from the hinge. The hinge appears to be quite flexible: the arms can open up to 180°, separating the terminal domains by 100 nm, or close to near 0°, bringing the terminal globular domains together.A surprising observation is that the ∼300–amino acid–long coiled coils are in an antiparallel arrangement. Known coiled coils are almost all parallel, and the longest antiparallel coiled coils known previously are 35–45 amino acids long. This antiparallel arrangement produces a symmetrical molecule with both an NH₂- and a COOH-terminal domain at each end. The SMC molecule therefore has two complete and identical functional domains at the ends of the long arms. The bifunctional symmetry and a possible scissoring action at the hinge should provide unique biomechanical properties to the SMC proteins.     

Ultrastructure of the zonula adherens revealed by rapid-freeze deep-etching

The zonula adherens (ZA) in adult chicken retinal pigment epithelium was examined with cryo-electron microscopic methods. Deep-etching of the cross-fractured ZA showed globules in the intercellular space. These globules apparently correspond to the electron-dense structure seen in thin sections. Deep-etching of obliquely fractured ZA further revealed rod-like structures extending from the extracellular surface into the intercellular space. These rods (mean approximately 9 nm thick, approximately 20 nm long) were straight and sometimes divided into two or three segments. The rods typically canted at approximately 60 degrees with respect to the plasma membrane, and they were often connected to the intercellular globules at their distal ends. When the rods are compared with the isolated cadherins reported previously, it is suggested that a combination of a rod and a globule may represent an extracellular part of cadherin. Membrane particles were observed on the P-face of the ZA plasma membrane, and their distribution density was approximately seven times that of the rods. The freeze-etching also revealed a characteristic particle complex on the ZA cytoplasmic surface, which may represent the cytosolic proteins linking cadherins to actin bundles.     

Pollen wall development in Eucommia ulmoides (Eucommiaceae)

In Eucommia the foot layer plays a prominent part in microspore wall development. Bacules (>100 nm) are rods originating within the foot layer. Small bacules (diameter ca. 10 nm) form at the same time as the foot layer. The tectum is considered to be made up of these microbacular rods. Spinules ar continuous with the bacules. An endexine is differentiated during a middle to late microspore stage. Except for the pore the furrow includes tectum and foot layer on the endexine. Since the furrow consists of a region of reduced foot layer and the reduction is gradual near the polar ends of furrows, assessment of furrow length depends to some extent upon variations in exine infolding. The pore is well defined, but, because it is crossed by lamellations of the endexine and foot layer and often overlaid by tongues and bridges of foot layer plus tectum (including spinules) it is obscured from view using either light microscopy or scanning electron microscopy.     

Nephrotic syndrome and subepithelial deposits in a mouse model of immune-mediated anti-podocyte glomerulonephritis

Subepithelial immune complex deposition in glomerular disease causes local inflammation and proteinuria by podocyte disruption. A rat model of membranous nephropathy, the passive Heymann nephritis, suggests that Abs against specific podocyte Ags cause subepithelial deposit formation and podocyte foot process disruption. In this study, we present a mouse model in which a polyclonal sheep anti-mouse podocyte Ab caused subepithelial immune complex formation. Mice developed a nephrotic syndrome with severe edema, proteinuria, hypoalbuminemia, and elevated cholesterol and triglycerides. Development of proteinuria was biphasic: an initial protein loss was followed by a second massive increase of protein loss beginning at approximately day 10. By histology, podocytes were swollen. Electron microscopy revealed 60-80% podocyte foot process effacement and subepithelial deposits, but no disruption of the glomerular basement membrane. Nephrin and synaptopodin staining was severely disrupted, and podocyte number was reduced in anti-podocyte serum-treated mice, indicating severe podocyte damage. Immunohistochemistry detected the injected anti-podocyte Ab exclusively along the glomerular filtration barrier. Immunoelectron microscopy localized the Ab to podocyte foot processes and the glomerular basement membrane. Similarly, immunohistochemistry localized mouse IgG to the subepithelial space. The third complement component (C3) was detected in a linear staining pattern along the glomerular basement membrane and in the mesangial hinge region. However, C3-deficient mice were not protected from podocyte damage, indicating a complement-independent mechanism. Twenty proteins were identified as possible Ags to the sheep anti-podocyte serum by mass spectrometry. Together, these data establish a reproducible model of immune-mediated podocyte injury in mice with subepithelial immune complex formation.     

Isolation and characterization of P1 adhesin, a leg protein of the gliding bacterium Mycoplasma pneumoniae

Mycoplasma pneumoniae, a pathogen causing human pneumonia, binds to solid surfaces at its membrane protrusion and glides by a unique mechanism. In this study, P1 adhesin, which functions as a "leg" in gliding, was isolated from mycoplasma culture and characterized. Using gel filtration, blue-native polyacrylamide gel electrophoresis (BN-PAGE), and chemical cross-linking, the isolated P1 adhesin was shown to form a complex with an accessory protein named P90. The complex included two molecules each of P1 adhesin and P90 (protein B), had a molecular mass of about 480 kDa, and was observed by electron microscopy to form 20-nm-diameter spheres. Partial digestion of isolated P1 adhesin by trypsin showed that the P1 adhesin molecule can be divided into three domains, consistent with the results from trypsin treatment of the cell surface. Sequence analysis of P1 adhesin and its orthologs showed that domain I is well conserved and that a transmembrane segment exists near the link between domains II and III.