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1.  Bacteriophage P23-77 Capsid Protein Structures Reveal the Archetype of an Ancient Branch from a Major Virus Lineage 
Structure(London, England:1993)  2013;21(5):718-726.
It has proved difficult to classify viruses unless they are closely related since their rapid evolution hinders detection of remote evolutionary relationships in their genetic sequences. However, structure varies more slowly than sequence, allowing deeper evolutionary relationships to be detected. Bacteriophage P23-77 is an example of a newly identified viral lineage, with members inhabiting extreme environments. We have solved multiple crystal structures of the major capsid proteins VP16 and VP17 of bacteriophage P23-77. They fit the 14 Å resolution cryo-electron microscopy reconstruction of the entire virus exquisitely well, allowing us to propose a model for both the capsid architecture and viral assembly, quite different from previously published models. The structures of the capsid proteins and their mode of association to form the viral capsid suggest that the P23-77-like and adeno-PRD1 lineages of viruses share an extremely ancient common ancestor.
•High-resolution structures of the two major capsid proteins of bacteriophage P23-77•P23-77 capsid proteins exhibit a conserved single β-barrel core fold•P23-77 is an ancient relative of the double β-barrel lineage of viruses•Capsid model illustrates that P23-77 uses a novel method of organization
Rissanen et al. propose a model for the architecture and assembly of bacteriophage P23-77 quite different from those previously published. The capsid proteins and their mode of association to form the virus particle suggest that P23-77 share a common evolutionary origin with the PRD1/Adenovirus lineage.
PMCID: PMC3919167  PMID: 23623731
2.  Crystallization screening test for the whole-cell project on Thermus thermophilus HB8 
Three conventional robots were subjected to a crystallization screening test involving 18 proteins from T. thermophilus HB8 using the sitting- and hanging-drop vapour-diffusion and microbatch methods. The number of diffraction-quality crystals and the amount of time required to obtain visible crystals depended greatly on the robots used. The combined use of different robots, especially for protein samples exhibiting low crystallization success rates, significantly increased the chance of obtaining diffraction-quality crystals.
It was essential for the structural genomics of Thermus thermophilus HB8 to efficiently crystallize a number of proteins. To this end, three conventional robots, an HTS-80 (sitting-drop vapour diffusion), a Crystal Finder (hanging-drop vapour diffusion) and a TERA (modified microbatch) robot, were subjected to a crystallization condition screening test involving 18 proteins from T. thermophilus HB8. In addition, a TOPAZ (microfluidic free-interface diffusion) designed specifically for initial screening was also briefly examined. The number of diffraction-quality crystals and the time of appearance of crystals increased in the order HTS-80, Crystal Finder, TERA. With the HTS-80 and Crystal Finder, the time of appearance was short and the rate of salt crystallization was low. With the TERA, the number of diffraction-quality crystals was high, while the time of appearance was long and the rate of salt crystallization was relatively high. For the protein samples exhibiting low crystallization success rates, there were few crystallization conditions that were common to the robots used. In some cases, the success rate depended greatly on the robot used. The TOPAZ showed the shortest time of appearance and the highest success rate, although the crystals obtained were too small for diffraction studies. These results showed that the combined use of different robots significantly increases the chance of obtaining crystals, especially for proteins exhibiting low crystallization success rates. The structures of 360 of 944 purified proteins have been successfully determined through the combined use of an HTS-80 and a TERA.
PMCID: PMC2496871  PMID: 18540056
protein crystallization; crystallization screening; crystallization success rates
3.  Preliminary crystallographic analysis of the major capsid protein P2 of the lipid-containing bacteriophage PM2 
The viral capsid protein P2 of bacteriophage PM2 has been crystallized. Preliminary X-ray analysis demonstrates the position and orientation of the two trimers in the asymmetric unit.
PM2 (Corticoviridae) is a dsDNA bacteriophage which contains a lipid membrane beneath its icosahedral capsid. In this respect it resembles bacteriophage PRD1 (Tectiviridae), although it is not known whether the similarity extends to the detailed molecular architecture of the virus, for instance the fold of the major coat protein P2. Structural analysis of PM2 has been initiated and virus-derived P2 has been crystallized by sitting-nanodrop vapour diffusion. Crystals of P2 have been obtained in space group P21212, with two trimers in the asymmetric unit and unit-cell parameters a = 171.1, b = 78.7, c = 130.1 Å. The crystals diffract to 4 Å resolution at the ESRF BM14 beamline (Grenoble, France) and the orientation of the non-crystallographic threefold axes, the spatial relationship between the two trimers and the packing of the trimers within the unit cell have been determined. The trimers form tightly packed layers consistent with the crystal morphology, possibly recapitulating aspects of the arrangement of subunits in the virus.
PMCID: PMC1952355  PMID: 16511151
virus crystallography; lipid-containing bacteriophages; PRD1-adenoviral lineage
4.  Closely Related Archaeal Haloarcula hispanica Icosahedral Viruses HHIV-2 and SH1 Have Nonhomologous Genes Encoding Host Recognition Functions 
Journal of Virology  2012;86(9):4734-4742.
Studies on viral capsid architectures and coat protein folds have revealed the evolutionary lineages of viruses branching to all three domains of life. A widespread group of icosahedral tailless viruses, the PRD1-adenovirus lineage, was the first to be established. A double β-barrel fold for a single major capsid protein is characteristic of these viruses. Similar viruses carrying genes coding for two major capsid proteins with a more complex structure, such as Thermus phage P23-77 and haloarchaeal virus SH1, have been isolated. Here, we studied the host range, life cycle, biochemical composition, and genomic sequence of a new isolate, Haloarcula hispanica icosahedral virus 2 (HHIV-2), which resembles SH1 despite being isolated from a different location. Comparative analysis of these viruses revealed that their overall architectures are very similar except that the genes for the receptor recognition vertex complexes are unrelated even though these viruses infect the same hosts.
PMCID: PMC3347397  PMID: 22357274
5.  Draft Crystal Structure of the Vault Shell at 9-Å Resolution 
PLoS Biology  2007;5(11):e318.
Vaults are the largest known cytoplasmic ribonucleoprotein structures and may function in innate immunity. The vault shell self-assembles from 96 copies of major vault protein and encapsulates two other proteins and a small RNA. We crystallized rat liver vaults and several recombinant vaults, all among the largest non-icosahedral particles to have been crystallized. The best crystals thus far were formed from empty vaults built from a cysteine-tag construct of major vault protein (termed cpMVP vaults), diffracting to about 9-Å resolution. The asymmetric unit contains a half vault of molecular mass 4.65 MDa. X-ray phasing was initiated by molecular replacement, using density from cryo-electron microscopy (cryo-EM). Phases were improved by density modification, including concentric 24- and 48-fold rotational symmetry averaging. From this, the continuous cryo-EM electron density separated into domain-like blocks. A draft atomic model of cpMVP was fit to this improved density from 15 domain models. Three domains were adapted from a nuclear magnetic resonance substructure. Nine domain models originated in ab initio tertiary structure prediction. Three C-terminal domains were built by fitting poly-alanine to the electron density. Locations of loops in this model provide sites to test vault functions and to exploit vaults as nanocapsules.
Author Summary
Vaults are large barrel-shaped particles found in the cytoplasm in all mammalian cells, which may function in innate immunity. As naturally occurring nanoscale capsules, vaults may be useful objects to engineer as delivery vehicles. In this study, we propose an atomic structure for the thin outer shell of the vault. Using x-ray diffraction and computer modeling, we have inferred a draft atomic model for the major vault protein, which forms the shell-like enclosure of the vault. The shell is made up of 96 identical protein chains, each of 873 amino acid residues, folded into 14 domains. Each chain forms an elongated stave of half the vault, as well as the cap of the barrel-like shell. Our draft atomic model is essentially an atomic-level model for the entire 9.3-MDa vault shell, which offers a guide for protein engineering to test vault functions and to exploit vault particles as nanocapsules.
A draft atomic structure has been determined for the 9.3-MDa protein shell of the vault cytoplasmic particle, revealing stave-like polypeptides forming the barrel-like structure of the vault.
PMCID: PMC2229873  PMID: 18044992
6.  Insights into the Evolution of a Complex Virus from the Crystal Structure of Vaccinia Virus D13 
Structure(London, England:1993)  2011;19(7-12):1011-1020.
The morphogenesis of poxviruses such as vaccinia virus (VACV) sees the virion shape mature from spherical to brick-shaped. Trimeric capsomers of the VACV D13 protein form a transitory, stabilizing lattice on the surface of the initial spherical immature virus particle. The crystal structure of D13 reveals that this major scaffolding protein comprises a double β barrel “jelly-roll” subunit arranged as pseudo-hexagonal trimers. These structural features are characteristic of the major capsid proteins of a lineage of large icosahedral double-stranded DNA viruses including human adenovirus and the bacteriophages PRD1 and PM2. Structure-based phylogenetic analysis confirms that VACV belongs to this lineage, suggesting that (analogously to higher organism embryogenesis) early poxvirus morphogenesis reflects their evolution from a lineage of viruses sharing a common icosahedral ancestor.
► Poxvirus D13 acts as a scaffold for the morphogenesis of spherical immature virions ► D13 has a double “jelly-roll” structure, like other large DNA virus capsid proteins ► Structure-based phylogenetics places D13 into an icosahedral viral lineage ► Poxvirus morphogenesis reflects Vaccinia virus evolution from an icosahedral ancestor
PMCID: PMC3136756  PMID: 21742267
7.  Crystallization and initial X-ray diffraction studies of scaffolding protein (gp7) of bacteriophage ϕ29 
ϕ29 bacteriophage scaffolding protein (gp7) has been overproduced in E. coli, purified, crystallized and characterized by X-ray diffraction. Two distinct crystal forms were obtained and a diffraction data set was collected to 1.8 Å resolution.
The Bacillus subtilis bacteriophage ϕ29 scaffolding protein (gp7) has been crystallized by the hanging-drop vapour-diffusion method at 293 K. Two new distinct crystal forms that both differed from a previously crystallized and solved scaffolding protein were grown under the same conditions. Form I belongs to the primitive tetragonal space group P41212, with unit-cell parameters a = b = 77.13, c = 37.12 Å. Form II crystals exhibit an orthorhombic crystal form, with space group C222 and unit-cell parameters a = 107.50, b = 107. 80, c = 37.34 Å. Complete data sets have been collected to 1.78 and 1.80 Å for forms I and II, respectively, at 100 K using Cu Kα X-rays from a rotating-anode generator. Calculation of a V M value of 2.46 Å3 Da−1 for form I suggests the presence of one molecule in the asymmetric unit, corresponding to a solvent content of 50.90%, whereas form II has a V M of 4.80 Å3 Da−1 with a solvent content of 48.76% and two molecules in the asymmetric unit. The structures of both crystal forms are being determined by the molecular-replacement method using the coordinates of the published crystal structure of gp7.
PMCID: PMC1952437  PMID: 16511059
scaffolding protein; bacteriophage ϕ29
8.  Crystallization and preliminary X-ray diffraction studies of the prototypal homologue of mitoNEET (Tth-NEET0026) from the extreme thermophile Thermus thermophilus HB8 
A thermophilic bacterial homologue of mitoNEET (a mammalian mitochondrial outer membrane protein) from T. thermophilus HB8 (open reading frame TTHA0026; Tth-NEET0026) has been identified as a water-soluble prototypal [2Fe–2S] protein and crystallized. The bipyramidal crystals of the recombinant Tth-NEET0026 diffracted to 1.80 Å resolution using synchrotron radiation.
MitoNEET (a mammalian mitochondrial outer membrane protein) is a potential pharmacological and clinical target of the insulin-sensitizer pioglitazone. The thermophilic homologue of mitoNEET (TTHA0026) from Thermus thermophilus HB8 has been heterologously overproduced in Escherichia coli and purified as a water-soluble prototypal protein containing the mitoNEET-like [2Fe–2S] cluster. The resultant recombinant protein, named Tth-NEET0026, has been crystallized in its oxidized form by the hanging-drop vapour-diffusion method using 17%(w/v) polyethylene glycol 4000, 8.5%(v/v) 2-­propanol, 15%(v/v) glycerol and 0.085 M HEPES–NaOH pH 7.2. The dark reddish crystals diffracted to 1.80 Å resolution and belonged to the tetragonal space group P43212, with unit-cell parameters a = 45.51, c = 84.26 Å. The asymmetric unit contains one protein molecule.
PMCID: PMC2593688  PMID: 19052371
mitoNEET; [2Fe–2S] clusters; type II diabetes; Thermus thermophilus
9.  Cloning, expression, crystallization and preliminary crystallographic analysis of a pentapeptide-repeat protein (Rfr23) from the bacterium Cyanothece 51142 
A construct of the Cyanothece 51142 pentapeptide-repeat protein Rfr23 (Thr27–Asp174) has been cloned, overexpressed in E. coli and purified. Diffraction data that extend to 2.5 Å and preliminary analyses are reported for selenomethionine-labeled crystals that belong to space group I41.
A unique feature of cyanobacteria genomes is the abundance of genes that code for hypothetical proteins containing tandem pentapeptide repeats approximately described by the consensus motif A(N/D)LXX. To date, the structures of two pentapeptide-repeat proteins (PRPs) have been determined, with the tandem pentapeptide-repeat sequences observed to adopt a novel type of right-handed quadrilateral β-helix, or Rfr-fold, in both structures. One structure, Mycobacterium tuberculosis MfpA, is a 183-residue protein that contains 30 consecutive pentapeptide repeats and appears to offer antibiotic resistance by acting as a DNA mimic. The other structure, Cyanothece 51142 Rfr32, is a 167-residue protein that contains 21 consecutive pentapeptide repeats. The function of Rfr32, like the other 35 hypothetical PRPs identified in the genome of Cyanothece, is unknown. In an effort to understand the role of PRPs in cyanobacteria and to better characterize the structural properties of Rfr-folds with different amino-acid sequences, a second PRP from Cyanothece 51142, Rfr23, has been cloned, expressed and purified. Selenomethione-substituted protein was crystallized by vapor diffusion in hanging drops. Nearly complete SAD and native diffraction data sets were collected from these crystals to 2.5 and 2.1 Å resolution, respectively, using synchrotron radiation. The crystals belonged to space group I41, with unit-cell parameters a = b = 106.61, c = 53.37 Å, and one molecule per asymmetric unit. Preliminary analysis of the electron-density map from the SAD data shows that Rfr23 contains an Rfr-fold.
PMCID: PMC2225376  PMID: 17142909
cyanobacteria; pentapeptide-repeat protein; Rfr-fold; Cyanothece
10.  Crystallization and preliminary X-ray diffraction analysis of the multidrug efflux transporter NorM from Neisseria gonorrhoeae  
The multidrug efflux transporter NorM from N. gonorrhoeae has been crystallized and X-ray diffraction data have been collected to a resolution of 6.5 Å.
The crystallization and preliminary X-ray data analysis of the NorM multidrug efflux pump produced by Neisseria gonorrhoeae are reported. NorM is a cytoplasmic membrane protein that consists of 459 amino-acid residues. It is a member of the recently classified multidrug and toxic compound extrusion (MATE) family of transporters and recognizes a number of cationic toxic compounds such as ethidium bromide, acriflavin, 2-N-methylellipticinium and ciprofloxacin. Recombinant NorM protein was expressed in Escherichia coli and purified by metal-affinity and gel-filtration chromatography. The protein was crystallized using hanging-drop vapor diffusion. X-ray diffraction data were collected from cryocooled crystals at a synchrotron light source. The best crystal diffracted anisotropically to 3.8 Å and diffraction data were complete to 6.5 Å resolution. The space group was determined to be C2, with unit-cell parameters a = 81.5, b = 164.4, c = 111.5 Å.
PMCID: PMC2374251  PMID: 18391429
NorM; multidrug efflux transporter; drug resistance; Neisseria gonorrhoeae
11.  Sequence and Structural Characterization of Great Salt Lake Bacteriophage CW02, a Member of the T7-Like Supergroup 
Journal of Virology  2012;86(15):7907-7917.
Halophage CW02 infects a Salinivibrio costicola-like bacterium, SA50, isolated from the Great Salt Lake. Following isolation, cultivation, and purification, CW02 was characterized by DNA sequencing, mass spectrometry, and electron microscopy. A conserved module of structural genes places CW02 in the T7 supergroup, members of which are found in diverse aquatic environments, including marine and freshwater ecosystems. CW02 has morphological similarities to viruses of the Podoviridae family. The structure of CW02, solved by cryogenic electron microscopy and three-dimensional reconstruction, enabled the fitting of a portion of the bacteriophage HK97 capsid protein into CW02 capsid density, thereby providing additional evidence that capsid proteins of tailed double-stranded DNA phages have a conserved fold. The CW02 capsid consists of bacteriophage lambda gpD-like densities that likely contribute to particle stability. Turret-like densities were found on icosahedral vertices and may represent a unique adaptation similar to what has been seen in other extremophilic viruses that infect archaea, such as Sulfolobus turreted icosahedral virus and halophage SH1.
PMCID: PMC3421657  PMID: 22593163
12.  Crystallization and preliminary X-ray analysis of bacteriophage T4 UvsY recombination mediator protein 
UvsY, the recombination mediator protein of bacteriophage T4, has been crystallized in both native and selenium-substituted forms. X-ray diffraction data have been collected to 2.2 Å.
Bacteriophage T4 UvsY protein is considered to be the prototype of recombination mediator proteins, a class of proteins which assist in the loading of recombinases onto DNA. Wild-type and Se-substituted UvsY protein have been expressed and purified and crystallized by hanging-drop vapor diffusion. The crystals diffract to 2.4 Å using in-house facilities and to 2.2 Å at NSLS, Brookhaven National Laboratory. The crystals belong to space group P422, P4222, P4212 or P42212, the ambiguity arising from pseudo-centering, with unit-cell parameters a = b = 76.93, c = 269.8 Å. Previous biophysical characterization of UvsY indicates that it exists primarily as a hexamer in solution. Along with the absence of a crystallographic threefold, this suggests that the asymmetric unit of these crystals is likely to contain either three monomers, giving a solvent content of 71%, or six monomers, giving a solvent content of 41%.
PMCID: PMC2225191  PMID: 17012799
bacteriophage T4; UvsY; recombination mediator proteins
13.  Recombinant portal protein from Staphylococcus epidermidis bacteriophage CNPH82 is a 13-subunit oligomer 
Crystals of the portal protein from Staphylococcus epidermidis bacteriophage CNPH82, diffracting to ∼4.2 Å resolution, have been obtained. The protein is a 13-subunit oligomer both in solution and in the crystal.
The portal protein cn3 of bacteriophage CNPH82 is predicted to serve as a gateway for translocation of viral genome into preformed pro-capsid, like portal proteins from other double-stranded DNA tailed bacteriophages. The host of bacteriophage CNPH82 is the opportunistic human pathogenic bacterium Staphylococcus epidermidis, a major cause of nosocomial infections. The portal protein of this phage has been cloned, overexpressed and purified. Size-exclusion chromatography–multi-angle laser light scattering analysis has indicated that the portal protein contains ∼13 subunits. Crystals of the portal protein, diffracting to 4.2 Å, have been obtained. These crystals belong to the space group C2221 with the unit-cell parameters of a = 252.4, b = 367.0, c = 175.5 Å. The self-rotation function revealed the presence of a single 13-subunit oligomer in the asymmetric unit.
PMCID: PMC3490468  PMID: 23027764
portal protein; DNA translocation; bacteriophage CNPH82; oligomeric state
14.  The putative small terminase from the thermophilic dsDNA bacteriophage G20C is a nine-subunit oligomer 
The putative small terminase protein from the thermostable bacteriophage G20C has been produced, purified and crystallized.
The assembly of double-stranded DNA bacteriophages is dependent on a small terminase protein that normally plays two important roles. Firstly, the small terminase protein specifically recognizes viral DNA and recruits the large terminase protein, which makes the initial cut in the dsDNA. Secondly, once the complex of the small terminase, the large terminase and the DNA has docked to the portal protein, and DNA translocation into a preformed empty procapsid has begun, the small terminase modulates the ATPase activity of the large terminase. Here, the putative small terminase protein from the thermostable bacteriophage G20C, which infects the Gram-negative eubacterium Thermus thermophilus, has been produced, purified and crystallized. Size-exclusion chromatography–multi-angle laser light scattering data indicate that the protein forms oligomers containing nine subunits. Crystals diffracting to 2.8 Å resolution have been obtained. These belonged to space group P212121, with unit-cell parameters a = 94.31, b = 125.6, c = 162.8 Å. The self-rotation function and Matthews coefficient calculations are consistent with the presence of a nine-subunit oligomer in the asymmetric unit.
PMCID: PMC3729163  PMID: 23908032
putative small terminase; Thermus thermophilus; bacteriophage G20C
15.  Purification, crystallization and preliminary X-ray crystallographic study of the l-fuculose-1-phosphate aldolase (FucA) from Thermus thermophilus HB8 
The crystallization and preliminary X-ray diffraction analysis of the l-fuculose-1-phosphate aldolase (FucA) from T. thermophilus HB8. Native diffraction data set was collected to a resolution of 1.9 Å.
Fuculose phosphate aldolase catalyzes the reversible cleavage of l-fuculose-1-­phosphate to dihydroxyacetone phosphate and l-lactaldehyde. The protein from Thermus thermophilus HB8 is a biological tetramer with a subunit molecular weight of 21 591 Da. Purified FucA has been crystallized using sitting-drop vapour-diffusion and microbatch techniques at 293 K. The crystals belong to space group P4, with unit-cell parameters a = b = 100.94, c = 45.87 Å. The presence of a dimer of the enzyme in the asymmetric unit was estimated to give a Matthews coefficient (V M) of 2.7 Å3 Da−1 and a solvent content of 54.2%(v/v). Three-wavelength diffraction MAD data were collected to 2.3 Å from zinc-containing crystals. Native diffraction data to 1.9 Å resolution have been collected using synchrotron radiation at SPring-8.
PMCID: PMC1978142  PMID: 16511238
fuculose phosphate aldolase; class II aldolases; Thermus thermophilus HB8
16.  Crystallization and molecular-replacement studies of the monoclonal antibody mAbR310 specific for the (R)-HNE-modified protein 
Antigen-free Fab fragment of mAbR310, which recognizes (R)-HNE modified protein, has been crystallized. Initial phases have been obtained by molecular replacement.
4-Hydroxy-2-nonenal (HNE), a major racemic product of lipid peroxidation, reacts with histidine to form a stable HNE–histidine Michael addition-type adduct possessing three chiral centres in the cyclic hemiacetal structure. Monoclonal antibodies against HNE-modified protein have been widely used for assessing oxidative stress in vitro and in vivo. Here, the purification, crystallization and preliminary crystallographic analysis of a Fab fragment of novel monoclonal antibody R310 (mAbR310), which recognizes (R)-HNE-modified protein, are reported. The Fab fragment of mAbR310 was obtained by digestion with papain, purified and crystallized. Using hanging-drop vapour-diffusion crystallization techniques, crystals of mAbR310 Fab were obtained. The crystal belongs to the monoclinic space group C2 (unit-cell parameters a = 127.04, b = 65.31, c = 64.29 Å, β = 118.88°) and diffracted X-rays to a resolution of 1.84 Å. The asymmetric unit contains one molecule of mAbR310, with a corresponding crystal volume per protein weight of 2.51 Å3 Da−1 and a solvent content of 51.0%.
PMCID: PMC2243084  PMID: 16754982
mAbR310; monoclonal antibodies; Fab fragments
17.  Cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of the regulator AcrR from Escherichia coli  
The transcriptional regulator AcrR from Escherichia coli has been cloned, overexpressed, purified and crystallized and X-ray diffraction data have been collected to a resolution of 2.5 Å.
This paper describes the cloning, expression, purification and preliminary X-ray data analysis of the AcrR regulatory protein. The Escherichia coli AcrR is a member of the TetR family of transcriptional regulators. It regulates the expression of the AcrAB multidrug transporter. Recombinant AcrR with a 6×His tag at the C-terminus was expressed in E. coli and purified by metal-affinity chromatography. The protein was crystallized using hanging-drop vapor diffusion. X-ray diffraction data were collected from cryocooled crystals at a synchrotron light source. The best crystal diffracted to 2.5 Å. The space group was determined to be P32, with unit-cell parameters a = b = 46.61, c = 166.16 Å.
PMCID: PMC2225224  PMID: 17077502
AcrR; regulators
18.  Mechanism of Membranous Tunnelling Nanotube Formation in Viral Genome Delivery 
PLoS Biology  2013;11(9):e1001667.
Abrescia and colleagues demonstrate how the bacteriophage PRD1, a model membrane-containing virus, generates a self-polymerizing protein-lipid nanotube to deliver its viral genome to a host cell.
In internal membrane-containing viruses, a lipid vesicle enclosed by the icosahedral capsid protects the genome. It has been postulated that this internal membrane is the genome delivery device of the virus. Viruses built with this architectural principle infect hosts in all three domains of cellular life. Here, using a combination of electron microscopy techniques, we investigate bacteriophage PRD1, the best understood model for such viruses, to unveil the mechanism behind the genome translocation across the cell envelope. To deliver its double-stranded DNA, the icosahedral protein-rich virus membrane transforms into a tubular structure protruding from one of the 12 vertices of the capsid. We suggest that this viral nanotube exits from the same vertex used for DNA packaging, which is biochemically distinct from the other 11. The tube crosses the capsid through an aperture corresponding to the loss of the peripentonal P3 major capsid protein trimers, penton protein P31 and membrane protein P16. The remodeling of the internal viral membrane is nucleated by changes in osmolarity and loss of capsid-membrane interactions as consequence of the de-capping of the vertices. This engages the polymerization of the tail tube, which is structured by membrane-associated proteins. We have observed that the proteo-lipidic tube in vivo can pierce the gram-negative bacterial cell envelope allowing the viral genome to be shuttled to the host cell. The internal diameter of the tube allows one double-stranded DNA chain to be translocated. We conclude that the assembly principles of the viral tunneling nanotube take advantage of proteo-lipid interactions that confer to the tail tube elastic, mechanical and functional properties employed also in other protein-membrane systems.
Author Summary
Viral survival and propagation depend on the ability of the viruses to transfer their genetic material to a host cell. Viral genome delivery has been described for viruses that directly enclose their genome in a capsid or nucleocapsid, but not for internal membrane-containing viruses in which the genome is protected by a lipid vesicle enclosed by the icosahedral capsid. The latter infect organisms across the three domains of life. We use a range of electron microscopy techniques to reveal how one such virus, the bacteriophage PRD1, which uses gram negative bacteria as its host, delivers its double-stranded DNA to the bacteria across the cell envelope. The PRD1 bacteriophage is special in that it doesn't carry a rigid tail; rather it creates a tube tail when needed at the time of infection to pass its DNA through to the host. We now show that this tube formation is accomplished via concerted restructuring of the icosahedral capsid and remodeling of the internal icosahedral protein-rich virus membrane. We also find that this tail tube is studded with membrane-associated proteins and its internal diameter allows one double-stranded DNA chain to be injected. Finally, we capture PRD1 in 3-D with the proteo-lipidic tail piercing the gram-negative bacterial cell and shuttling its viral genome in vivo. These results provide insights into a new mechanism of viral genome delivery.
PMCID: PMC3782422  PMID: 24086111
19.  Purification, crystallization and preliminary characterization of a putative LmbE-like deacetylase from Bacillus cereus  
The BC1534 protein from B. cereus was purified and crystallized and a native X-ray diffraction data set was collected to 2.5 Å using synchrotron radiation.
The Bacillus cereus BC1534 protein, a putative deacetylase from the LmbE family, has been purified to homogeneity and crystallized using the hanging-drop vapour-diffusion method. Crystals of the 26 kDa protein grown from MPD and acetate buffer belong to space group R32, with unit-cell parameters a = b = 76.7, c = 410.5 Å (in the hexagonal setting). A complete native data set was collected to a resolution of 2.5 Å from a single cryoprotected crystal using synchrotron radiation. As BC1534 shows significant sequence homology with an LmbE-like protein of known structure from Thermus thermophilus, molecular replacement will be used for crystal structure determination.
PMCID: PMC2197166  PMID: 16511317
BC1534; deacetylases; LmbE family
20.  Evolutionary connection between the catalytic subunits of DNA-dependent RNA polymerases and eukaryotic RNA-dependent RNA polymerases and the origin of RNA polymerases 
The eukaryotic RNA-dependent RNA polymerase (RDRP) is involved in the amplification of regulatory microRNAs during post-transcriptional gene silencing. This enzyme is highly conserved in most eukaryotes but is missing in archaea and bacteria. No evolutionary relationship between RDRP and other polymerases has been reported so far, hence the origin of this eukaryote-specific polymerase remains a mystery.
Using extensive sequence profile searches, we identified bacteriophage homologs of the eukaryotic RDRP. The comparison of the eukaryotic RDRP and their homologs from bacteriophages led to the delineation of the conserved portion of these enzymes, which is predicted to harbor the catalytic site. Further, detailed sequence comparison, aided by examination of the crystal structure of the DNA-dependent RNA polymerase (DDRP), showed that the RDRP and the β' subunit of DDRP (and its orthologs in archaea and eukaryotes) contain a conserved double-psi β-barrel (DPBB) domain. This DPBB domain contains the signature motif DbDGD (b is a bulky residue), which is conserved in all RDRPs and DDRPs and contributes to catalysis via a coordinated divalent cation. Apart from the DPBB domain, no similarity was detected between RDRP and DDRP, which leaves open two scenarios for the origin of RDRP: i) RDRP evolved at the onset of the evolution of eukaryotes via a duplication of the DDRP β' subunit followed by dramatic divergence that obliterated the sequence similarity outside the core catalytic domain and ii) the primordial RDRP, which consisted primarily of the DPBB domain, evolved from a common ancestor with the DDRP at a very early stage of evolution, during the RNA world era. The latter hypothesis implies that RDRP had been subsequently eliminated from cellular life forms and might have been reintroduced into the eukaryotic genomes through a bacteriophage. Sequence and structure analysis of the DDRP led to further insights into the evolution of RNA polymerases. In addition to the β' subunit, β subunit of DDRP also contains a DPBB domain, which is, however, distorted by large inserts and does not harbor a counterpart of the DbDGD motif. The DPBB domains of the two DDRP subunits together form the catalytic cleft, with the domain from the β' subunit supplying the metal-coordinating DbDGD motif and the one from the β subunit providing two lysine residues involved in catalysis. Given that the two DPBB domains of DDRP contribute completely different sets of active residues to the catalytic center, it is hypothesized that the ultimate ancestor of RNA polymerases functioned as a homodimer of a generic, RNA-binding DPBB domain. This ancestral protein probably did not have catalytic activity and served as a cofactor for a ribozyme RNA polymerase. Subsequent evolution of DDRP and RDRP involved accretion of distinct sets of additional domains. In the DDRPs, these included a RNA-binding Zn-ribbon, an AT-hook-like module and a sandwich-barrel hybrid motif (SBHM) domain. Further, lineage-specific accretion of SBHM domains and other, DDRP-specific domains is observed in bacterial DDRPs. In contrast, the orthologs of the β' subunit in archaea and eukaryotes contains a four-stranded α + β domain that is shared with the α-subunit of bacterial DDRP, eukaryotic DDRP subunit RBP11, translation factor eIF1 and type II topoisomerases. The additional domains of the RDRPs remain to be characterized.
Eukaryotic RNA-dependent RNA polymerases share the catalytic double-psi β-barrel domain, containing a signature metal-coordinating motif, with the universally conserved β' subunit of DNA-dependent RNA polymerases. Beyond this core catalytic domain, the two classes of RNA polymerases do not have common domains, suggesting early divergence from a common ancestor, with subsequent independent domain accretion. The β-subunit of DDRP contains another, highly diverged DPBB domain. The presence of two distinct DPBB domains in two subunits of DDRP is compatible with the hypothesis that the ultimate ancestor of RNA polymerases was a RNA-binding DPBB domain that had no catalytic activity but rather functioned as a homodimeric cofactor for a ribozyme polymerase.
PMCID: PMC151600  PMID: 12553882
21.  Structure of Bombyx mori Densovirus 1, a Silkworm Pathogen▿‡ 
Journal of Virology  2011;85(10):4691-4697.
Bombyx mori densovirus 1 (BmDNV-1), a major pathogen of silkworms, causes significant losses to the silk industry. The structure of the recombinant BmDNV-1 virus-like particle has been determined at 3.1-Å resolution using X-ray crystallography. It is the first near-atomic-resolution structure of a virus-like particle within the genus Iteravirus. The particles consist of 60 copies of the 55-kDa VP3 coat protein. The capsid protein has a β-barrel “jelly roll” fold similar to that found in many diverse icosahedral viruses, including archaeal, bacterial, plant, and animal viruses, as well as other parvoviruses. Most of the surface loops have little structural resemblance to other known parvovirus capsid proteins. In contrast to vertebrate parvoviruses, the N-terminal β-strand of BmDNV-1 VP3 is positioned relative to the neighboring 2-fold related subunit in a “domain-swapped” conformation, similar to findings for other invertebrate parvoviruses, suggesting domain swapping is an evolutionarily conserved structural feature of the Densovirinae.
PMCID: PMC3126189  PMID: 21367906
22.  Thermostable multicopper oxidase from Thermus thermophilus HB27: crystallization and preliminary X-ray diffraction analysis of apo and holo forms 
A thermostable multicopper oxidase from Thermus thermophilus HB27 (Tth-MCO) has been successfully crystallized using the sitting-drop and hanging-drop vapour-diffusion methods.
A thermostable multicopper oxidase from Thermus thermophilus HB27 (Tth-MCO) was successfully crystallized using the sitting-drop and hanging-drop vapour-diffusion methods. Crystallization conditions and preliminary X-ray diffraction data to 1.5 Å resolution obtained using synchrotron radiation at 100 K are reported. The crystals belonged to space group C2221, with unit-cell parameters a = 93.6, b = 110.3, c = 96.3 Å. A monomer in the asymmetric unit yielded a Matthews coefficient (V M) of 2.60 Å3 Da−1 and a solvent content of 53%. An inactive enzyme form, apo-Tth-MCO, was also crystallized and diffraction data were collected to 1.7 Å resolution. In addition, a second inactive form of the enzyme, Hg-Tth-MCO, was obtained by soaking apo-Tth-MCO crystals with mercury(II) chloride and data were collected to a resolution of 1.7 Å.
PMCID: PMC3232148  PMID: 22139175
laccases; multicopper oxidases; Thermus thermophilus
23.  Cloning, purification crystallization and preliminary X-ray characterization of a conserved hypothetical protein XC6422 from Xanthomonas campestris  
A conserved hypothetical protein XC6422 from X. campestris pv. campestris has been overexpressed in E. coli, purified and crystallized. Crystals obtained from the purified recombinant protein showed a variety of forms that diffracted to at least 1.6 Å resolution.
Xanthomonas campestris pv. campestris is a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. Its genome contains approximately 4500 genes, roughly one third of which have no known structure and/or function. However, some genes of unknown function are highly conserved among several different bacterial genuses. XC6422 is one such conserved hypothetical protein and has been overexpressed in Escherichia coli, purified and crystallized in a variety of forms using the hanging-drop vapour-diffusion method. Crystals grew to approximately 2 × 1.5 × 0.4 mm in size after one week and diffracted to at least 1.6 Å resolution. They belong to the monoclinic space group C2, with one molecule per asymmetric unit and unit-cell parameters a = 75.8, b = 79.3, c = 38.2 Å, β = 109.4°. Determination of this structure may provide insights into the protein’s function.
PMCID: PMC1952462  PMID: 16511134
conserved hypothetical proteins; Xanthomonas campestris; structural genomics
24.  The Structure of a Putative Scaffolding Protein of Immature Poxvirus Particles as Determined by Electron Microscopy Suggests Similarity with Capsid Proteins of Large Icosahedral DNA Viruses▿  
Journal of Virology  2007;81(20):11075-11083.
Orf virus, the prototype parapoxvirus, is responsible for contagious ecthyma in sheep and goats. The central region of the viral genome codes for proteins highly conserved among vertebrate poxviruses and which are frequently essential for viral proliferation. Analysis of the recently published genome sequence of orf virus revealed that among such essential proteins, the protein orfv075 is an orthologue of D13, the rifampin resistance gene product critical for vaccinia virus morphogenesis. Previous studies showed that D13, arranged as “spicules,” is necessary for the formation of vaccinia virus immature virions, a mandatory intermediate in viral maturation. We have determined the three-dimensional structure of recombinant orfv075 at ∼25-Å resolution by electron microscopy of two-dimensional crystals. orfv075 organizes as trimers with a tripod-like main body and a propeller-like smaller domain. The molecular envelope of orfv075 shows unexpectedly good agreement to that of a distant homologue, VP54, the major capsid protein of Paramecium bursaria Chlorella virus type 1. Our structural analysis suggests that orfv075 belongs in the double-barreled capsid protein family found in many double-stranded DNA icosahedral viruses and supports the hypothesis that the nonicosahedral poxviruses and the large icosahedral DNA viruses are evolutionarily related.
PMCID: PMC2045580  PMID: 17670837
25.  Expression, purification and preliminary crystallographic analysis of human thyroid hormone responsive protein 
Human Thrsp has been crystallized as a prelude to the determination of its three-dimensional structure by X-ray crystallography.
Thyroid hormone responsive protein (Thrsp, also known as Spot 14 and S14) is a carbohydrate-inducible and thyroid-hormone-inducible nuclear protein specific to liver, adipose and lactating mammary tissues. Thrsp functions to activate genes encoding fatty-acid synthesis enzymes. Recent studies have shown that in some cancers human Thrsp (hS14) localizes to the nucleus and is amplified, suggesting that it plays a role in the regulation of lipogenic enzymes during tumourigenesis. Thrsp, a member of the Spot 14 superfamily, is an acidic homodimeric protein with no sequence similarity to other mammalian gene products and its biochemical function is elusive. To shed light on the structure–function relationship of this protein, human Thrsp was crystallized. Recombinant human Thrsp (hThrsp), the N-terminally truncated human Thrsp10–146 (hThrsp9) and their selenomethionyl (SeMet) derivatives were expressed in Escherichia coli, purified and crystallized using the hanging-drop vapour-diffusion method. Diffraction-quality crystals were grown at 293 K using Li2SO4 as a precipitant. Using synchrotron radiation, data for the hThrsp SeMet derivative, hThrsp9 and its SeMet derivative were collected to 4.0, 3.0 and 3.6 Å resolution, respectively, at 100 K. The crystals of full-length hThrsp and its SeMet derivative belonged to space group P41212, with approximate unit-cell parameters a = b = 123.9, c = 242.1 Å, α = β = γ = 90.0°. In contrast, the crystals of the truncated hThrsp9 and its SeMet derivative belonged to space group P212121, with approximate unit-cell parameters a = 91.6, b = 100.8, c = 193.7 Å, α = β = γ = 90.0°. A molecular-replacement solution calculated using a murine Spot 14 structure as a search model indicated the presence of six molecules per asymmetric unit, comprising three hThrsp homodimers.
PMCID: PMC3151134  PMID: 21821901
breast cancer; lipogenesis; Spot 14; S14; Thrsp

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