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PubMed1996

A hydrophobic heptad repeat of the core protein of woodchuck hepatitis virus is required for capsid assembly.

The capsid particle of hepadnaviruses is assembled from its dimer precursors. However, the mechanism of the protein-protein interaction is still poorly understood. A small region in the capsid protein of woodchuck hepatitis virus (WHV) contains four hydrophobic residues, including leucine 101, leucine 108, valine 115, and phenylalanine 122, that are conserved and spaced every seventh residue in the primary sequence to form a hydrophobic heptad repeat (hhr). A hydrophobic force often plays an important role in the interaction of proteins. Therefore, to investigate the role of this region in capsid assembly, we individually changed the codons specifying these four hydrophobic amino acids to codons specifying alanine or proline. In addition, we examined the in vivo infectivity of a WHV genome bearing a naturally occurring single amino acid change (histidine 104-->proline) in the hhr region. The phenotype of each altered genome was determined in both eukaryotic and prokaryotic systems by a capsid protein assay and electron microscopic examination. We show that replacement of any one of the four hydrophobic residues with alanine did not prevent capsid assembly. However, assembled capsid particles were not detected if combinations of any two of the four residues were substituted with alanines or if the spacing of these four hydrophobic residues was changed. An individual introduction of a proline (which dramatically changes the secondary structure of proteins) into different positions of this small region also abolished capsid assembly in vitro or viral replication in vivo. These results suggested that the hhr region of the core protein of WHV was critical for capsid assembly.

Amino Acid Sequence↗
PubMed1999

Packaging-competent capsids of a herpes simplex virus temperature-sensitive mutant have properties similar to those of in vitro-assembled procapsids.

Newcomb and coworkers (W. W. Newcomb, F. L. Homa, D. R. Thomsen, F. P. Booy, B. L. Trus, A. C. Steven, J. V. Spencer, and J. C. Brown, J. Mol. Biol. 263:432-446, 1996; W. W. Newcomb, F. L. Homa, D. R. Thomsen, Z. Ye, and J. C. Brown, J. Virol. 68:6059-6063, 1994) have recently described an in vitro herpes simplex virus (HSV) capsid assembly product which, because of certain parallels between its properties and those of bacteriophage proheads, they have designated the procapsid. As in their bacteriophage counterparts, there are marked differences between the structures of the two types of particle, and conversion from the procapsid to the capsid form requires extensive reconfiguration of the subunits. This reconfiguration occurs spontaneously upon extended in vitro incubation. One of the distinctive features of the HSV procapsids is that, unlike mature capsids, they are unstable and disassemble upon storage at 2 degrees C. Using a mutant of HSV type 1 (ts1201), which has a lesion in the protease responsible for maturational cleavage of the scaffolding protein, we have demonstrated that capsids present within cells infected at nonpermissive temperatures are also cryosensitive and disappear if the cells are incubated at 0 degrees C. This suggests that ts1201 capsids may resemble procapsids in structure. However, ts1201 capsids remain cryosensitive following extended incubation at an elevated temperature and, therefore, do not appear to undergo the spontaneous reconfiguration seen with in vitro-assembled procapsids. The lesion in ts1201 is reversible, and capsids formed at the nonpermissive temperature can undergo maturational cleavage and go on to form infectious virions following downshift to permissive temperatures. The sensitivity of ts1201 capsids to low temperatures is closely correlated with the cleavage status of the scaffolding protein, suggesting that proteolysis may act to trigger their conversion to the stable form. The experiments described here provide the firmest evidence yet that the procapsid has a biologically relevant role in the virus life cycle.

Animals↗
PubMed2000

Rubella virus capsid associates with host cell protein p32 and localizes to mitochondria.

Togavirus nucleocapsids have a characteristic icosahedral structure and are composed of multiple copies of a capsid protein complexed with genomic RNA. The assembly of rubella virus nucleocapsids is unique among togaviruses in that the process occurs late in virus assembly and in association with intracellular membranes. The goal of this study was to identify host cell proteins which may be involved in regulating rubella virus nucleocapsid assembly through their interactions with the capsid protein. Capsid was used as bait to screen a CV1 cDNA library using the yeast two-hybrid system. One protein that interacted strongly with capsid was p32, a cellular protein which is known to interact with other viral proteins. The interaction between capsid and p32 was confirmed using a number of different in vitro and in vivo methods, and the site of interaction between these two proteins was shown to be at the mitochondria. Interestingly, overexpression of the rubella virus structural proteins resulted in clustering of the mitochondria in the perinuclear region. The p32-binding site in capsid is a potentially phosphorylated region that overlaps the viral RNA-binding domain of capsid. Our results are consistent with the possibility that the interaction of p32 with capsid plays a role in the regulation of nucleocapsid assembly and/or virus-host interactions.

Animals↗
PubMed2000

A null mutation in the UL36 gene of herpes simplex virus type 1 results in accumulation of unenveloped DNA-filled capsids in the cytoplasm of infected cells.

The UL36 open reading frame (ORF) encodes the largest herpes simplex virus type 1 (HSV-1) protein, a 270-kDa polypeptide designated VP1/2, which is also a component of the virion tegument. A null mutation was generated in the UL36 gene to elucidate its role in the virus life cycle. Since the UL36 gene specifies an essential function, complementing cell lines transformed for sequences encoding the UL36 ORF were made. A mutant virus, designated KDeltaUL36, that encodes a null mutation in the UL36 gene was isolated and propagated in these cell lines. When noncomplementing cells infected with KDeltaUL36 were analyzed, both terminal genomic DNA fragments and DNA-containing capsids (C capsids) were detected; therefore, UL36 is not required for cleavage or packaging of DNA. Sedimentation analysis of lysates from mutant-infected cells revealed the presence of particles that have the physical characteristics of C capsids. In agreement with this, polypeptide profiles of the mutant particles revealed an absence of the major envelope and tegument components. Ultrastructural analysis revealed the presence of numerous unenveloped DNA containing capsids in the cytoplasm of KDeltaUL36-infected cells. The UL36 mutant particles were tagged with the VP26-green fluorescent protein marker, and their movement was monitored in living cells. In KDeltaUL36-infected cells, extensive particulate fluorescence corresponding to the capsid particles was observed throughout the cytosol. Accumulation of fluorescence at the plasma membrane which indicated maturation and egress of virions was observed in wild-type-infected cells but was absent in KDeltaUL36-infected cells. In the absence of UL36 function, DNA-filled capsids are produced; these capsids enter the cytosol after traversing the nuclear envelope and do not mature into enveloped virus. The maturation and egress of the UL36 mutant particles are abrogated, possibly due to a late function of this complex polypeptide, i.e., to target capsids to the correct maturation pathway.

Capsid↗
PubMed2001

Identification of a conserved residue of foamy virus Gag required for intracellular capsid assembly.

In contrast to all retroviruses but similar to the hepatitis B virus, foamy viruses (FV) require expression of the envelope protein for budding of intracellular capsids from the cell, suggesting a specific interaction between the Gag and Env proteins. Capsid assembly occurs in the cytoplasm of infected cells in a manner similar to that for the B- and D-type viruses; however, in contrast to these retroviruses, FV Gag lacks an N-terminal myristylation signal and capsids are not targeted to the plasma membrane (PM). We have found that mutation of an absolutely conserved arginine (Arg) residue at position 50 to alanine (R50A) of the simian foamy virus SFV cpz(hu) inhibits proper capsid assembly and abolishes viral budding even in the presence of the envelope (Env) glycoproteins. Particle assembly and extracellular release of virus can be restored to this mutant with the addition of an N-terminal Src myristylation signal (Myr-R50A), presumably by providing an alternate site for assembly to occur at the PM. In addition, the strict requirement of Env expression for capsid budding can be bypassed by addition of a PM-targeting signal to Gag. These results suggest that intracellular capsid assembly may be mediated by a signal akin to the cytoplasmic targeting and retention signal CTRS found in Mason-Pfizer monkey virus and that FV Gag has the inherent ability to assemble capsids at multiple sites like conventional retroviruses. The necessity of Env expression for particle egress is most probably due to the lack of a membrane-targeting signal within FV Gag to direct capsids to the PM for release and indicates that Gag-Env interactions are essential to drive particle budding.

Amino Acid Sequence↗
PubMed2002

The VP1 N-terminal sequence of canine parvovirus affects nuclear transport of capsids and efficient cell infection.

The unique N-terminal region of the parvovirus VP1 capsid protein is required for infectivity by the capsids but is not required for capsid assembly. The VP1 N terminus contains a number of groups of basic amino acids which resemble classical nuclear localization sequences, including a conserved sequence near the N terminus comprised of four basic amino acids, which in a peptide can act to transport other proteins into the cell nucleus. Testing with a monoclonal antibody recognizing residues 2 to 13 of VP1 (anti-VP1-2-13) and with a rabbit polyclonal serum against the entire VP1 unique region showed that the VP1 unique region was not exposed on purified capsids but that it became exposed after treatment of the capsids with heat (55 to 75 degrees C), or urea (3 to 5 M). A high concentration of anti-VP1-2-13 neutralized canine parvovirus (CPV) when it was incubated with the virus prior to inoculation of cells. Both antibodies blocked infection when injected into cells prior to virus inoculation, but neither prevented infection by coinjected infectious plasmid DNA. The VP1 unique region could be detected 4 and 8 h after the virus capsids were injected into cells, and that sequence exposure appeared to be correlated with nuclear transport of the capsids. To examine the role of the VP1 N terminus in infection, we altered that sequence in CPV, and some of those changes made the capsids inefficient at cell infection.

Amino Acid Sequence↗
PubMed2006

[Development of intranuclear inclusions of human polyomavirus JC. Capsid proteins are assembled into virions at the PML nuclear bodies].

Human polyomavirus JC (JCV) is a causative agent for progressive multifocal leukoencephalopathy, a fatal demyelinating disorder. The viruses form intranuclear viral inclusions in infected oligodendrocytes. The outer capsid of JCV is thought to be composed of 360 molecules of major capsid protein VP1, and minor capsid proteins VP2 and VP3 in an appropriate ratio. However, the regulatory mechanisms of gene expression for the capsid proteins, their nuclear transport, and formation of viral inclusions are not well understood. We have recently clarified the following regarding the mechanism underlying JCV virion assembly; (i) major and minor capsid proteins are synthesized from messenger RNAs, the expression ratio of which is determined by alternative splicing, (ii) messenger RNAs for the major and minor capsid proteins are polycistronic, and their translation occurs downstream of the regulatory protein, agnoprotein, (iii) major and minor capsid proteins are translocated to the nucleus in a cooperative manner and accumulate at the dot-shaped intranuclear structures called promyelocytic leukemia nuclear bodies (PML-NBs), (iv) efficient viral replication can occur at the PML-NBs, where capsid assembly is likely to be associated with viral DNA replication. PML-NBs are the sites for expression of important nuclear functions for the host cells. The finding that the target of JCV infection is the PML-NB may contribute greatly to our understanding of the mechanism underlying cellular degeneration, which occurs after the formation of intranuclear viral inclusions.

Capsid Proteins↗
PubMed1980

The rolling circle . capsid complex as an intermediate in phi X DNA replication and viral assembly.

Late in the life cycle of the single-stranded DNA phage phi X, the synthesis of positive strand DNA is coupled to the maturation of progeny virions. DNA synthesis and packaging take place in a replication-assembly complex, which we have purified to homogeneity and characterized. The following conclusions can be drawn: 1. The DNA component of the replication-assembly complex is a rolling circle with a single-stranded DNA tail which is less than or equal to genome length. 2. The major protein component of the replication-assembly complex is an intact viral capsid, as shown by gel analysis of 35S-labeled complexes. As replication proceeds at the DNA growing point, the positive strand tail of the rolling circle is displaced directly into the capsid. In addition to the capsid, the complex contains at least 1 molecule of the phi X gene A nicking protein, which appears to be covalently linked to the DNA. 3. The rolling circle . capsid complex can be purified to homogeneity by taking advantage of its uniform sedimentation velocity (35 S) and its uniform density upon equilibrium centrifugation in CsCl (1.50 g/cc). 4. The replication-assembly complex can be visualized in the electron microscope. An electron-dense particle, which has the dimensions of a viral capsid, is observed attached to a rolling circle at the DNA growing point. 5. Hybridization of specific phi X restriction fragments to the deproteinized, single-stranded tails of intact rolling circles has allowed the use of these replicating intermediates to determine both the origin/terminus and the direction of phi X positive strand DNA synthesis. The ends of the rolling circle tails map in the Hae III restriction Fragment Z6b, at the position on the phi X genome at which the gene A endonuclease is known to cut. This result indicates that this endonuclease participates in the "termination" of each round of synthesis by cutting off unit-length viral genomes. 6. Rolling circle . capsid complexes were also isolated from two other icosahedral, single-stranded DNA phages: G4 and St-1. The rolling circle . capsid complex seen in the case of the single-stranded DNA phages represents the first example of a structure in which DNA synthesis and viral assembly occur in a coupled manner. This tight coordination explains why double-stranded DNA circles are the net product of synthesis early in the viral life cycle while only single-stranded DNA circles are produced later. The single-stranded tails of the rolling circle intermediates are available for conversion to the duplex state at early times, whereas the concentration of preformed capsids later is high enough to bind to all of the replicating molecules and package the emerging positive strand DNA.

Bacteriophage phi X 174↗
PubMed1975

Chemical accessibility of tyrosyl and lysyl residues in turnip yellow mosaic virus capsids.

The chemical accessibility of tyrosyl residues in TYMV capsids was studied by spectrophotometric titration and with the nitrating agent tetranitromethane. That of the lysyl residues was probed with trinitrobenzenesulfonate. Attempts to test their accessibility in virions were also made. Since some of these reactions were accompanied by structural changes, degradation of the particles were monitored with ultracentrifugation and light-scattering measurements. Alkaline titration of TYMV capsids induced ionization of two of the three tyrosyl residues per subunit at pH 11.3, but the third tyrosyl ionized with an apparent pK of 12.65, concomitantly with the degradation of the capsids. Reaction with tetranitromethane suggested that one tyrosyl residue per subunit can easily be nitrated and initiates degradation, after which the remaining residues also react. In intact capsids, five out of seven lysyl residues per subunit reacted readily with trinitrobenzenesulfonate. The other two lysyl residues were trinitrophenylated only after degradation of the capsids. On the other hand, all seven lysyl residues per subunit were easily trinitrophenylated in virions, during which reaction the virions disintegrated. The demonstrated chemical inaccessibility of specific numbers of tyrosyl and lysyl residues in TYMV capsids and the observed structural consequences to the capsids when the residues were made to react are consistent with previously published properties of the cysteinyl and tryptophanyl residues. The findings suggest that in the capsid the central region of the TYMV polypeptide chain is buried and might represent a site of contact between neighboring subunits.

Binding Sites↗
PubMed2004

Sindbis virus nucleocapsid assembly: RNA folding promotes capsid protein dimerization.

In Sindbis virus, initiation of nucleocapsid core assembly begins with recognition of the encapsidation signal of the viral RNA genome by capsid protein. This nucleation event drives the recruitment of additional capsid proteins to fully encapsidate the genome, generating an icosahedral nucleocapsid core. The encapsidation signal of the Sindbis virus genomic RNA has previously been localized to a 132-nucleotide region of the genome within the coding region of the nsP1 protein, and the RNA-binding activity of the capsid was previously mapped to a central region of the capsid protein. It is unknown how capsid protein binding to encapsidation signal leads to ordered oligomerization of capsid protein and nucleocapsid core assembly. To address this question, we have developed a mobility shift assay to study this interaction. We have characterized a 32 amino acid peptide capable of recognizing the Sindbis virus encapsidation signal RNA. Using this peptide, we were able to observe a conformational change in the RNA induced by capsid protein binding. Binding is tight (K(d)(app) = 12 nM), and results in dimerization of the capsid peptide. Mutational analysis reveals that although almost every predicted secondary structure within the encapsidation signal is required for efficient protein binding, the identities of the bases within the helices and hairpin turns of the RNA do not need to be maintained. In contrast, two purine-rich loops are essential for binding. From these data, we have developed a model in which the encapsidation signal RNA adopts a highly folded structure and this folding process directs early events in nucleocapsid assembly.

Base Pairing↗
PubMed2006

Electrostatic properties of cowpea chlorotic mottle virus and cucumber mosaic virus capsids.

Electrostatic properties of cowpea chlorotic mottle virus (CCMV) and cucumber mosaic virus (CMV) were investigated using numerical solutions to the Poisson-Boltzmann equation. Experimentally, it has been shown that CCMV particles swell in the absence of divalent cations when the pH is raised from 5 to 7. CMV, although structurally homologous, does not undergo this transition. An analysis of the calculated electrostatic potential confirms that a strong electrostatic repulsion at the calcium-binding sites in the CCMV capsid is most likely the driving force for the capsid swelling process during the release of calcium. The binding interaction between the encapsulated genome material (RNA) inside of the capsid and the inner capsid shell is weakened during the swelling transition. This probably aids in the RNA release process, but it is unlikely that the RNA is released through capsid openings due to unfavorable electrostatic interaction between the RNA and capsid inner shell residues at these openings. Calculations of the calcium binding energies show that Ca(2+) can bind both to the native and swollen forms of the CCMV virion. Favorable binding to the swollen form suggests that Ca(2+) ions can induce the capsid contraction and stabilize the native form.

Binding Sites↗
PubMed1995

The capsid size-determining protein Sid forms an external scaffold on phage P4 procapsids.

Although the phages P2 and P4 build their capsids from the same precursor, the product of the P2 N gene, the two capsids differ in size: P2 builds a 60 nm, T = 7 capsid from 420 subunits, whereas P4 makes a 45 nm, T = 4 capsid from 240 subunits. This difference leads to substantial changes in shell geometry and subunit interactions. Previous results have demonstrated that the P4 sid gene is responsible for the assembly of P4-sized shells. We have used cryo-electron microscopy and image reconstruction to determine the structure of a putative assembly intermediate of P4 capsids, produced in vivo from cloned genes. We demonstrate that Sid forms a P4-specific scaffold with icosahedral symmetry on the outside of the procapsid-like particles. The Sid molecules (60 or 120 copies) form lofty arches that interact with the gpN hexamers on the icosahedral 2-fold axes, and connect as trimers over the 3-fold axes, forming a continuous dodecahedrally shaped outer cage. The gpN shell inside the Sid cage is approximately 40 nm wide, consistent with the previously suggested maturational expansion. The main difference with respect to the mature P4 capsids is found in the hexamers, which appear strongly elongated and more protruding than in the mature shell. These and previous results are discussed in the light of a model for regulation of capsid size.

Amino Acid Sequence↗
PubMed1996

Capsid targeting sequence targets foreign proteins into bacteriophage T4 and permits proteolytic processing.

A membrane-independent morphogenetic viral signal peptide is identified within bacteriophage T4 internal protein III (IPIII). Utilizing a phagederived expression-packaging-processing system, which packages foreign proteins fused with IPIII into the phage capsid, a synthetic cleavage site introduced at the C terminus of IPIII, is demonstrated to be functional and permits processing of fusion proteins. IPIII, which possesses a native P21 cleavage site at its N terminus, is altered to possess a second P21 cleavage site at its C terminus where cleavage occurs by means of the scaffold proteinase P21 within the capsid. The altered IPIII was inserted into an expression vector to permit the creation of fusion proteins with staphylococcal nuclease, EcoRI endonuclease, beta-globin, and luciferase. Western immunoblot analysis of packaged T4eG326 indicates that the IPIII:fusion-proteins are packaged into phage and proteolytically processed, thus the synthetic P21 cleavage site positioned at the C terminus of IPIII is demonstrated to be functional, and 20 to 200 protein molecules are packaged per capsid. Truncation experiments identified the minimal portion of IPIII required to achieve targeting into the phage capsid as a ten amino acid residue from the N terminus, which includes the N-terminal methionine residue and the proteinase P21 cleavage site, designated the CTS (capsid targeting sequence). The addition of the CTS to a fragment of luciferase permits the protein to be packaged and processed, which demonstrates that the CTS is by itself sufficient to target foreign protein to the capsid. The imputed dual function of the CTS is supported by site-directed PCR mutagenesis, which reveals two functionally separate domains of the CTS for targeting and processing. The CTS appears to function in a core-related targeting mechanism that directs a polymorphic set of proteins into the T-even capsid or scaffold. Although structure formation is often assumed to involve extended protein interfaces, the analysis shows that a limited but specific sequence, the CTS, drives the interaction required to achieve targeting.

Amino Acid Sequence↗
PubMed1993

Expression of the polyomavirus VP2 and VP3 proteins in insect cells: coexpression with the major capsid protein VP1 alters VP2/VP3 subcellular localization.

During polyomavirus infection the capsid proteins are synthesized in the cytoplasm and transported into the nucleus were virion assembly occurs. Expression of the major capsid protein VP1 in Sf9 insect cells results in the accumulation of capsid-like particles in the nucleus, independent of the presence of the minor capsid proteins VP2 and VP3 or the viral DNA (Montross et al., J. Virol. 65, 4991-4998, 1991). Sf9 cells infected with baculovirus vectors expressing the polyomavirus minor capsid proteins VP2 and VP3 were examined. VP2 was myristylated in Sf9 cells, as seen during polyomavirus infection of mouse cells. Immunoprecipitation of lysates from co-infected cells demonstrated an association between VP1 and VP2. As determined by immunogold electron microscopy, when expressed alone VP2 was associated with membrane structures in the cytoplasm and VP3 was diffusely localized in the cytoplasm. When co-infected with a VP1 expressing baculovirus, both VP2 and VP3 became predominantly localized to the nucleus in association with capsid-like structures. Thus, the polyomavirus capsid proteins interact in vivo and alter their subcellular localization as a consequence.

Animals↗
PubMed1995

Analysis of the cell and erythrocyte binding activities of the dimple and canyon regions of the canine parvovirus capsid.

Canine parvovirus (CPV) binds to a number of cell and erythrocyte receptors, some of which are involved in cell infection, while others are used for other viral functions. Little is known about the regions of the virus capsid which bind to the cell receptors. CPV binds sialic acid through a region within or adjacent to the dimple on the surface of the capsid (Barbis, D. P., Chang, S-F., and Parrish, C. R., 1992, Virology 191, 301-308). In order to map the sialic acid binding site in more detail and to examine other regions of the capsid for cell receptor binding, a variety of mutant capsids were analyzed which had changes in two depressions within the surface of the capsid--the "canyon" and "dimple." In most cases recombinant VP1 and VP2 proteins were stably expressed together in canine A72 cells from a plasmid expression vector. The purified empty capsids were tested for their ability to bind sialic acid and thereby hemagglutinate (HA) erythrocytes and for binding to permissive host cells. In addition, the ability of neutralizing monoclonal antibodies to block cell attachment was also examined. Mutations of amino acids on a wall of the dimple eliminated or severely decreased HA. Changing various residues within the canyon had no effect on binding to either sialic acids or other receptors on feline lymphoblastoid cells, suggesting that the canyon is not the site of cell receptor attachment. Neutralizing monoclonal antibodies against both major antigenic determinants had variable effects on cell binding, but no consistent inhibition of binding was observed by antibodies directed against either of those two major antigenic determinants of the capsid.

Amino Acid Sequence↗
PubMed2000

Molecular architecture of bacteriophage T4 capsid: vertex structure and bimodal binding of the stabilizing accessory protein, Soc.

T4 encodes two dispensable proteins that bind to the outer surface of the mature capsid. Soc (9 kDa) stabilizes the capsid against extremes of alkaline pH and temperature, but Hoc (40 kDa) has no perceptible effect. Both proteins have been developed as display platforms. Their positions on the hexagonal surface lattice of gp23*, the major capsid protein, were previously defined by two-dimensional image averaging of negatively stained electron micrographs of elongated variant capsids. We have extended these observations by reconstructing cryo-electron micrographs of isometric capsids produced by a point mutant in gene 23, for both Hoc+.Soc+ and Hoc+.Soc- phages. The expected T = 13 lattice was observed, with a single Hoc molecule at the center of each gp23* hexamer. The vertices are occupied by pentamers of gp24*: despite limited sequence similarity with gp23*, the respective monomers are similar in size and shape, suggesting they may have the same fold. However, gp24* binds neither Hoc nor Soc; in situ, Soc is visualized as trimers at the trigonal points of the gp23* lattice and as monomers at the sites closest to the vertices. In solution, Soc is a folded protein ( approximately 10% alpha-helix and 50-60% beta sheet) that is monomeric as determined by analytic ultracentrifugation. Thus its trimerization on the capsid surface is imposed by a template of three symmetry-related binding sites. The observed mode of Soc binding suggests that it stabilizes the capsid by a clamping mechanism and offers a possible explanation for the phenotype of osmotic shock resistance.

Bacteriophage T4↗
PubMed2005

High plasticity of the hepatitis B virus capsid revealed by conformational stress.

Hepatitis B virus (HBV) replicates through reverse transcription inside its icosahedral nucleocapsid. The internal genome status is signaled to the capsid surface, predicting regulated conformational changes in the capsid structure. To probe their nature and extent, we imposed local conformational stress on the outer surface of HBV capsid-like particles, and monitored its consequences by electron cryomicroscopy and image reconstruction. The capsid structure had an enormous flexibility and robustness as a whole, as well as within the subunits, whose spikes were able to rotate by as much as 40 degrees against the distal interdimer contact sites. The likely hinge for the swiveling movement was the conserved Gly111 residue at the inner surface of the capsid. The stress imposed from the outside also affected the internal capsid organization, implying a specific route for the flow of conformational information between capsid interior and exterior as required for signaling of the genome status.

Amino Acid Sequence↗
PubMed2005

A quasi-atomic model of human adenovirus type 5 capsid.

Adenoviruses infect a wide range of vertebrates including humans. Their icosahedral capsids are composed of three major proteins: the trimeric hexon forms the facets and the penton, a noncovalent complex of the pentameric penton base and trimeric fibre proteins, is located at the 12 capsid vertices. Several proteins (IIIa, VI, VIII and IX) stabilise the capsid. We have obtained a 10 A resolution map of the human adenovirus 5 by image analysis from cryo-electron micrographs (cryoEMs). This map, in combination with the X-ray structures of the penton base and hexon, was used to build a quasi-atomic model of the arrangement of the two major capsid components and to analyse the hexon-hexon and hexon-penton interactions. The secondary proteins, notably VIII, were located by comparing cryoEM maps of native and pIX deletion mutant virions. Minor proteins IX and IIIa are located on the outside of the capsid, whereas protein VIII is organised with a T=2 lattice on the inner face of the capsid. The capsid organisation is compared with the known X-ray structure of bacteriophage PRD1.

Adenoviruses, Human↗