Assembly of adenovirus major capsid protein is mediated by a nonvirion protein

Viral evolution revealed by bacteriophage PRD1 and human adenovirus coat protein structures

The unusual bacteriophage PRD1 features a membrane beneath its icosahedral protein coat. The crystal structure of the major coat protein, P3, at 1.85 A resolution reveals a molecule with three interlocking subunits, each with two eight-stranded viral jelly rolls normal to the viral capsid, and putative membrane-interacting regions. Surprisingly, the P3 molecule closely resembles hexon, the equivalent protein in human adenovirus. Both viruses also have similar overall architecture, with identical capsid lattices and attachment proteins at their vertices. Although these two dsDNA viruses infect hosts from very different kingdoms, their striking similarities, from major coat protein through capsid architecture, strongly suggest their evolutionary relationship.

Mechanism of scaffolding-directed virus assembly suggested by comparison of scaffolding-containing and scaffolding-lacking P22 procapsids

Assembly of certain classes of bacterial and animal viruses requires the transient presence of molecules known as scaffolding proteins, which are essential for the assembly of the precursor procapsid. To assemble a procapsid of the proper size, each viral coat subunit must adopt the correct quasiequivalent conformation from several possible choices, depending upon the T number of the capsid. In the absence of scaffolding protein, the viral coat proteins form aberrantly shaped and incorrectly sized capsids that cannot package DNA. Although scaffolding proteins do not form icosahedral cores within procapsids, an icosahedrally ordered coat/scaffolding interaction could explain how scaffolding can cause conformational differences between coat subunits. To identify the interaction sites of scaffolding protein with the bacteriophage P22 coat protein lattice, we have determined electron cryomicroscopy structures of scaffolding-containing and scaffolding-lacking procapsids. The resulting difference maps suggest specific interactions of scaffolding protein with only four of the seven quasiequivalent coat protein conformations in the T = 7 P22 procapsid lattice, supporting the idea that the conformational switching of a coat subunit is regulated by the type of interactions it undergoes with the scaffolding protein. Based on these results, we propose a model for P22 procapsid assembly that involves alternating steps in which first coat, then scaffolding subunits form self-interactions that promote the addition of the other protein. Together, the coat and scaffolding provide overlapping sets of binding interactions that drive the formation of the procapsid.

A helper-independent adenovirus vector with E1, E3, and fiber deleted: structure and infectivity of fiberless particles

The adenovirus (Ad) fiber protein largely determines viral tropism through interaction with specific cell surface receptors. This molecule may also be involved in virion assembly or maturation, as some previously characterized fiber mutants were defective for processing of viral structural proteins. We previously described packaging cell lines that express Ad type 5 (Ad5) fiber and can complement the temperature-sensitive Ad fiber mutant H5ts142. We have now used these packaging cells to construct a new adenoviral vector (Ad5.betagal.DeltaF) with E1, E3, and L5 (fiber) deleted and analyzed the fiber null phenotype. Ad5.betagal.DeltaF growth was completely helper independent, and fiberless particles were produced by a single final round of growth in 293 cells. Cryoelectron microscopic studies and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that the structure and composition of these particles was nearly identical to those of first-generation Ad vectors. As expected, fiberless particles had reduced infectivity on epithelial cells, but they retained the ability to infect monocytic cells via an integrin-dependent pathway. These studies provide a novel approach to developing retargeted Ad gene therapy vectors.

The herpes simplex virus triplex protein, VP23, exists as a molten globule

Two proteins, VP19C (50,260 Da) and VP23 (34,268 Da), make up the triplexes which connect adjacent hexons and pentons in the herpes simplex virus type 1 capsid. VP23 was expressed in Escherichia coli and purified to homogeneity by Ni-agarose affinity chromatography. In vitro capsid assembly experiments demonstrated that the purified protein was functionally active. Its physical status was examined by differential scanning calorimetry, ultracentrifugation, size exclusion chromatography, circular dichroism, fluorescence spectroscopy, and 8-anilino-1-naphthalene sulfonate binding studies. These studies established that the bacterially expressed VP23 exhibits properties consistent with its being in a partially folded, molten globule state. We propose that the molten globule represents a functionally relevant intermediate which is necessary to allow VP23 to undergo interaction with VP19C in the process of capsid assembly.

Virus assembly and disassembly: the adenovirus cysteine protease as a trigger factor

Viruses are efficient carriers of genetic material between cells. They specifically recognise a target cell and utilise host functions for genome delivery to the replication site. A mature viral capsid emerging from an infected cell serves at least three distinct functions. It enables virus egress from the infected cell, protects the extracellular genome against chemical and physical stress and mediates virus entry into a non-infected cell. How can a virus particle be stably assembled in an infected cell and moments later-after passing through the extracellular milieu-be disintegrated by a non-infected cell? In this review I discuss how adenovirus, a DNA virus, recruits cellular and viral factors and makes use of its own cysteine protease to regulate capsid assembly and disassembly. Copyright 1998 John Wiley & Sons, Ltd.

Nucleotide and amino acid sequence analysis of the 100K protein of a serotype 3 porcine adenovirus

The genomic region between map units 69 and 78 of a type 3 porcine adenovirus (PAV3) was sequenced and analysed. An open reading frame (ORF) of 2514 nucleotides encoding a polypeptide of 838 amino acids and approximately 94.1 kDa was found. The size and location of the ORF suggested it was the PAV3 homologue of the 100K gene and this was confirmed by nucleotide sequence comparison with the 100K of human adenovirus type 2. Amino acid sequence alignment of the predicted polypeptide with the sequences of the 100K proteins of four human adenoviruses and type 10 fowl adenovirus revealed sequence identities of between 31% and 52%. Although amino acid conservation was present throughout the entire sequences compared, lower identity was noted in both the amino- and carboxy-termini.

Sequence of the mouse adenovirus type-1 DNA encoding the 100-kDa, 33-kDa and DNA-binding proteins

The genomic nucleotide sequence for the region of 66 to 77 map units (m.u.) of mouse adenovirus type 1 (MAV-1) was determined and predicted to encode proteins homologous to the human adenovirus (Ad) 100-kDa, 33kDa and DNA-binding proteins (DBP). The putative MAV-1 100-kDa protein has 65-70% amino-acid similarity to 100-kDa proteins from five different human Ad serotypes. The mRNA for the putative 33-kDa protein is internally spliced within the coding sequence, as are its human Ad counterparts [Oosterom-Dragon and Anderson, J. Virol 45 (1983) 251-263]. The N-terminal region of the putative MAV-1 33-kDa protein has 41-44% similarity to two human Ad 33-kDa N-termini, and the C-terminal regions are more conserved, with 60-65% similarity. The MAV-1 DBP is predicted to be encoded in this region and was compared to six different human Ad DBP N-and C-termini. The N-termini of the MAV-1 and Ad DBP were 33-48% similar and the C-termini were 56-60% similar. The MAV-1 DBP contains conserved regions (CR) 1,2 and 3, and it retains important residues for a putative zinc finger (Zf) motif identified in Ad DBP [Eagle and Klessig, Virology 187 (1992) 777-787]. Additional sequence features of these proteins have also been identified.

Human adenovirus type 5 variants with sequence alterations flanking the E2A gene: effects on E2 expression and DNA replication

The human adenovirus type 5 (Ad5) E2 transcription unit is divided into a promoter-proximal region, E2A, and a distal region, E2B, each with its own polyadenylation site. Together these regions encode the three virus-derived proteins necessary for genome replication. Ad5 variants were produced that carried linker insertion mutations immediately 5' and/or 3' to the coding sequence for the E2A gene DNA binding protein (DBP). Two variants carrying solely a 5' lesion showed decreased usage of the adjacent 3' splice site, via which the DBP mRNA is produced, and an increased usage of the alternative downstream splice sites in the E2B region, wherein viral DNA polymerase and terminal protein precursor are encoded; these viruses showed somewhat reduced growth. A variant carrying a 3' lesion showed a marginal increase in DBP expression and slightly accelerated growth. When lesions 5' and 3' to the DBP coding sequence were combined in cis, the resulting virus was severely defective for growth and expressed E2B products to the virtual exclusion of E2A DBP. These data indicate that interactions must occur between the E2A 3' splice site and polyadenylation site before this region can be treated as an exon by the RNA processing machinery, and that a sequence alteration at the polyadenylation site that alone has only minor effects on the pattern of RNA processing can drastically affect terminal exon usage when placed in cis with a mutation that reduces splicing efficiency at the upstream 3' splice site. The data further indicate that, in vivo, Ad5 DNA replication is limited by prevailing DBP levels rather than by levels of polymerase or terminal protein precursor.

Stability of wild-type and temperature-sensitive protein subunits of the phage P22 capsid

Temperature-sensitive folding (tsf) mutants of the phage P22 coat protein prevent newly synthesized polypeptide chains from reaching the conformation competent for capsid assembly in cells, and can be rescued by the GroEL chaperone (Gordon, C., Sather, S., Casjens, S., and King, J. (1994) J. Biol. Chem. 269, 27941-27951). Here we investigate the stabilities of wild-type and four tsf mutant unpolymerized subunits. Wild-type coat protein subunits denatured at 40 degrees C, with a calorimetric enthalpy of approximately 600 kJ/mol. Comparison with coat protein denaturation within the shell lattice (Tm = 87 degrees C, delta H approximately 1700 kJ/mol) (Galisteo, M.L., and King, J. (1993) Biophys. J. 65, 227-235) indicates that protein-protein interactions within the capsid provide enormous stabilization. The melting temperatures of the subunits carrying tsf substitutions were similar to wild-type. At low temperatures, the tsf mutants, but not the wild-type, formed non-covalent dimers, which were dissociated at temperatures above 30 degrees C. Spectroscopic and calorimetric studies indicated that the mutant proteins have reduced amounts of ordered structure at low temperature, as compared to the wild-type protein. Although complex, the in vitro phenotypes are consistent with the in vivo finding that the mutants are defective in folding, rather than subunit stability. These results suggest a role for incompletely folded subunits as precursors in viral capsid assembly, providing a mechanism of reaching multiple conformations in the polymerized form.

Protection of turkeys against haemorrhagic enteritis by monoclonal antibody and hexon immunization

Virus-neutralizing monoclonal antibodies specific for the hexon of haemorrhagic enteritis virus (HEV), a turkey adenovirus, were examined for their ability to confer passive protection against haemorrhagic enteritis (HE) in turkeys. A high dose of antibody prevented clinical disease and reduced virus replication in experimentally infected birds. This suggests that virus neutralization might be an important mechanism for protection against HE. Subsequently, the use of the hexon protein as a subunit vaccine was investigated by immunizing birds with affinity-purified HEV hexon. The birds were tested for the appearance of hexon-specific antibodies in their sera, for protection from clinical disease, and prevention of virus replication after challenge with virulent HEV (HEV-V). Regardless of whether birds were immunized with native or denatured hexon, high ELISA antibody titres were produced to each immunogen. A virus-neutralizing antibody response was induced by immunization with the native hexon but not by immunization with the denatured protein. All turkeys twice immunized with a dose of at least 1 microgram, and four out of five birds immunized with two doses of 0.3 micrograms of purified native hexon, were protected against virus-induced disease and virus replication. In contrast, birds inoculated with denatured hexon were not protected. These results demonstrate the importance of the native (trimeric) structure of the hexon protein for eliciting a protective immune response. The impact of these results on the development of a vaccine for HE in turkeys produced by recombinant DNA technology is discussed.

The role of proteolytic processing in the morphogenesis of virus particles

Proteinases are encoded by many RNA viruses, all retroviruses and several DNA viruses. They play essential roles at various stages in viral replication, including the coordinated assembly and maturation of virions. Most of these enzymes belong to one of three (Ser, Cys or Asp) of the four major classes of proteinases, and have highly substrate-selective and cleavage specific activities. They can be thought of as playing one of two general roles in viral morphogenesis. Structural proteins are encoded by retroviruses and many RNA viruses as part of large polyproteins. Their proteolytic release is a prerequisite to particle assembly; consequent structural rearrangement of the capsid domains serves to regulate and direct association and assembly of capsid subunits. The second general role of proteolysis is in assembly-dependent maturation of virus particles, which is accompanied by the acquisition of infectivity.

A microscale analytical batch chromatographic method for detecting soluble viral DNA-binding proteins in crude extracts

We describe a micro-method for determining the presence in crude cellular extracts of soluble proteins which can bind to immobilized DNA, using the DNA-binding protein of human adenovirus as an example. Batch chromatography of radiolabeled proteins is performed in microcentrifuge tubes containing 50 microliters packed volume of commercially available denatured calf thymus DNA-cellulose. Eluted single-stranded DNA-binding proteins are then visualized by fluorography following gel electrophoresis. The batch procedure gives yields of adenovirus DNA-binding protein which are comparable to those obtained with a mini-column of similar adsorbent volume. The scale of the procedure makes it convenient for simultaneously analyzing multiple samples.

The adenovirus L4 100-kilodalton protein is necessary for efficient translation of viral late mRNA species

When screening a number of adenovirus type 5 (Ad5) temperature-sensitive mutants for defects in viral gene expression, we observed that H5ts1-infected 293 cells accumulated reduced levels of newly synthesized viral late proteins. Pulse-labeling and pulse-chase experiments were used to establish that the late proteins synthesized in H5ts1-infected cells under nonpermissive conditions were as stable as those made in Ad5-infected cells. H5ts1-infected cells contained normal levels of viral late mRNAs. Because these observations implied that translation of viral mRNA species was defective in mutant virus-infected cells, the association of viral late mRNAs with polyribosomes was examined during the late phase of infection at a nonpermissive temperature. In Ad5-infected cells, the majority of the viral L2, L3, L4, pIX, and IVa2 late mRNA species were polyribosome bound. By contrast, these same mRNA species were recovered from H5ts1-infected cells in fractions nearer the top of polyribosome gradients, suggesting that initiation of translation was impaired. During the late phase of infection, neither the polyribosome association nor the translation of most viral early mRNA species was affected by the H5ts1 mutation. This lesion, mapped by marker rescue to the L4 100-kilodalton (kDa) nonstructural protein, has been identified as a single base pair substitution that replaces Ser-466 of the Ad5 100-kDa protein with Pro. A set of temperature-independent revertants of H5ts1 was isolated and characterized. Either true reversion of the H5ts1 mutation or second-site mutation of Pro-466 of the H5ts1 100-kDa protein to Thre, Leu, or His restored both temperature-independent growth and the efficient synthesis of viral late proteins. We therefore conclude that the Ad5 L4 100-kDa protein is necessary for efficient initiation of translation of viral late mRNA species during the late phase of infection.

Adenovirus serotype 3 fibre protein is expressed as a trimer in Escherichia coli

The adenovirus serotype 3 (Ad3) fibre has been expressed in Escherichia coli as an insoluble protein. The protein was solubilized by extraction with urea. Slow removal of urea during the purification procedure resulted in a soluble Ad3 fibre preparation. Polyacrylamide gel analysis of the purified fibre protein, as well as cross-linking experiments performed on cellular debris of expressing cells, suggest that the recombinant Ad3 fibre self-assembles as a trimer from identical polypeptide chains. Gel filtration gave the same exclusion volume for the purified recombinant fibre and for the native fibre in the protein mixture extracted from the Ad3-infected cells. The recombinant fibre was partially resistant to proteolytic degradation, suggesting a folded structure.

Adenovirus proteins associated with mRNA and hnRNA in infected HeLa cells

The proteins that interact with cytoplasmic and nuclear polyadenylated RNA in adenovirus type 5 (Ad5) infection of HeLa cells were examined by UV-induced RNA-protein cross-linking in intact cells. The Ad5 100-kilodalton late nonvirion protein (100K protein) was cross-linked to both host and viral polyadenylated cytoplasmic RNA (mRNA). The cross-linking of the 100K protein to mRNA appears to correlate with productive infection, because the protein is not cross-linked to mRNA in abortive infection of wild-type Ad5 in monkey cells (CV-1) even though normal amounts of it are produced. However, when CV-1 cells are infected with Ad5 hr404, and Ad5 mutant which overcomes the host restriction to wild-type Ad5 infection in these cells, the 100K protein is cross-linked to mRNA. To identify and obtain antibodies to RNA-contacting proteins, a mouse was immunized with oligo(dT)-selected cross-linked RNA-protein complexes from Ad5-infected cells and the serum was used for immunoblotting experiments. It was found that in addition to the 100K protein, the Ad5 72K DNA-binding protein is also associated with RNA in the infected cells. The 72K DNA-binding protein is cross-linked to polyadenylated nuclear RNA sequences. These findings indicate that adenovirus proteins interact with RNAs in the infected cell and suggest possible mechanisms for the effects of the virus on mRNA metabolism.

Hexon trimerization occurring in an assembly-defective, 100K temperature-sensitive mutant of adenovirus 2

Analysis of 100K-defective temperature-sensitive adenovirus mutants confirmed the multifunctional character of the nonstructural, virus-coded 100K protein. In addition to its function in hexon trimerization (altered in H5ts1), and its possible direct or indirect role in hexon transport to nucleus (mutated in H2ts118), genetic and biochemical evidence was presented that 100K play some critical role in the scaffolding process of adenovirus capsid. This function appeared to be defective in H2ts107 and to map between coordinates 69.0 and 69.9, leftward from the H5ts1 lesion (70-73 map units; Arrand, 1978). This corresponded to the central domain of the 100K protein, between amino acid 300 and 400 from the N end. DNA sequencing of cloned fragments of H2ts107 DNA overlapping the mutation revealed two point mutations on the same codon at nucleotide 25,082 and 25,083 (GAC----GCA), corresponding to a nonconservative amino acid change (aspartic acid----alanine) at position 324 in the 100K sequence. 100K of adenovirus 2 wild type (WT) was found to bind in significant amounts to novobiocin-affinity column, and to be coeluted with hexon, penton, IIIa, and cellular topoisomerase II activity, by novobiocin- or ATP-Mg2+-containing buffers. H2ts107 100K also bound to novobiocin column, but the elution pattern differed from that of WT, suggesting some alteration in the affinity of the mutated 100K for novobiocin. The same behavior on affinity column as H2ts107 100K was observed for 90K, a cleavage product of the 100K, found in great abundance in H2ts107 at 39.5 degrees and corresponding to the C-terminal moiety of the 100K molecule. This implied that the "novobiocin-binding" domain of the 100K was not confined at its N terminus, and was altered in the H2ts107 mutant.

Three-dimensional structure of the adenovirus major coat protein hexon

The three-dimensional crystal structure of the adenovirus major coat protein is presented. Adenovirus type 2 hexon, at 967 residues, is now the longest polypeptide whose structure has been determined crystallographically. Taken with our model for hexon packing, which positions the 240 trimeric hexons in the capsid, the structure defines 60% of the protein within the 150 X 10(6) dalton virion. The assembly provides the first details of a DNA-containing animal virus that is 20 times larger than the spherical RNA viruses previously described. Unexpectedly, the hexon subunit contains two similar beta-barrels whose topology is identical to those of the spherical RNA viruses, but whose architectural role in adenovirus is very different. The hexon structure reveals several distinctive features related to its function as a stable protective coat, and shows that the type-specific immunological determinants are restricted to the virion surface.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.

The structure of the adenovirus capsid. I. An envelope model of hexon at 6 A resolution

The structure of hexon, the major coat protein of adenovirus, has been determined by X-ray crystallography. Electron density maps were obtained with phases based on five heavy-atom derivatives at 2.9 A resolution. The main experimental finding derives from a low resolution electron density map calculated at 6.0 A resolution, but based on phases determined by the multiple isomorphous replacement method at 2.9 A resolution. Hexon consists of three subunits together forming two major components of different morphological symmetry. A triangular top with three towers of density is superimposed on a more bulky pseudo-hexagonal base. The symmetry of the top is in accord with the trimeric nature of hexon, but that of the base derives from the molecular function, which is to provide a densely packed impenetrable protective outer layer for the virion. A close-packed array of hexons forms a planar facet of the icosahedral capsid, with the tops presenting a spiky appearance that is consistent with electron micrographs of the adenovirus capsid. Hexon is hollow, permitting it to occupy a larger volume than normal for the same quantity of protein. The polypeptide chain has been traced in the 2.9 A electron density map for several non-contiguous stretches, allowing C alpha co-ordinates to be measured for 820 out of the 967 amino acid residues. The overall folding pattern confirms the assignment of shape, but the lack of connectivity so far precludes its complete description. The modest amount of alpha-helix and beta-sheet present is in accord with spectroscopic results.

The structure of the adenovirus capsid. II. The packing symmetry of hexon and its implications for viral architecture

The orientation and location of the 240 hexons comprising the outer protein shell of adenovirus have been determined. Electron micrographs of the capsid and its fragments were inspected for the features of hexon known from the X-ray crystallographic model as described in the accompanying paper. A capsid model is proposed with each facet comprising a small p3 net of 12 hexons, arranged as a triangular sextet with three outer hexon pairs. The sextet is centrally placed about the icosahedral threefold axis, with its edges parallel to those of the facet. The outer pairs project over the facet edges on one side of the icosahedral twofold axes at each edge. The model capsid is defined by the underlying icosahedron, of edge 445 A, upon which hexons are arranged. The hexons are thus bounded by icosahedra with insphere radii of 336 A and 452 A. A quartet of hexons forms the asymmetric unit of an icosahedral hexon shell, which can be closed by the addition of pentons at the 12 vertices. Considering the hexon trimer as a complex structure unit, its interactions in the four topologically distinct environments are very similar, with conservation of at least two-thirds of the inter-hexon bonding. The crystal-like construction explains the flat facets and sharp edges characteristic of adenovirus. Larger "adenovirus-like" capsids of any size could be formed using only one additional topologically different environment. The construction of adenovirus illustrates how an impenetrable protein shell can be formed, with highly conserved intermolecular bonding, by using the geometry of an oligomeric structure unit and symmetry additional to that of the icosahedral point group. This contrasts with the manner suggested by Caspar & Klug (1962), in which the polypeptide is the structure unit, and for which the number of possible bonding configurations required of a structure unit tends to infinity as the continuously curved capsid increases in size. The known structures of polyoma and the plant viruses with triangulation number equal to 3 are evaluated in terms of hexamer-pentamer packing, and evidence is presented for the existence of larger subunits than the polypeptide in both cases. It is suggested that spontaneous assembly can occur only when exact icosahedral symmetry relates structure units or sub-assemblies, which would themselves have been formed by self-limiting closed interactions. Without such symmetry, the presence of scaffolding proteins or nucleic acid is necessary to limit aggregation.

Alterations in chromatin conformation are accompanied by reorganization of nonchromatin domains that contain U-snRNP protein p28 and nuclear protein p107

The intranuclear distribution of nuclear matrix-associated protein p107 and the 28-kD Sm antigen of U-snRNPs have been studied using double-label immunofluorescence and immunoperoxidase electron microscopy. In interphase nuclei of HeLa cells, Novikoff hepatoma cells, and rat kangaroo kidney cells, p107 was confined to discrete interchromatin domains. The domains had an irregular contour, with an average diameter of 1-1.5 micron. Each domain appeared to be composed of interconnected granules. The Sm antigen colocalized and appeared concentrated in these domains but also showed some general nucleoplasmic distribution. During mitosis, the interchromatin domains disassembled such that the Sm portion redistributed to the perichromosomal and spindle regions and the p107 component redistributed throughout the mitotic cytoplasm. During anaphase, p107 assembled into discrete clusters throughout the mitotic cytoplasm. The Sm antigen was not a component of these clusters. Double-label immunofluorescence with anti-p107 and the anti-DNA tight-binding protein, AhNa1, showed that the extranuclear p107 domains assumed an interchromatin localization only after the chromosomes had decondensed. The correlation between chromosome decondensation and the occurrence of p107 within interchromatin domains was also observed during chicken erythrocyte nuclear reactivation. We propose that the discrete interchromatin domains that contain p107 and p28 may be important for processing and splicing of RNA and that their structural assembly within nuclei is sensitive to the presence of the transcriptionally active conformation of chromatin.

Morphogenesis of human adenovirus type 2: sequence of entry of proteins into previral and viral particles

The initial steps of adenovirus capsid morphogenesis and the sequence of entry of structural and nonstructural proteins into assembly-intermediate (IM) particles were investigated by pulse-chase labeling, temperature shifts, and cycloheximide inhibition of particle formation. The experiments were performed on wild-type and two assembly-defective, temperature-sensitive mutants, H2 ts 112 and H2 ts 107. The sequence of events in the adenovirus assembly can be schematized as follows. (i) Hexons, pentons, and protein IX assembled with scaffolding proteins 100K, PVIII, and PVII, precursor to the major core protein, to form a previral particle banding at a density of 1.285 in CsCl; (ii) additional incorporation of maturation and/or stabilization proteins IIIa, 50K, 39K, 28K, and PVI led to 1.295 IM; (iii) exit of 100K, 39K, and 28K, and entry of viral DNA gave rise to 1.370 IM; (iv) dephosphorylation and/or exit of 50K and exchange with core protein V and processing of precursors to VII, VI, VIII, and DNA-terminal protein resulted in formation of infectious 1.345 virion. The polypeptide composition of the new class of assembly-intermediate particles elicited by H2 ts 107 (1.285 IM), suggested that 100K, PVIII, and also PVII might serve as scaffold components for adenovirus capsid building.

Electron microscopic studies on the connections between capsomers in the adenovirus capsid

Electron microscopic observation of negatively stained virions suggests that the interhexonal spaces within the adenovirus capsid are bridged by interhexonal connective elements. Similar connections are also detected between pentons and the five peripentonal hexons. Examinations of electron micrographs by Markham's rotational integration technique reveals the presence of a regular network of the connective elements between the capsomers in the whole capsid. The length of these connections corresponds to about one-third of the diameter of the hexons. The connections consist of two approximately parallel elements, thus every hexon is joined to its six nearest neighbours by 12 connections. A possible mutual orientation pattern of the connective elements in the whole capsid is presented on a tentative virus model.

The application of monoclonal antibodies in the study of viruses

This chapter discusses the applications of monoclonal antibodies in virology. A single monoclonal antibody can provide information on protein “relatedness,” structure, function, synthesis, processing, and cellular or tissue distribution and on the association among molecules. The use of monoclonal antibodies provides valuable insight into the working of the protein both as an enzyme and as a target for the host immune response, evolving in reaction to that response. Monoclonal antibodies find application in two main areas: (1) in the field of rapid diagnosis of virus disease in man, animals, and plants and (2) in the extension of virus taxonomy. Monoclonal antibodies may be used to analyze the role of a protein. This ability to distinguish related proteins can be used to provide a genetic marker in recombination experiments. Monoclonal antibodies can detect low amounts of individual virus proteins within the infected cell. They can, thus, provide information concerning the temporal and spatial separation of protein formation and accumulation, and data on protein modification and processing in the infected cell.

Analysis of Ad5 hexon and 100K ts mutants using conformation-specific monoclonal antibodies

Adenovirus type 5 ts mutants deficient in hexon metabolism were investigated using conformation-specific monoclonal antibodies directed against hexon capsomeres and the viral 100K protein. The ts mutants map either in the hexon structural gene or in the gene encoding the 100K protein, a major, late nonstructural protein. All of the mutants examined (ts1, ts2, ts3, ts4, ts17, and ts20 of J. F. Williams, M. Gharpure, S. Ustacelebi, and S. McDonald (1971). J. Gen. Virol. 11, 95-101) were unable to produce the capsomeric form of hexon (a trimer of three hexon monomers) at the nonpermissive temperature. However, all of the mutants retained the ability to produce a complex of 100K and hexon which has been demonstrated to play a major role in the assembly of hexon trimers. The mutants accumulated nontrimerized hexon in this ts complex in the perinuclear region of the cell. Several of the mutants (ts1, ts2, ts3) were found to successfully assemble hexon synthesized at the nonpermissive temperature upon shift down to the permissive temperature, even in the presence of a protein synthesis inhibitor. The mutant, ts2, which maps in the hexon structural gene, was found to be dependent on protein synthesis for transport of hexon trimers into the nucleus during temperature shift down, while the 100K ts mutants, ts1 and ts3, were independent of protein synthesis for both hexon assembly and transport.

Aberrant distribution of human adenovirus type 2 late proteins in monkey kidney cells

Monkey kidney cells (CV-C) infected with adenovirus type 2 displayed an aberrant distribution of 100K, 100K-hexon complex, hexon monomers, hexon trimers, penton base, and fiber proteins, relative to the patterns observed in adenovirus type 2-infected human cells. Human cell patterns were observed in CV-C cells when mutants selected for growth on monkey cells were used.