Isolation and characterization of a slow-migrating class of adenovirus type 2 hexons

Preparation and characterization of monoclonal antibodies to enteric adenovirus types 40 and 41

We have prepared monoclonal antibodies to each of the enteric adenoviruses types 40 and 41. Three different hybridoma cell lines were selected which produced antibody found to react by radioimmunoprecipitation with adenovirus (Ad) hexon antigens. One was specific for Ad 40, another for Ad 41, and a third one reacted with both types. When tested in an enzyme immunoassay against all 41 known human Ad types, the type-specific monoclonal antibody against Ad 40 reacted homotypically, as did the monoclonal antibody against Ad 41. In addition, these monoclonal antibodies neutralized the homologous enteric Ad type. The monoclonal antibody which reacted with both enteric Ad types also showed lower levels of reactivity with the group C adenoviruses types 2, 5, and 6. The monoclonal antibodies produced will provide a definitive means for rapid identification of specific Ad types, and will be useful in defining the relationship of enteric adenoviruses to other types.

Purification of adenovirus hexon protein by high-performance liquid chromatography

Adenovirus (Type 2)-infected HeLa cells were sonicated and treated with 1,1,2-trichlorotrifluoroethane. The water-soluble extract was ultracentrifuged and the supernatant, containing the dissociated proteins, was subjected to anion-exchange high-performance liquid chromatography on a Mono Q column in 50 mM bis-Tris-HCl (pH 6.5). Elution with a linear salt gradient resulted in a major peak, containing the hexon protein.

Cytoskeletal proteins associated with intracytoplasmic human adenovirus at an early stage of infection

Taking advantage of the sedimentation properties of adenovirus particles, adenovirus-infected baby hamster kidney (BHK21) cells were reversibly fixed with cleavable diimidoester dimethyl 3,3'-dithiobispropionimidate (DTBP) at early times of infection (30 min). Cytoskeletal proteins associated with/or in close vicinity to virions were isolated as a complex cross-linked with carrier virus. Four major cellular proteins were thus found to co-purify with adenovirus particles. They were characterized by their coordinates on 2D maps and immunological reactivity. Two of them were identified as alpha-tubulin (58 kD), and vimentin subunits (56 kD). The two other species 68 and 66 kD might correspond to stress proteins. Affinity blotting on gels showed that both alpha-tubulin and vimentin were capable of binding with intact and penton-less adenovirions. Adenovirus components involved in the binding seemed to be mainly core proteins V and VII, and to a lesser extent, hexon. Analysis of neighbor relationships among proteins of the adenovirus-cytoskeletal protein cross-linked complex suggested that some capsid alterations occurred upon/or after entry of the virus into the cell, and that these structural modifications preferentially concerned the vertex components penton and IIIa, and the core protein V.

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.

Limited proteolysis and a reactive cysteine residue define accesible regions in the native conformation of the adenovirus hexon protein

The sulfhydryl group of one cysteine residue in the adenovirus hexon protein is accessible to alkylation in the absence of denaturating agents. This residue was [14C]carboxymethylated and characterized in peptides after proteolytic treatments. It was identified in two different types of digest and corresponded to the cysteine residue most accessible to oxidation. It is located close to residue 670 in the tentative amino acid sequence of the entire polypeptide. Trypsin has a highly limited action on the native hexon protein. Only three peptide bonds are fully susceptible to cleavage. These were found to occur in the N-terminal third of the protein, and were characterized as cleavages close to positions 150, 180 and 320 in the tentative structure of the polypeptide. Trypsin therefore liberates four fragments in major yield, with approximate Mr of 3000, 15000, 17000 and 78000. The smallest fragment corresponds to a highly acidic region. A limited, but less restricted susceptibility of hexon to chymotryptic cleavage was also found, and four major sites were characterized. The cysteine labelling and the proteolytic treatments define accessible regions in the native conformation of hexon. Together with previously assigned residue distributions, they suggest the subdivision of the entire polypeptide chain into three approximately equal parts, with accessible sites in the border regions, as well as in the middle of at least the N-terminal third. One model compatible with all data is presented.