Structural study of adenovirus type 2 fibre using anti-fibre and anti-peptide sera

Characterization of the Adenovirus Fiber Protein

Entry of the adenovirus (Ad) capsids during the early stages of infection is a multistep process that includes initial attachment of the virus capsid to the cell surface followed by internalization of the virus into early endosomes. The Ad fiber protein, a complex of three apparently identical subunits, mediates the initial attachment step. In this chapter, methods for the purification and characterization of the Ad fiber protein are presented. Chromatographic methods for the isolation of the protein from infected cells can yield substantial quantities of protein for biochemical analysis. Protocols for characterization of the protein by Western blot and by indirect immunofluorescence of infected cells are also presented. The specificity of different monoclonal and polyclonal antibodies that recognize Ad fiber is also discussed. Ad fiber from a number of serotypes also contains a posttranslational modification, O-linked N-acetyl-glucosamine; methods for detection and characterization of this modification are also provided. With these tools and protocols, one can address important questions about this protein, which helps direct the tissue tropism of Ad.

Inhibition of cell adhesion to the virus by synthetic peptides of fiber knob of human adenovirus serotypes 2 and 3 and virus neutralisation by anti-peptide antibodies

The fiber knob of adenovirus (Ad) causes the first step in the interaction of adenovirus with cell membrane receptors. To obtain information on the receptor binding site(s) several synthetic peptides derived from Ad2 and Ad3 fiber head sequences and their antisera were tested for interference with virus attachment to HeLa and FL cells and cell adhesion to viruses. The anti-peptide sera were also evaluated in ELISA and virus neutralisation test. Ad2 (of subgroup C) and Ad3 (of subgroup B) attachment was not significantly inhibited by peptides corresponding to the amino acid residues 535-554, 555-573, 562-582 of Ad2 fiber or 210-225, 267-283, 291-306 and 300-319 of Ad3 fiber. However, microplate pre-adsorbed Ad3 fiber residues 210-225 and 267-283 could bind FL and HeLa cells, and 1 mg/ml of Ad3 fiber residues 267-283 inhibited the cell adhesion to Ad3 virus to approximately 90%. This peptide may participate in the receptor binding site of Ad3 fiber. ELISA reactive anti-peptide antibodies against the homologous peptide and virus did not significantly reduce the cell adhesion to the immobilised virus or the virus attachment to cells, but in the neutralisation assay antibodies raised to Ad2 fiber residues 555-573 and 562-582 and Ad3 fiber residues 210-225 caused neutralisation of the homologous virus at serum dilutions of 1:500 and 1:32, respectively. The corresponding peptides and one further peptide of Ad2 fiber and two of Ad3 fiber seem to contain neutralisation epitopes.

Human adenovirus serotype 3 (Ad3) and the Ad3 fiber protein bind to a 130-kDa membrane protein on HeLa cells

The fiber protein of adenovirus mediates the interaction of adenovirus with cell membrane receptors. We have produced the Ad3 fiber protein in the baculovirus expression system. Biochemical, morphological and functional analyses showed that the recombinant fiber was properly folded and functionally competent. The specific binding of Ad3 virus to two HeLa membrane proteins of 130 and 100 kDa was demonstrated with an overlay protein binding assay. In the same assay, Ad3 fiber only recognized the 130-kDa protein. Divalent cations seemed to be important for the interaction of both virus and fiber with these proteins.

Deletion analysis of functional domains in baculovirus-expressed adenovirus type 2 fiber

Various forms of Ad2 fiber were expressed in insect cells using recombinant baculoviruses and phenotypically characterized with respect to the following properties: trimerization, binding to penton base, nuclear targeting, and glycosylation. The morphology and dimensions of full-length fiber produced by invertebrate cells were indistinguishable from those observed in extracts from lytically infected mammalian cells. The domain required for trimer formation was mapped to the C-terminus, between amino acids 541 and 582. The N-terminal domain, between amino acids 1 and 16, negatively influenced the trimerization efficiency. Fiber gene products reduced to the shaft portion of the fiber capsomer formed significant amounts of stable dimers. Recognition with penton base only occurred with trimeric forms of fiber and was apparently not affected by deletion of the first 60 amino acids from the N-terminus. Fiber deleted of the Met1-Gly60 sequence was found to localize within the nucleus at levels similar to those of full-length fiber. All recombinant fibers, including tail-and-know-deleted forms, were found to be glycosylated using three separate assays, (i) in vivo labeling with [3H]glucosamine, (ii) binding to WGA, and (iii) reaction with monoclonal antibody RL2 directed against O-GlcNAc-containing glycopeptide. This implied that Ad2 fiber is a substrate for GlcNAc O-seryl transferase in insect cell cytoplasm and that at least one major glycosylation site is located in the shaft domain, between Met61 and Asn410.

Functional and structural effects of an Ala to Val mutation in the adenovirus serotype 2 fibre

H2ts125 is a fibre-defective, temperature-sensitive mutant of adenovirus serotype 2. H2ts125 fibre is unstable at the non-permissive temperature (ts phenotype), and does not migrate in the same way as the wild-type fibre in an SDS/polyacrylamide gel (elm phenotype). Sequence analysis has shown that H2ts125 carries two mutations on the fibre gene: Leu105 to Phe, and Ala434 to Val. Analysis of the structural modifications occurring in H2ts125 fibre was performed using peptide finger-printing and antipeptide sera as immunological probes. We found that all the detectable structural alterations in the mutant fibre were due to the substitution on codon 434. In addition, the ts phenotype was rescued by a wild-type DNA fragment containing the 3' moiety of the fibre gene and overlapping the 434th codon. Morphological analysis of fibre molecules observed under the electron microscope showed minor but statistically significant differences in the fibre length between mutant and wild-type. The mutant fibre was found to be slightly longer (308.8 +/- 1.9 A) than the wild-type fibre (300.1 +/- 2.1 A). Thus both ts and elm phenotypes were carried by the same Ala434 to Val mutation which probably resulted from a change in the three-dimensional structure of the fibre protein, and not from some proteolytic cleavage.

Fiber sequence heterogeneity in subgroup F adenoviruses

The fiber gene of adenovirus type 41 was sequenced and compared to the fiber gene sequence of adenovirus type 40 (A. H. Kidd and M. J. Erasmus, 1989, Virology 172, 134-144), the other known member of subgroup F. The open reading frame, from map units 87 through 92 with transcription from the r-strand, comprised 1686 bases and was 45 bases longer than its counterpart on the Ad40 genome. The 45-base difference appears to have resulted from a block deletion on the Ad40 sequence. Apart from this one region, the Ad40 and Ad41 fiber genes showed remarkably high homology (95.6%), indicating a relatively recent evolutionary divergence. The deduced amino acid sequence of the Ad41 fiber polypeptide was analyzed according to the model of N. M. Green et al. (1983, EMBO J. 2, 1357-1365) for the structure of the adenovirus fiber. Ad41 had one more 15-residue repeat in the shaft region than Ad40, there being 22 repeat motifs. A detailed study of various Ad40 and Ad41 strains with proven genome differences indicated that the 15-amino acid difference in polypeptide length at the 14th repeat motif is a type-specific difference among the subgroup F adenoviruses. However, two uncommon Ad41 strains belonging to 2 of the 16 Ad41 genome types tested had a 15-amino acid block deletion which was different to that of the Ad40 polypeptide. The implication from this work is that the Ad40 fiber gene probably arose from its Ad41 counterpart, but the fiber gene sequences of both types of subgroup F adenovirus are so similar that genetic recombination between strains could occur with some frequency.

O-linked GlcNAc in serotype-2 adenovirus fibre

Serotype-2 adenovirus fibre is shown to possess an O-linked GlcNAc residue and to have affinity for wheat germ agglutinin. The cytoplasmic and nuclear fibres are both glycosylated. Glycosylation seems to take place in the cytoplasm since most of the [14C]GlcN-labelled fibre is found in this compartment, little label being associated with the microsomes. Glycosylation of the fibre was not affected by inhibitors of N- and O-glycosylation. A variation in fibre glycosylation is observed among adenovirus. Among the serotypes tested, only serotype-5 adenovirus (another subgroup C virus) also incorporated [14C]GlcN into its fibre, but did not possess affinity for wheat-germ agglutinin. The GlcNAc is located in the N-terminal two-thirds of the fibre and more probably in the N-terminal one-third. The free or penton-base-associated fibres are similarly glycosylated. These results suggest that glycosylation is not involved in viral adsorption and in assembly with the capsid penton base. Thus, glycosylation might be a characteristic feature of subgroup C viruses.

Sequence characterization of the adenovirus 40 fiber gene

The fiber gene of human adenovirus type 40 has been characterized. The 6.1-kbp EcoRI fragment C of the Ad40 genome, from map units 74 through 92, was cloned and the right-most 2.8 kbp from 84 map units was sequenced. By analogy with Ad2, this region would be expected to contain the gene specifying the Ad40 fiber polypeptide. Sequencing revealed an open reading frame of 1641 bases on the r-strand, the first 53 bases of which had marked homology with the corresponding L5 (fiber) regions of Ad2 (77.2%), Ad5 (75.0%), and Ad3 (64.3%). In addition, base positions 1114 to 1146 of this open reading frame had 85% homology with base positions 1198 to 1230 of the Ad2 fiber gene. The predicted polypeptide sequence of 547 amino acids showed marked homology with the Ad2, Ad5, and Ad3 fiber polypeptides in two regions, in the first 55 amino acids from the N-terminus and from amino acids 372 through 382. Analysis of hydrophobic amino acid positions revealed a repeating pattern of approximately 15 residues between positions 42 and 374, with 21 repeats. The sequence of the Ad40 polypeptide thus fits the model of Green et al. [1983), EMBO J. 2, 1357-1365) for the structure of the adenovirus fiber, but is 35 amino acids shorter than the Ad2 fiber polypeptide, with one less 15-residue repeat in the shaft region. According to this model, the regions of highest homology between the Ad40 fiber polypeptide and those of Ad2, Ad5, and Ad3 correspond to the tail of the shaft and the base of the knob. The results of this analysis are in agreement with previously published EM data on the fiber length of subgroup F adenoviruses.

Complementary peptide sequences in partner proteins of the adenovirus capsid

Peptides quite similar to the signal peptide of secretory proteins were found in adenovirus serotype 2 fibre. This was surprising because this protein does not have this property. Because the N-end fibre is implicated in the penton base-fibre interaction and complementary peptides to the N-sequence fibre were found in penton base protein, these complementary peptides might be involved in the protein-protein recognition and interaction process. So far, known structural data agree with such an hypothesis.