Biochemical and genetical characterization of a fiber-defective temperature-sensitive mutant of type 2 adenovirus

Baculovirus display of single chain antibody (scFv) using a novel signal peptide

Background

Cells permissive to virus can become refractory to viral replication upon intracellular expression of single chain fragment variable (scFv) antibodies directed towards viral structural or regulatory proteins, or virus-coded enzymes. For example, an intrabody derived from MH-SVM33, a monoclonal antibody against a conserved C-terminal epitope of the HIV-1 matrix protein (MAp17), was found to exert an inhibitory effect on HIV-1 replication.

Results

Two versions of MH-SVM33-derived scFv were constructed in recombinant baculoviruses (BVs) and expressed in BV-infected Sf9 cells, N-myristoylation-competent scFvG2/p17 and N-myristoylation-incompetent scFvE2/p17 protein, both carrying a C-terminal HA tag. ScFvG2/p17 expression resulted in an insoluble, membrane-associated protein, whereas scFvE2/p17 was recovered in both soluble and membrane-incorporated forms. When coexpressed with the HIV-1 Pr55Gag precursor, scFvG2/p17 and scFvE2/p17 did not show any detectable negative effect on virus-like particle (VLP) assembly and egress, and both failed to be encapsidated in VLP. However, soluble scFvE2/p17 isolated from Sf9 cell lysates was capable of binding to its specific antigen, in the form of a synthetic p17 peptide or as Gag polyprotein-embedded epitope. Significant amounts of scFvE2/p17 were released in the extracellular medium of BV-infected cells in high-molecular weight, pelletable form. This particulate form corresponded to BV particles displaying scFvE2/p17 molecules, inserted into the BV envelope via the scFv N-terminal region. The BV-displayed scFvE2/p17 molecules were found to be immunologically functional, as they reacted with the C-terminal epitope of MAp17. Fusion of the N-terminal 18 amino acid residues from the scFvE2/p17 sequence (N18E2) to another scFv recognizing CD147 (scFv-M6-1B9) conferred the property of BV-display to the resulting chimeric scFv-N18E2/M6.

Conclusion

Expression of scFvE2/p17 in insect cells using a BV vector resulted in baculoviral progeny displaying scFvE2/p17. The function required for BV envelope incorporation was carried by the N-terminal octadecapeptide of scFvE2/p17, which acted as a signal peptide for BV display. Fusion of this peptide to the N-terminus of scFv molecules of interest could be applied as a general method for BV-display of scFv in a GP64- and VSV-G-independent manner.

Protein crystals in Adenovirus type 5-infected cells: requirements for intranuclear crystallogenesis, structural and functional analysis

Intranuclear crystalline inclusions have been observed in the nucleus of epithelial cells infected with Adenovirus serotype 5 (Ad5) at late steps of the virus life cycle. Using immuno-electron microscopy and confocal microscopy of cells infected with various Ad5 recombinants modified in their penton base or fiber domains, we found that these inclusions represented crystals of penton capsomers, the heteromeric capsid protein formed of penton base and fiber subunits. The occurrence of protein crystals within the nucleus of infected cells required the integrity of the fiber knob and part of the shaft domain. In the knob domain, the region overlapping residues 489-492 in the FG loop was found to be essential for crystal formation. In the shaft, a large deletion of repeats 4 to 16 had no detrimental effect on crystal inclusions, whereas deletion of repeats 8 to 21 abolished crystal formation without altering the level of fiber protein expression. This suggested a crucial role of the five penultimate repeats in the crystallisation process. Chimeric pentons made of Ad5 penton base and fiber domains from different serotypes were analyzed with respect to crystal formation. No crystal was found when fiber consisted of shaft (S) from Ad5 and knob (K) from Ad3 (heterotypic S5-K3 fiber), but occurred with homotypic S3K3 fiber. However, less regular crystals were observed with homotypic S35-K35 fiber. TB5, a monoclonal antibody directed against the Ad5 fiber knob was found by immunofluorescence microscopy to react with high efficiency with the intranuclear protein crystals in situ. Data obtained with Ad fiber mutants indicated that the absence of crystalline inclusions correlated with a lower infectivity and/or lower yields of virus progeny, suggesting that the protein crystals might be involved in virion assembly. Thus, we propose that TB5 staining of Ad-infected 293 cells can be used as a prognostic assay for the viability and productivity of fiber-modified Ad5 vectors.

Adenovirus type 5 fiber knob domain has a critical role in fiber protein synthesis and encapsidation

Adenovirus serotype 5 (Ad5) vectors carrying knobless fibers designed to remove their natural tropism were found to have a lower fiber content than recombinant Ad5 with wild-type (WT) capsid, implying a role for the knob-coding sequence or/and the knob domain in fiber encapsidation. Experimental data using a variety of fiber gene constructs showed that the defect did not occur at the fiber mRNA level, but at the protein level. Knobless fiber proteins were found to be synthesized at a significant slower rate compared with knob-carrying fibers, and the trimerization process of knobless fibers paralleled their slow rate of synthesis. A recombinant Ad5 diploid for the fiber gene (referred to as Ad5/R7-ZZwt/E1 : WT-fiber) was constructed to analyse the possible rescue of the knobless low-fiber-content phenotype by co-expression of WT fiber. Ad5/R7-ZZwt/E1 : WT-fiber contained a knobless fiber gene in its natural location (L5) in the viral genome and an additional WT fiber gene in an ectopic position in E1. Knobless fiber was still synthesized at low levels compared with the co-expressed E1 : WT fiber and the recovery of the two fiber species in virus progeny reflected their respective amounts in the infected cells. Our results suggested that deletion of the fiber knob domain had a negative effect on the translation of the fiber mRNA and on the intracellular concentration of fiber protein. They also suggested that the knob control of fiber protein synthesis and encapsidation occurred as aciseffect, which was not modified by WT fiber protein providedin transby the same Ad5 genome.

Epitopes and hemagglutination binding domain on subgenus B:2 adenovirus fibers

The adenovirus fiber serves as a ligand between the virus and the host cell receptor and manifests hemagglutination (HA) activity and antigenic domains. We have screened both the antigenic and immunogenic epitopes on the adenovirus fibers of subgenus B:2 by using recombinant fiber proteins (rfibers) expressed in Escherichia coli, synthesized peptides (P1 to P8), and the corresponding antisera. The results indicated that P4 (amino acids [aa] 201 to 220), P5 (aa 231 to 250), and P7 (aa 275 to 295) presented both antigenic and immunogenic epitopes in adenovirus type 11 prototype (Ad11p), Ad34a, and Ad11a fibers. P6 (aa 251 to 270) presented both epitopes in Ad11a fiber but only an antigenic epitope in other fibers. The C-terminal 20 amino acids of the fiber, corresponding to P8, manifested an epitope of low-level immunogenicity. P5, localized at the N-terminal aa 231 to 250, displayed an epitope that reacted with fibers of all the members of subgenus B analyzed. The rfibers of Ad11p and Ad34a displayed HA activity with monkey erythrocytes, though those of Ad11a did not. Mutagenesis of the rfibers revealed that neither the fragment replacements, 11p20211a, llp26011a,and 11a28011p, nor the Ad11p rfiber with the substitutions of Tyr-260-->H (Tyr260H)and Arg279Q displayed HA activity. The Ad11a fiber knob was sensitive to proteolytic digestion, whereas that of Ad11p was resistant. The results demonstrated that the decisive HA binding domain was presented at aa 260 to 280 and was conformation dependent. Nearby amino acids, aa 283 and 284, may also affect the HA function.

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.

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

Peptides corresponding to the N- and C-extremities of the adenovirus 2 fibre polypeptide were synthesized, coupled to tetanus toxoid and injected into rabbits. Two sera were obtained: the anti-NTT serum and the anti-CTT serum. These sera and an anti-native-fibre serum were used to study fragments generated by hydrochloric acid cleavage of the fibre. The 44-Kd fragment corresponding to the 2/3 N-terminal part of the molecule retained its antigenic reactivity. This is consistent with a shaft structure for this part of the fibre. The anti-peptide sera were used to orientate the fibre, i.e., to determine the site of anchorage of this protein in the penton base. First, immunorevelation of blots of enzymatic digests of native or dissociated penton suggested that the N-extremity of the fibre was involved in the assembly of this protein in the penton base. Second, attempts were made to determine the accessibility of the fibre ends in the penton structure by ELISA assays and by immunorevelation of penton in Western blots. The results agreed with the proposed orientation derived from study of the enzymatic digests. Since the 2 anti-peptide sera and the peptides were unable to affect viral adsorption, it was not possible to determine how the fibre is orientated with respect to the cell receptor. However, the anti-peptide sera were found to inhibit viral production slightly.

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.

Human adenovirus 2 temperature-sensitive mutant 112 contains three mutations in the protein IIIa gene

The temperature-sensitive (ts) mutant 112 of human adenovirus 2 is defective in the late stage of virus maturation. The region of functional mutation has been localised by marker rescue. It was observed that the ts mutation can be rescued by the left-hand part of the wild-type gene (nucleotides 12,301-12,891). By nucleotide sequencing, two mutations, both C to T (at position 12,386 and 12,741), were found in this region. The first one, in the glycine 20 codon, is silent, whereas the second changes alanine 145 to valine. A third mutation, which changed C to A (nucleotide 13,613), was identified in the right-hand part of the gene, resulting in the replacement of alanine-436 by threonine.