Conserved regions of bovine adenovirus-3 pVIII contain functional domains involved in nuclear localization and packaging in mature infectious virions

Regions of Bovine Adenovirus-3 Protein VII Involved in Interactions with Viral and Cellular Proteins

The L 1 region of bovine adenovirus (BAdV)-3 encodes a multifunctional protein named protein VII. Anti-protein VII sera detected a protein of 26 kDa in transfected or BAdV-3-infected cells, which localizes to nucleus and nucleolus of infected/transfected cells. Analysis of mutant protein VII identified four redundant overlapping nuclear/nucleolar localization signals as deletion of all four potential nuclear/nucleolar localization signals localizes protein VII predominantly to the cytoplasm. The nuclear import of protein VII appears to use importin α (α-1), importin-β (β-1) and transportin-3 nuclear transport receptors. In addition, different nuclear transport receptors also require part of protein VII outside nuclear localization sequences for efficient interaction. Proteomic analysis of protein complexes purified from recombinant BAdV-3 expressing protein VII containing Strep Tag II identified potential viral and cellular proteins interacting with protein VII. Here, we confirm that protein VII interacts with IVa2 and protein VIII in BAdV-3-infected cells. Moreover, amino acids 91-101 and 126-137, parts of non-conserved region of protein VII, are required for interaction with IVa2 and protein VIII, respectively.

Bioinformatics and Functional Analysis of a New Nuclear Localization Sequence of the Influenza A Virus Nucleoprotein

Influenza viruses deliver their genome into the nucleus of infected cells for replication. This process is mediated by the viral nucleoprotein (NP), which contains two nuclear localization sequences (NLSs): NLS1 at the N-terminus and a recently identified NLS2 (²¹²GRKTR²¹⁶). Through mutagenesis and functional studies, we demonstrated that NP must have both NLSs for an efficient nuclear import. As with other NLSs, there may be variations in the basic residues of NLS2 in different strains of the virus, which may affect the nuclear import of the viral genome. Although all NLS2 variants fused to the GFP mediated nuclear import of GFP, bioinformatics showed that 98.8% of reported NP sequences contained either the wild-type sequence ²¹²GRKTR²¹⁶ or ²¹²GRRTR²¹⁶. Bioinformatics analyses used to study the presence of NLS2 variants in other viral and nuclear proteins resulted in very low hits, with only 0.4% of human nuclear proteins containing putative NLS2. From these, we studied the nucleolar protein 14 (NOP14) and found that NLS2 does not play a role in the nuclear import of this protein but in its nucleolar localization. We also discovered a functional NLS at the C-terminus of NOP14. Our findings indicate that NLS2 is a highly conserved influenza A NP sequence.

Adenovirus Core Proteins: Structure and Function

Adenoviruses have served as a model for investigating viral-cell interactions and discovering different cellular processes, such as RNA splicing and DNA replication. In addition, the development and evaluation of adenoviruses as the viral vectors for vaccination and gene therapy has led to detailed investigations about adenovirus biology, including the structure and function of the adenovirus encoded proteins. While the determination of the structure and function of the viral capsid proteins in adenovirus biology has been the subject of numerous reports, the last few years have seen increased interest in elucidating the structure and function of the adenovirus core proteins. Here, we provide a review of research about the structure and function of the adenovirus core proteins in adenovirus biology.

Nuclear and Nucleolar Localization of Bovine Adenovirus-3 Protein V

The L2 region of bovine adenovirus-3 (BAdV-3) encodes a Mastadenovirus genus-specific protein, designated as pV, which is important for the production of progeny viruses. Here, we demonstrate that BAdV-3 pV, expressed as 55 kDa protein, localizes to the nucleus and specifically targets nucleolus of the infected cells. Analysis of deletion mutants of pV suggested that amino acids 81–120, 190–210, and 380–389 act as multiple nuclear localization signals (NLS), which also appear to serve as the binding sites for importin α-3 protein, a member of the importin α/β nuclear import receptor pathway. Moreover, pV amino acids 21–50 and 380–389 appear to act as nucleolar localization signals (NoLs). Interestingly, amino acids 380–389 appear to act both as NLS and as NoLS. The presence of NoLS is essential for the production of infectious progeny virions, as deletion of both NoLs are lethal for the production of infectious BAdV-3. Analysis of mutant BAV.pVd1d3 (isolated in pV completing CRL cells) containing deletion/mutation of both NoLS in non-complementing CRL cells not only revealed the altered intracellular localization of mutant pV but also reduced the expression of some late proteins. However, it does not appear to affect the incorporation of viral proteins, including mutant pV, in BAV.pVd1d3 virions. Further analysis of CsCl purified BAV.pVd1d3 suggested the presence of thermo-labile virions with disrupted capsids, which appear to affect the infectivity of the progeny virions. Our results suggest that pV contains overlapping and non-overlapping NoLS/NLS. Moreover, the presence of both NoLS appear essential for the production of stable and infectious progeny BAV.pVd1d3 virions.

Bovine Adenovirus-3 Tropism for Bovine Leukocyte Sub-Populations

A number of characteristics including lack of virulence and the ability to grow to high titers, have made bovine adenovirus-3 (BAdV-3) a vector of choice for further development as a vaccine-delivery vehicle for cattle. Despite the importance of blood leukocytes, including dendritic cells (DC), in the induction of protective immune responses, little is known about the interaction between BAdV-3 and bovine blood leukocytes. Here, we demonstrate that compared to other leukocytes, bovine blood monocytes and neutrophils are significantly transduced by BAdV404a (BAdV-3, expressing enhanced yellow green fluorescent protein [EYFP]) at a MOI of 1-5 without a significant difference in the mean fluorescence of EYFP expression. Moreover, though expression of some BAdV-3-specific proteins was observed, no progeny virions were detected in the transduced monocytes or neutrophils. Interestingly, addition of the "RGD" motif at the C-terminus of BAdV-3 minor capsid protein pIX (BAV888) enhanced the ability of the virus to enter the monocytes without altering the tropism of BAdV-3. The increased uptake of BAV888 by monocytes was associated with a significant increase in viral genome copies and the abundance of EYFP and BAdV-3 19K transcripts compared to BAdV404a-transduced monocytes. Our results suggest that BAdV-3 efficiently transduces monocytes and neutrophils in the absence of viral replication. Moreover, RGD-modified capsid significantly increases vector uptake without affecting the initial interaction with monocytes.

Regions of bovine adenovirus-3 IVa2 involved in nuclear/nucleolar localization and interaction with pV

Bovine adenovirus-3 (BAdV-3) is a non enveloped, icosahedral DNA virus containing a genome of 34446 bps. The intermediate region of BAdV-3 encodes pIX and IVa2 proteins. Here, we report the characterization of BAdV-3 IVa2. Anti-IVa2 serum detected a 50 kDa protein at 24-48 h post infection in BAdV-3 infected cells. The IVa2 localizes to nucleus and nucleolus of BAdV-3 infected cells. Analysis of mutant IVa2 demonstrated that amino acids 1-25 and 373-448 are required for nuclear and nucleolar localization of IVa2, respectively. The nuclear import of IVa2 utilize importin α -1 of importin nuclear import pathway. Although deletion/substitution of amino acids 4-18 is sufficient to abrogate the nuclear localization of IVa2, amino acids 1-25 are required for nuclear localization of a cytoplasmic protein. Furthermore, we demonstrate that amino acids 1-25 and 120-140 of IVa2 interact with importin α-1 and pV proteins, respectively in BAdV-3 infected cells.

Domains of bovine adenovirus-3 protein 22K involved in interacting with viral protein 52K and cellular importins α-5/α-7

The L6 region of bovine adenovirus-3 (BAdV-3) encodes unspliced and spliced proteins named 22K and 33K, respectively. Earlier, anti-22K sera detected two proteins of 42- and 37-kDa in infected cells and 42-kDa protein in transfected cells. Here, we demonstrate that 22K protein localizes to the nucleus of BAdV-3 infected or transfected cells. Analysis of mutant 22K proteins suggested that amino acids 231-250 of non-conserved C-terminus of 22K are required for nuclear localization. The nuclear import of 22K appears to utilize multiple importin (α-5 and α-7) of importin α/β nuclear import pathway. Mutational analysis of 22K identified four basic residues ²³⁸RRRK²⁴¹, which apparently are essential for the nuclear localization of 22K. Our results suggest that the nuclear localization of 22K appear essential for virus replication and production of progeny BAdV-3. Furthermore, we demonstrate that N-terminus amino acid 35-65 conserved in 22K and 33K interact with 52K protein in BAdV-3 infected cells.

Proteolytic Cleavage of Bovine Adenovirus 3-Encoded pVIII

Proteolytic maturation involving cleavage of one nonstructural and six structural precursor proteins including pVIII by adenovirus protease is an important aspect of the adenovirus life cycle. The pVIII encoded by bovine adenovirus 3 (BAdV-3) is a protein of 216 amino acids and contains two potential protease cleavage sites. Here, we report that BAdV-3 pVIII is cleaved by adenovirus protease at both potential consensus protease cleavage sites. Usage of at least one cleavage site appears essential for the production of progeny BAdV-3 virions as glycine-to-alanine mutation of both protease cleavage sites appears lethal for the production of progeny virions. However, mutation of a single protease cleavage site of BAdV-3 pVIII significantly affects the efficient production of infectious progeny virions. Further analysis revealed no significant defect in endosome escape, genome replication, capsid formation, and virus assembly. Interestingly, cleavage of pVIII at both potential cleavage sites appears essential for the production of stable BAdV-3 virions as BAdV-3 expressing pVIII containing a glycine-to-alanine mutation of either of the potential cleavage sites is thermolabile, and this mutation leads to the production of noninfectious virions.IMPORTANCE Here, we demonstrated that the BAdV-3 adenovirus protease cleaves BAdV-3 pVIII at both potential protease cleavage sites. Although cleavage of pVIII at one of the two adenoviral protease cleavage sites is required for the production of progeny virions, the mutation of a single cleavage site of pVIII affects the efficient production of infectious progeny virions. Further analysis indicated that the mutation of a single protease cleavage site (glycine to alanine) of pVIII produces thermolabile virions, which leads to the production of noninfectious virions with disrupted capsids. We thus provide evidence about the requirement of proteolytic cleavage of pVIII for production of infectious progeny virions. We feel that our study has significantly advanced the understanding of the requirement of adenovirus protease cleavage of pVIII.

Bovine Adenovirus-3 pVIII Suppresses Cap-Dependent mRNA Translation Possibly by Interfering with the Recruitment of DDX3 and Translation Initiation Factors to the mRNA Cap

Earlier, targeting of DDX3 by few viral proteins has defined its role in mRNA transport and induction of interferon production. This study was conducted to investigate the function of bovine adenovirus (BAdV)-3 pVIII during virus infection. Here, we provided evidence regarding involvement of DDX3 in cap dependent cellular mRNA translation and demonstrated that targeting of DDX3 by adenovirus protein VIII interfered with cap-dependent mRNA translation function of DDX3 in virus infected cells. Adenovirus late protein pVIII interacted with DDX3 in transfected and BAdV-3 infected cells. pVIII inhibited capped mRNA translation in vitro and in vivo by limiting the amount of DDX3 and eIF3. Diminished amount of DDX3 and eIFs including eIF3, eIF4E, eIF4G, and PABP were present in cap binding complex in BAdV-3 infected or pVIII transfected cells with no trace of pVIII in cap binding complex. The total amount of eIFs appeared similar in uninfected or infected cells as BAdV-3 did not appear to degrade eIFs. The co-immunoprecipitation experiments indicated the absence of direct interaction between pVIII and eIF3, eIF4E, or PABP. These data indicate that interaction of pVIII with DDX3 interferes with the binding of eIF3, eIF4E and PABP to the 5′ Cap. We conclude that DDX3 promotes cap-dependent cellular mRNA translation and BAdV-3 pVIII inhibits translation of capped cellular mRNA possibly by interfering with the recruitment of eIFs to the capped cellular mRNA.

Deletion of pV affects integrity of capsid causing defect in the infectivity of bovine adenovirus-3

Members of the genus Mastadenovirus including bovine adenovirus 3 (BAdV-3) encode a genus-specific unique protein named pV. The pV encoded by BAdV-3 is a protein of 423 aa showing 40.9 % identity to pV of human adenovirus 2. Here, we report the construction and analysis of recombinant BAdV-3 (BAV.dV) containing deletion of pV. The BAV.dV could only be isolated in CRL.pV cells expressing pV, suggesting that pV appears essential for the infection of BAdV-3. Analysis of BAV.dV suggested that despite affecting some late gene expression in virus-infected cells, there was no significant difference in the incorporation of viral proteins in the mature virions. Moreover, analysis of mature virions revealed degraded capsids leading to change in morphology and infectivity of BAV.dV. Furthermore, analysis of the genome sequence of different clones of BAV.dV passaged in different cell lines revealed no mutations in core proteins pVII and pX\Mu suggesting that the replication defect may not be rescued. Our results suggest that pV is required for proper viral assembly of BAdV-3 as lack of pV produces aberrant capsids. Moreover, altered capsids lead to the production of non-infectious BAV.dV virions.

Bovine adenovirus-3 as a vaccine delivery vehicle

The use of vaccines is an effective and relatively inexpensive means of controlling infectious diseases, which cause heavy economic losses to the livestock industry through animal loss, decreased productivity, treatment expenses and decreased carcass quality. However, some vaccines produced by conventional means are imperfect in many respects including virulence, safety and efficacy. Moreover, there are no vaccines for some animal diseases. Although genetic engineering has provided new ways of producing effective vaccines, the cost of production for veterinary use is a critical criterion for selecting the method of production and delivery of vaccines. The cost effective production and intrinsic ability to enter cells has made adenovirus vectors a highly efficient tool for delivery of vaccine antigens. Moreover, adenoviruses induce both humoral and cellular immune responses to expressed vaccine antigens. Since nonhuman adenoviruses are species specific, the development of animal specific adenoviruses as vaccine delivery vectors is being evaluated. This review summarizes the work related to the development of bovine adenovirus-3 as a vaccine delivery vehicle in animals, particularly cattle.