Completion of the genomic sequence of the simian rotavirus SA11: nucleotide sequences of segments 1, 2, and 3

Sequences of the four larger proteins of a porcine group C rotavirus and comparison with the equivalent group A rotavirus proteins

The sequences of the four larger proteins of rotavirus group C (Cowden strain) are presented and compared with the sequences of the corresponding group A proteins. They exhibit a significant level of homology, allowing gene coding assignment for the group C rotavirus. The coding strategy of the group C virus RNA segment is the same as that for the group A large segments as one long open reading frame is present in each segment. The genome segment 1 encodes the structural protein VP1 which presents the RNA-dependent RNA polymerase consensus motifs. The VP1 protein is the most highly conserved between the rotaviruses of groups A and C. The genome segment 2 encodes the VP2 protein. The deduced protein sequence does not present the putative leucine zippers identified in the group A protein but its amino terminal is hydrophilic and highly charged as previously noted for the group A protein. The genome segment 3 encodes for a protein homologous to the group A outer capsid protein VP4. As observed among the various group A sequences, the amino terminal is the region presenting the fewest similarities. A cleavage region and a putative fusion motif similar to those present in the group A viruses have been identified. For this protein the comparison has been extended to the IDIRV [corrected] VP3 previously sequenced and indicates that groups A and C VP4 proteins are much more related to each other than to the group B equivalent. The genome segment 4 encodes for a protein showing an approximate 40% sequence identity to the minor core protein, VP3, of the group A rotavirus. This remarkable conservation of primary structures argues for severe functional constraint on the evolution of these proteins.

Infectious entry pathway for canine parvovirus

We have investigated whether canine parvovirus (CPV) infection involves a low pH-dependent entry pathway. The effects of two lysosomotropic bases, NH4Cl and chloroquine, on CPV entry were studied by immunofluorescence and ultrastructural and biochemical methods. In the presence of these reagents, input virions appear to accumulate in large vacuoles. Ultrastructural studies indicated that uptake of virions takes place predominantly in small uncoated vesicles that appear to fuse with larger vesicles. In the presence of NH4Cl, virions accumulate in the latter structures and their uncoating appears to be prevented. Viral DNA as well as antigen synthesis were found to be significantly inhibited in the presence of these reagents. In addition, inhibition of viral DNA and antigen synthesis appeared to be most extensive when NH4Cl was present from 30 min preinfection, whereas no significant inhibition was observed when the cells were treated after 2 hr postinfection. Thus, the results indicate that CPV requires exposure to low pH in an endosomal compartment to initiate a productive infection.

The S2 gene nucleotide sequences of prototype strains of the three reovirus serotypes: characterization of reovirus core protein sigma 2

The S2 gene nucleotide sequences of prototype strains of the three reovirus serotypes were determined to gain insight into the structure and function of the S2 translation product, virion core protein sigma 2. The S2 sequences of the type 1 Lang, type 2 Jones, and type 3 Dearing strains are 1,331 nucleotides in length and contain a single large open reading frame that could encode a protein of 418 amino acids, corresponding to sigma 2. The deduced sigma 2 amino acid sequences of these strains are very conserved, being identical at 94% of the sequence positions. Predictions of sigma 2 secondary structure and hydrophobicity suggest that the protein has a two-domain structure. A larger domain is suggested to be formed from the amino-terminal three-fourths of sigma 2 sequence, which is separated from a smaller carboxy-terminal domain by a turn-rich hinge region. The carboxy-terminal domain includes sequences that are more hydrophilic than those in the rest of the protein and contains sequences which are predicted to form an alpha-helix. A region of striking similarity was found between amino acids 354 and 374 of sigma 2 and amino acids 1008 and 1031 of the beta subunit of the Escherichia coli DNA-dependent RNA polymerase. We suggest that the regions with similar sequence in sigma 2 and the beta subunit form amphipathic alpha-helices which may play a related role in the function of each protein. We have also performed experiments to further characterize the double-stranded RNA-binding activity of sigma 2 and found that the capacity to bind double-stranded RNA is a property of the sigma 2 protein of prototype strains and of the S2 mutant tsC447.

Photoaffinity labeling of rotavirus VP1 with 8-azido-ATP: identification of the viral RNA polymerase

Rotavirus single-shelled particles have several enzymatic activities that are involved with the synthesis of capped mRNAs both in vivo and in vitro. Because single-shelled particles must be structurally intact to carry out transcription, it has proven to be difficult to identify the protein within such particles that possesses associated RNA polymerase activity. One approach for characterizing the function of the individual proteins within single-shelled particles is to use nucleotide analogs to specifically label those proteins, such as the viral RNA polymerase, that have affinity for nucleotides. In this study, 8-azido-ATP (azido-ATP), a photoreactable nucleotide analog, was used to identify the viral RNA polymerase on the basis of the ability of the analog to inhibit transcription activity associated with rotavirus particles on exposure to UV light. When single-shelled particles were treated with UV light in the presence of [alpha-32P]azido-ATP, the structural protein VP1 became radiolabeled because of cross-linking of the nucleotide analog, and there was a corresponding decrease in the ability of the particles to synthesize mRNA. In parallel experiments in which single-shelled particles were not exposed to UV light, VP1 was not radiolabeled and the particles successfully used azido-ATP as a substrate for the synthesis of viral mRNAs. Taken together, these results are consistent only with the conclusion that VP1 is the rotavirus RNA-dependent RNA polymerase.

Expression of rotavirus VP2 produces empty corelike particles

The complete VP2 gene of bovine rotavirus strain RF has been inserted into the baculovirus transfer vector pVL941 under the control of the polyhedrin promoter. Cotransfection of Spodoptera frugiperda 9 cells with wild-type baculovirus DNA and transfer vector DNA led to the formation of recombinant baculoviruses which contain bovine rotavirus gene 2. Infection of S. frugiperda cells with this recombinant virus resulted in the production of a protein similar in size and antigenic properties to the authentic rotavirus VP2. The protein binds double-stranded RNA and DNA in an overlay protein blot assay. Expressed VP2 assembles in the cytoplasm of infected cells in corelike particles 45 nm in diameter. These corelike particles were purified by sucrose gradient centrifugation and found to be devoid of nucleic acid. Coexpression of VP2 and VP6 from heterologous rotavirus strains (bovine and simian) resulted in the formation of single-shelled particles. These results definitively show the existence of an innermost protein shell in rotavirus which is formed independently of other rotavirus proteins. These results have implications for schemes of rotavirus morphogenesis.

Mammalian reoviruses contain a myristoylated structural protein

The structural protein mu 1 of mammalian reoviruses was noted to have a potential N-myristoylation sequence at the amino terminus of its deduced amino acid sequence. Virions labeled with [3H]myristic acid were used to demonstrate that mu 1 is modified by an amide-linked myristoyl group. A myristoylated peptide having a relative molecular weight (Mr) of approximately 4,000 was also shown to be a structural component of virions and was concluded to represent the 4.2-kDa amino-terminal fragment of mu 1 which is generated by the same proteolytic cleavage that yields the carboxy-terminal fragment and major outer capsid protein mu 1C. The myristoylated 4,000-Mr peptide was found to be present in reovirus intermediate subviral particles but to be absent from cores, indicating that it is a component of the outer capsid. A distinct large myristoylated fragment of the intact mu 1 protein was also identified in intermediate subviral particles, but no myristoylated mu-region proteins were identified in cores, consistent with the location of mu 1 in the outer capsid. Similarities between amino-terminal regions of the reovirus mu 1 protein and the poliovirus capsid polyprotein were noted. By analogy with other viruses that contain N-myristoylated structural proteins (particularly picornaviruses), we suggest that the myristoyl group attached to mu 1 and its amino-terminal fragments has an essential role in the assembly and structure of the reovirus outer capsid and in the process of reovirus entry into cells.