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Gene product 8 (gp8, 344 amino acids per monomer) of bacteriophage T4 is one of the baseplate structural proteins. We constructed an expression vector of gp8 and developed a method for purification of recombinant protein. CD spectroscopy showed that gp8 is an alpha/beta type structural protein. Its polypeptide chain consists of nearly 40% beta-structure and 15% alpha-helix. These data agree with results of prediction of secondary structure based on the amino acid sequence of the protein. The sedimentation coefficient under standard conditions (S20,w) is 4.6S. Analytical ultracentrifugation results demonstrated that gp8 in solution has two types of oligomers--dimer and tetramer. The tetramer of gp8 may be included in the wedge (1/6 of the baseplate), and the dimer may be an intermediate product of association.

Chlorella virus PBCV-1 topoisomerase II is the only functional type II enzyme known to be encoded by a virus that infects eukaryotic cells. However, it has not been established whether the protein is expressed following viral infection or whether the enzyme has any catalytic features that distinguish it from cellular type II topoisomerases. Therefore, the present study characterized the physiological expression of PBCV-1 topoisomerase II and individual reaction steps catalyzed by the enzyme. Results indicate that the topoisomerase II gene is widely distributed among Chlorella viruses and that the protein is expressed 60-90 min after viral infection of algal cells. Furthermore, the enzyme has an extremely high DNA cleavage activity that sets it apart from all known eukaryotic type II topoisomerases. Levels of DNA scission generated by the viral enzyme are approximately 30 times greater than those observed with human topoisomerase IIalpha. The high levels of cleavage are not due to inordinately tight enzyme-DNA binding or to impaired DNA religation. Thus, they most likely reflect an elevated forward rate of scission. The robust DNA cleavage activity of PBCV-1 topoisomerase II provides a unique tool for studying the catalytic functions of type II topoisomerases.

The human astroviruses (HAst) are increasingly recognized as an important cause of gastroenteritis. These viruses contain a 6.8-kb positive-sense, single-stranded RNA molecule that is infectious when transfected into permissive cells. The HAst gene 1 is composed of two open reading frames (ORFs 1a and 1b) connected by a ribosomal frameshift. Gene 1 is predicted to encode two nonstructural polyproteins (pp 1a and pp 1ab), and analysis of the HAst gene 1 sequence has resulted in predictions of a serine proteinase within the ORF1a polyprotein. However, none of the gene 1 proteins have been identified. To examine the expression and processing of the HAst2 gene 1 polyprotein, we have translated pp 1a and pp 1ab in vitro. These ongoing studies will provide the foundation for correlating gene 1 expression in vitro with proteins expressed in virus-infected cells.

The reovirus sigma1s protein is a 14-kDa nonstructural protein encoded by the S1 gene segment. The S1 gene has been linked to many properties of reovirus, including virulence and induction of apoptosis. Although the function of sigma1s is not known, the sigma1s open reading frame is conserved in all S1 gene sequences determined to date. In this study, we identified and characterized a variant of type 3 reovirus, T3C84-MA, which does not express sigma1s. To facilitate these experiments, we generated two monoclonal antibodies (MAbs) that bind different epitopes of the sigma1s protein. Using these MAbs in immunoblot and immunofluorescence assays, we found that L929 (L) cells infected with T3C84-MA do not contain sigma1s. To determine whether sigma1s is required for reovirus infection of cultured cells, we compared the growth of T3C84-MA and its parental strain, T3C84, in L cells and Madin-Darby canine kidney (MDCK) cells. After 48 h of growth, yields of T3C84-MA were equivalent to yields of T3C84 in L cells and were fivefold lower than yields of T3C84 in MDCK cells. After 7 days of growth following adsorption at a low multiplicity of infection, yields of T3C84-MA and T3C84 did not differ significantly in either L cells or MDCK cells. To determine whether sigma1s is required for apoptosis induced by reovirus infection, T3C84-MA and T3C84 were tested for their capacity to induce apoptosis, using an acridine orange staining assay. In these experiments, the percentages of apoptotic cells following infection with T3C84-MA and T3C84 were equivalent. These findings indicate that nonstructural protein sigma1s is not required for reovirus growth in cell culture and does not influence the capacity of reovirus to induce apoptosis. Therefore, reovirus replication does not require the full complement of virally encoded proteins.

By using a model system for cell transformation mediated by the cooperation of the activated H-ras oncogene and the inactivated p53 tumor suppressor gene, rCop-1 was identified by mRNA differential display as a gene whose expression became lost after cell transformation. Homology analysis indicates that rCop-1 belongs to an emerging cysteine-rich growth regulator family called CCN, which includes connective-tissue growth factor, CYR61, CEF10 (v-src inducible), and the product of the nov proto-oncogene. Unlike the other members of the CCN gene family, rCop-1 is not an immediate-early gene, it lacks the conserved C-terminal domain which was shown to confer both growth-stimulating and heparin-binding activities, and its expression is lost in cells transformed by a variety of mechanisms. Ectopic expression of rCop-1 by retroviral gene transfers led to cell death in a transformation-specific manner. These results suggest that rCop-1 represents a new class of CCN family proteins that have functions opposing those of the previously identified members.

Mutations selected in reoviruses isolated from persistently infected cultures (PI viruses) affect viral entry into cells. Unlike wild-type (wt) viruses, PI viruses can grow in the presence of ammonium chloride, a weak base that blocks acid-dependent proteolysis of viral outer-capsid proteins in cellular endosomes during viral entry. In this study, we show that E64, an inhibitor of cysteine proteases such as those present in the endocytic compartment, blocks growth of wt reovirus by inhibiting viral disassembly. To determine whether PI viruses can grow in the presence of an inhibitor of endocytic proteases, we compared yields of wt and PI viruses in cells treated with E64. Prototype PI viruses L/C, PI 2A1, and PI 3-1 produced substantially greater yields than wt viruses type 1 Lang (T1L) and type 3 Dearing (T3D) in E64-treated cells. To identify viral genes that segregate with growth of PI viruses in the presence of E64, we tested reassortant viruses isolated from independent crosses of T1L and each of the prototype PI viruses for growth in cells treated with E64. Growth of reassortant viruses in the presence of E64 segregated exclusively with the S4 gene, which encodes viral outer-capsid protein sigma3. These results suggest that mutations in sigma3 protein selected during persistent infection alter its susceptibility to cleavage during viral disassembly. To determine the temporal relationship of acid-dependent and protease-dependent steps in reovirus disassembly, cells were infected with wt strain T1L or T3D, and medium containing either ammonium chloride or E64d, a membrane-permeable form of E64, was added at various times after adsorption. Susceptibility to inhibition by both ammonium chloride and E64 was abolished when either inhibitor was added at times greater than 60 min after adsorption. These findings indicate that acid-dependent and protease-dependent disassembly events occur with similar kinetics early in reovirus replication, which suggests that these events take place within the same compartment of the endocytic pathway.

A cold-passaged (cp) temperature-sensitive (ts) mutant of human respiratory syncytial virus designated RSV cpts-248 was previously derived by random chemical mutagenesis of the non-ts mutant cp-RSV that possesses one or more host range mutations. We previously demonstrated in rodents and seronegative chimpanzees that the cpts-248 virus is more attenuated than cp-RSV and is more stable genetically than previously isolated RSV ts mutants. In the present study, we determined that the acquisition of the ts phenotype and the increased attenuation of the cpts-248 virus are associated with a single nucleotide substitution at nucleotide 10,989 that results in a change in the coding region (amino acid position 831) of the polymerase gene. The identification of this attenuating ts mutation is important because cpts-248 was used as the parent virus for the generation of a number of further attenuated mutants that are currently being evaluated as candidate vaccine strains in clinical trials in infants. Furthermore, technology now exists to rationally design new vaccine candidates by incorporating multiple attenuating mutations, such as the one identified here, into infectious viruses that are genetically stable and appropriately attenuated.

The reovirus attachment protein, sigma1, determines numerous aspects of reovirus-induced disease, including viral virulence, pathways of spread, and tropism for certain types of cells in the central nervous system. The sigma1 protein projects from the virion surface and consists of two distinct morphologic domains, a virion-distal globular domain known as the head and an elongated fibrous domain, termed the tail, which is anchored into the virion capsid. To better understand structure-function relationships of sigma1 protein, we conducted experiments to identify sequences in sigma1 important for viral binding to sialic acid, a component of the receptor for type 3 reovirus. Three serotype 3 reovirus strains incapable of binding sialylated receptors were adapted to growth in murine erythroleukemia (MEL) cells, in which sialic acid is essential for reovirus infectivity. MEL-adapted (MA) mutant viruses isolated by serial passage in MEL cells acquired the capacity to bind sialic acid-containing receptors and demonstrated a dependence on sialic acid for infection of MEL cells. Analysis of reassortant viruses isolated from crosses of an MA mutant virus and a reovirus strain that does not bind sialic acid indicated that the sigma1 protein is solely responsible for efficient growth of MA mutant viruses in MEL cells. The deduced sigma1 amino acid sequences of the MA mutant viruses revealed that each strain contains a substitution within a short region of sequence in the sigma1 tail predicted to form beta-sheet. These studies identify specific sequences that determine the capacity of reovirus to bind sialylated receptors and suggest a location for a sialic acid-binding domain. Furthermore, the results support a model in which type 3 sigma1 protein contains discrete receptor binding domains, one in the head and another in the tail that binds sialic acid.

Reoviruses isolated from persistently infected cultures (PI viruses) can grow in the presence of ammonium chloride, a weak base that blocks acid-dependent proteolysis of viral outer-capsid proteins during viral entry into cells. We used reassortant viruses isolated from crosses of wild-type (wt) reovirus strain, type 1 Lang, and three independent PI viruses, L/C, PI 2A1, and PI 3-1, to identify viral genes that segregate with the capacity of PI viruses to grow in cells treated with ammonium chloride. Growth of reassortant viruses in ammonium chloride-treated cells segregated with the S1 gene of L/C and the S4 gene of PI 2A1 and PI 3-1. The S1 gene encodes viral attachment protein sigma1, and the S4 gene encodes outer-capsid protein sigma3. To identify mutations in sigma3 selected during persistent reovirus infection, we determined the S4 gene nucleotide sequences of L/C, PI 2A1, PI 3-1, and four additional PI viruses. The deduced amino acid sequences of sigma3 protein of six of these PI viruses contained a tyrosine-to-histidine substitution at residue 354. To determine whether mutations selected during persistent infection alter cleavage of the viral outer capsid, the fate of viral structural proteins was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after treatment of virions of wt and PI viruses with chymotrypsin in vitro. Proteolysis of PI virus outer-capsid proteins sigma3 and mu1C occurred with faster kinetics than proteolysis of wt virus outer-capsid proteins. These results demonstrate that mutations in either the S1 or S4 gene alter acid-dependent disassembly of the reovirus outer capsid and suggest that increased efficiency of proteolysis of viral outer-capsid proteins is important for maintenance of persistent reovirus infections of cultured cells.