The mechanisms of reovirus uncoating and gene activation in vivo

Shythesis of reovirus oligo adenylic acid in vivo and in vitro

The formation of reovirus double-stranded (ds) RNA and of oligo adenylic acid (oligo A) is inhibited by 5 mug of actinomycin D per ml added at the time of viral infection. Viral proteins are synthesized and assembled into dsRNA-deficient particles under these conditions. The addition of cycloheximide to infected cells during the mid-logarithmic phase of viral replication terminates protein and dsRNA synthesis, but allows continued oligo A synthesis for about 1 h. The (3)H-labeled oligo A formed in the presence of cycloheximide is incorporated into particles whose density in CsCl is identical to that of reovirions. Using the large particulate or virus factory-containing cytoplasmic fraction of infected L-cells, we have established an in vitro system for the synthesis of oligo A. The in vitro product migrates slightly faster in sodium dodecyl sulfate acrylamide gels than marker oligo A. Oligo A synthesis in vitro continues for about 1 h, requires, the presence of only one ribonucleoside triphosphate (ATP), is not inhibited by DNase or RNase, but is abruptly terminated by the addition of chymotrypsin to the reaction mixture. Oligo A formed both in vivo and in vitro is released from the factory fraction by chymotrypsin digestion. The enzymes which catalyze the synthesis of oligo A, dsRNA, and single-stranded RNA all exhibit a similar temperature dependence with an optimum of approximately 45 C. These results indicate that oligo A is formed within the core of the nascent virion after the completion of dsRNA synthesis; they suggest that the oligo A polymerase is an alternative activity of the virion-bound transcriptase and that it is regulated by outer capsomere proteins.

Early events in cell-animal virus interactions

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New intermediate subviral particles in the in vitro uncoating of reovirus virions by chymotrypsin

Reovirus virions, grown in suspension cultures of L cells and extensively purified by density gradient and velocity gradient centrifugation after their release from cell debris by fluorocarbon extraction, are characterized by a mean particle diameter of 73 nm and a density in CsCl of 1.36 to 1.37 g/cm(3). Treatment of intact virions by chymotrypsin (CHT) digestion in vitro converts them to subviral particles (SVP) having characteristics which are determined by the species of monovalent cation present during the digestion. In the presence of Cs(+) ions, CHT converts the virions to SVP of mean diameter 51 nm and density 1.43 to 1.44 g/cm(3). In the presence of K(+) ions, the conversion is to SVP of diameter 51 nm and density 1.39 to 1.40 g/cm(3). The SVP made in the presence of either Cs(+) or K(+) possess an extremely active RNA polymerase and nucleoside triphosphate phosphohydrolase (NTPase) activity in vitro and are resistant to further digestion by CHT. Treatment of intact virions with CHT in the presence of Na(+) or Li(+) ions results in their conversion to SVP of mean diameter 64 nm and density 1.37 to 1.38 g/cm(3). Such SVP are not active in in vitro RNA synthesis or NTP hydrolysis and are resistant to further digestion by CHT even during prolonged exposure to high concentrations of enzyme. Addition of Cs(+) or K(+) ions to the digestion mixture allows conversion of the 64-nm diameter SVP to 51-nm diameter SVP in which the RNA polymerase and NTPase are active in vitro. Analysis of the proteins present in intact virions and in the different SVP reveals clear differences which indicate that the conversions are accomplished by removal or cleavage of particular species of polypeptides.

Isolation and properties of an RNA polymerase from influenza virus-infected cells

Structures with RNA polymerase activity were isolated from influenza virus-infected cells, and consisted of ribonucleoprotein (RNP) complexes, similar in morphology to the viral internal component or nucleocapsid. The isolation procedure involved fractionation of infected cells in a discontinuous sucrose gradient, in which enzyme activity was concentrated in a fraction of intermediate density which contains both smooth and rough cytoplasmic membranes. The RNPs with polymerase activity were further purified in a velocity gradient, after which the peak fractions showed a 35-fold purification of the polymerase activity when compared with cytoplasmic extracts. The NP polypeptide, which is the subunit of the virion RNP, was the only virus-specific polypeptide detected in these RNP structures.

The enzymatic iodination of the red cell membrane

An enzymatic iodination procedure utilizing lactoperoxidase (LPO), radioactive iodide, and hydrogen peroxide generated by a glucose oxidase-glucose system has been described and utilized for a study of the red cell membrane. 97% of the incorporated isotope is in the erythrocyte ghost and 3% is associated with hemoglobin. No significant labeling of the red cell membrane occurs in the absence of LPO or by the deletion of any of the other reagents. A 6 million-fold excess of chloride ions inhibits iodination by no more than 50%. Incorporation of up to 1 x 10(6) iodide atoms into a single erythrocyte membrane results in no significant cell lysis. The incorporated label is exclusively in tyrosine residues as monoiodotyrosine. 10-15% of the trichloroacetic acid-precipitable radioactivity can be extracted with lipid solvents but is present as either labeled protein or (125)I. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized membrane proteins reveals only two labeled protein bands out of the 15 present, and the presence of 50-1 x 10(6) iodide atoms per ghost does not alter this pattern. Component a has a molecular weight of 110,000, is carbohydrate poor, and represents 40% of the total label. Component b has an apparent molecular weight of 74,000, contains all of the demonstrable sialic acid, and accounts for 60% of the total label. Trypsinization of iodinated, intact red cells results in the disappearance of only component b, the appearance of labeled glycopeptides in the medium, and the absence of smaller, labeled peptides remaining in the membrane. Pronase treatment hydrolyzes component b in a similar fashion, but also cleaves component a to a 72,000 mol wt peptide which is retained in the membrane. A combination of protease treatment and double labeling with (125)I and (131)I does not reveal the appearance of previously unexposed proteins.

Comparison of the structure and polypeptide composition of three double-stranded ribonucleic acid-containing viruses (diplornaviruses): cytoplasmic polyhedrosis virus, wound tumor virus, and reovirus

Iodination of reovirus, cytoplasmic polyhedrosis virus (CPV), and wound tumor virus (WTV), and their respective subviral forms, followed by analysis of the labeled polypeptides by using polyacrylamide gel electrophoresis, has been used to compare the protein contents of these three diplornaviruses. This approach, when combined with electron microscopy and buoyant density determinations, appears capable of localizing individual polypeptides in some of the viral and subviral forms. CPV (p = 1.435 g/cm(3)) seems to resemble reovirus cores (p = 1.440 g/cm(3)) in both ultrastructure and polypeptide composition. CPV is composed of five polypeptides with molecular weights of about 151,000, 142,000, 130,000, 67,000, and 33,000. The polyhedral matrix, which in nature encapsulates the virions, is, in turn, composed mainly of two polypeptide species with molecular weights of about 30,000 and 20,000, and several minor proteins. The proteins of WTV consist mainly of four species of polypeptide with molecular weights of about 156,000, 122,000, 63,000, and 44,000, and several minor components. These molecular weight determinations are consistent with the hypothesis that, as has been suggested for reovirus, the viral proteins of CPV and WTV seem to be coded for by monocistronic mes senger RNA molecules transcribed from distinct segments of the double-stranded RNA viral genomes.

Transcription by infectious subviral particles of reovirus

Digestion of purified reovirus with chymotrypsin in the presence of 0.15 M NaCl converts virions to infectious subviral particles (SVP(i)). The SVP(i) have an active ribonucleic acid (RNA) polymerase and are similar in composition to the partially uncoated virions which have been isolated from infected L cells. SVP(i) have a buoyant density of 1.40 g/ml in CsCl and sediment at 420S as compared to 1.37 g/ml and 630S for virions. They consist of 30% less protein and include the polypeptides of the inner structural layer, lambda(1), lambda(2), and sigma(3), and a polypeptide derived by cleavage of mu(2), a constituent of the outer shell. The genome RNA is retained within SVP(i), but more than 60% of the "adenine-rich," single-stranded RNA is released by the proteolytic treatment. Infection of L cells with SVP(i) or virions results in the transcription of all 10 genome segments. In cycloheximide-treated SVP(i)-infected cells, transcription occurs predominantly from one medium and two small genome segments, the same pattern of early messenger RNA (mRNA) observed in virion-infected cells. In contrast, SVP(i) incubated in vitro synthesize mRNA corresponding to all genome segments.