Calf rotavirus: detection of outer capsid glycoproteins by lectins

Glycosylation, an important modifier of rotavirus antigenicity

Mutants of a non-glycosylated strain of SA 11 rotavirus (clone 28), were selected using a monoclonal antibody directed against the VP 7 protein. These mutants possessed an amino acid substitution at residue 238 of VP 7, whereas mutants of wild type SA 11 selected with the same antibody have previously been shown to contain a substitution at residue 211 (i.e., in the antigenic C region). In both cases the mutations produce new potential glycosylation sites, and these were found to be utilized. The mutations also lead to gross antigenic changes, and these were found to be reversible upon removal of the attached carbohydrate. The results suggest an important role for carbohydrate in influencing the exposure of antigenic determinants of the rotavirus serotype-specific protein, VP 7.

Patterns of polypeptide synthesis in human rotavirus infected cells

Polypeptide analysis of three strains of human rotavirus (KUN, Wa and MO) were conducted using a hypertonic culture which suppressed host protein synthesis and unmasked rotavirus specific protein synthesis. As a result, eleven human rotavirus specific polypeptides (Vp 1--Vp 11) were detected by pulselabeling infected cells with [14C]-leucine. Among the 11 polypeptides, three polypeptides (Vp 7, Vp 10 and Vp 11) underwent post-translational processing, and two (Vp 7 and Vp 10) were glycosylated. Six polypeptides (Vp 1, 2, 3, 4, 6 and 7) were identified as viral structural proteins. Comparisons of three strains of different serotypes revealed that their polypeptide profiles differed from each other in electrophoretic mobility; in particular, profiles of the glycosylated polypeptide, Vp 7, were distinct among the three strains.

Two types of glycoprotein precursors are produced by the simian rotavirus SA11

The rotavirus genome codes for two glycoproteins: an outer capsid structural glycoprotein (VP7, apparent molecular weight 38,000 (38K)) and a nonstructural glycoprotein (NS28K). The synthesis of these glycoproteins was analyzed in infected cells and in a cell-free system derived from rabbit reticulocyte lysates supplemented with dog pancreatic microsomes. The data showed a 37K product synthesized in the cell-free system is the precursor to the 38K glycoprotein and that the 37K polypeptide contains a cleavable signal sequence (apparent molecular weight 1.5K). The 37K polypeptide was glycosylated in vitro in the presence of microsomal membranes to a polypeptide of 38K that was immunoprecipitated by monospecific antiserum to VP7. Endo H digestion of the 38K polypeptides from either infected cells or the cell-free system produced polypeptides of identical molecular weight, 35.5K (the glycoprotein precursor lacking the signal sequence). These results were confirmed by comparative studies with a variant of SA11 that is defective in glycosylation of VP7. Similar experiments with the 20K precursor to the 29K nonstructural glycoprotein showed the 20K polypeptide contains a noncleavable signal sequence. Both glycoproteins were inserted into microsomal membranes and were processed via oligosaccharide trimming.

Effects of tunicamycin on rotavirus morphogenesis and infectivity

The functions of the two rotavirus glycoproteins were investigated by using tunicamycin and a variant of SA11 rotavirus having nonglycosylated VP7. Results showed that glycosylation of VP7 is not required for normal viral morphogenesis and infectivity and suggested that the nonstructural glycoprotein is involved in assembly of the outer capsid.

Biosynthesis of virus-specific proteins in cells infected with infectious bursal disease virus and their significance as structural elements for infectious virus and incomplete particles

It has previously been shown that infectious bursal disease virus is a naked icosahedral particle with a diameter of about 60 nm and a genome consisting of two segments of double-stranded RNA (Müller et al., J. Virol. 31:584-589, 1979). One of the two major structural polypeptides (molecular weight, 40,000) of this virus could not be found in lysates of infected cells; it is derived from a precursor polypeptide demonstrable inside the cells in relatively large quantities and seems to be processed during virus assembly or later. The precursor molecule is regularly present in the infectious virus particle (buoyant density, 1.33 g/ml) in minor proportions, but it represents an outstanding structural element of incomplete noninfectious particles ("top components"; buoyant density, 1.29 g/ml) which contain viral RNA. This type of incomplete particles is mainly produced by chicken embryo fibroblasts in contrast to lymphoid cells from the bursa of Fabricius. Precursor-product relationships also seem to exist in the biosynthesis of the other viral polypeptides. In contrast to some other viruses with a segmented double-stranded RNA genome, none of the structural proteins of infectious bursal disease virus is appreciably glycosylated.

Purification and characterization of bovine rotavirus cores

Using the chaotropic effect generated by a high concentration of CaCl2, we converted calf rotavirus particles into cores of 40 nm in diameter. These cores were purified by rate zonal centrifugation in sucrose gradients and by isopycnic gradients. They had a sedimentation coefficient of 280S +/- 20S and a density of 1.44 g/ml in CsCl. When analyzed by polyacrylamide gel electrophoresis, they contained three polypeptides (VP125, VP89, and VP78). The major internal polypeptide of the virion (VP39) was recovered in a purified and soluble form in the top fractions of the sucrose gradients. From this stepwise degradation, it appears that VP39 is the most external polypeptide of dense particles. In contrast to reovirus cores, calf rotavirus cores did not exhibit transcriptase activity. Purified VP39 also did not exhibit transcriptase activity when tested after being mixed with purified rotavirus genome RNA as a template. Transcriptase activity was partially recovered when ionic conditions were adjusted to permit the reassociation of VP39 with the cores.

Identification, synthesis, and modifications of simian rotavirus SA11 polypeptides in infected cells

The synthesis and processing of simian rotavirus SA11 polypeptides was investigated after infection of MA104 cells. [35S]methionine- or 3H-amino acid-labeled cell extracts were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Viral protein synthesis was maximal 3 to 5 h postinfection, and 12 major viral polypeptides were detected. Immunoprecipitation and peptide mapping experiments, demonstrated five viral structural proteins (125,000 daltons [125K], 94K, 88K, 41K, and 38K). Three proteins (53K, 35K, and 34K) were identified as nonstructural by comparison of their partial proteolysis maps with those from polypeptides of similar molecular weight synthesized in vitro from viral RNA transcripts. Assignment as to structural or nonstructural status of two other primary gene products (26K and 20K) remains tentative. Pulse-chase experiments and tunicamycin blockage of glycosylation revealed cotranslational or post-translational modifications (or both) and precursor-product relationships of several of the polypeptides. Tunicamycin inhibition of glycosylation identified a 35.5K polypeptide which was proven to be the precursor to the 38K structural glycoprotein by immunoprecipitation and peptide mapping analyses. Tunicamycin treatment of infected cells also resulted in the disappearance of other glycoprotein species (23K to 29K) and in the concomitant build-up of an unglycosylated 20K polypeptide, suggesting a precursor-product relationship between those polypeptides. Labeling with [3H]glucosamine or [3H]mannose suggested that the rotavirus glycoproteins contained high mannose oligosaccharides. The effects of amino acid analogs on rotavirus polypeptide synthesis and processing were also investigated.

Activation of rotavirus RNA polymerase by calcium chelation

Two types of particles were isolated during purification of rotavirus. Dense (D) particles have a density of 1.38 in CsCl and exhibit spontaneously a fully active endogenous transcriptase. Light (L) particles (density of 1.36 in CsCl) need to be treated with chelating agents to show a polymerase activity. The activation process of L particles was studied under strictly controlled monovalent, divalent, and hydrogen ion concentrations. These experiments demonstrate that i) activation is not affected by the ionic strength ii) activation occurs only at a pH higher than 7.1 iii) a low concentration of chelating agent (40 muM EDTA) is sufficient to activate the enzyme. Treatment of particles with EGTA, which chelates selectively Ca2+, leads to unmasking even in the presence of magnesium, indicating that the concentration of free calcium ions plays a major role in the activation process. Various glycosidases, detergents, and chelating agents were tested in respect to unmasking properties. Of these compound only chelating agents turned out to be efficient. Following activation, two glycopeptides were solubilized. These glycopeptides have an apparent molecular weight of 34,000 and 31,000 daltons and react with concanavalin A. The role of Ca2+ upon the stability of virus particles, and the activation of the endogenous transcriptase in vitro and in the infected cells is discussed.