One-step separation of the components of Semliki Forest virus by cation exchange chromatography

Semliki Forest virus capsid protein inhibits the initiation of translation by upregulating the double-stranded RNA-activated protein kinase (PKR)

We investigated the possible translational role which elevated concentrations of highly purified Semliki Forest virus (SFV) capsid (C)-protein molecules may play in a cell-free translation system. Here we demonstrate that in the absence of double-stranded RNA high concentrations of C protein triggered the phosphorylation of the interferon-induced, double-stranded RNA-activated protein kinase, PKR. Activated PKR in turn phosphorylated its natural substrate, the alpha subunit of eukaryotic initiation factor 2 (eIF-2), thereby inhibiting initiation of host cell translation. These findings were further strengthened by experiments showing that during natural infection with SFV the maximum phosphorylation of PKR coincided with the maximum synthesis of C protein 4-9 hours post infection. Thus, our results demonstrate that high concentrations of C-protein molecules may act in a hitherto novel mechanism on PKR to inhibit host cell protein synthesis during viral infection.

Nucleolar accumulation of core protein in cells naturally infected with Semliki Forest virus. Quantitative aspects

The core (C) protein of Semliki Forest virus (SFV) is known to exert several important functions with regard to both the virus and the host. This paper shows that migration of parental and progeny C protein to the nucleolus is a common feature in infected vertebrate and invertebrate cells. The amount of C protein accumulating to the nucleolus is small, always less than 1% of the intra- and extracellular C protein at various times post infection. Migration to the nucleolus is a fast process; 1.5 h post infection a prominent amount of parental C protein is already incorporated into nucleolar fractions.

Semliki Forest virus capsid protein acts as a pleiotropic regulator of host cellular protein synthesis

The Semliki Forest virus capsid (C) protein was introduced into various target cells by electroporation-, liposome-, and erythrocyte-ghost-mediated delivery. Data are presented which show that the incorporated C protein is biologically active and, at low concentrations (10(3) to 10(4) molecules per cell), markedly induces host cellular protein synthesis (average value, up to 90%). On the other hand, high concentrations (10(5) to 10(6) molecules per cell) led to a significant inhibition (average value, up to 60%). The cellular response to C protein was found to be identical in P3X63Ag8 suspension cells, CV-1 cells, and GpBind4 cells. Following electroporation-mediated delivery of C-protein molecules, both induction and repression of cellular protein synthesis were immediate, whereas with liposome-mediated delivery these events were delayed by about 1 h. Maximum stimulation and repression occurred between 0 and 1 h after delivery of C protein and decreased thereafter to reach control values at about 4 h. The analysis of the proteins synthesized suggests that low amounts of microinjected C protein are responsible for the induction of classes with specific Mrs, whereas high amounts lead to an inhibition of overall protein synthesis.

The use of sulfite to study the mechanism of membrane fusion induced by E1 of Semliki Forest virus

In this study we have used 35S-labeled sulfite to modify the disulfide bonds of the proteins at the cell surface of Semliki Forest-infected Aedes albopictus cells before and after low pH treatment. This reagent specifically cleaves disulfide bonds and concomitantly reacts with the newly formed cysteines, thereby labeling the respective protein. Treatment of the infected cells with sulfite led to inhibition of the fusion activity only when applied after low pH exposure. These cells exhibited substantial incorporation of the label into the viral E1 glycoprotein as compared to the E2 glycoprotein. These results provide direct evidence for the low pH-induced conformational change of E1 during the generation of its fusogenic potential.

Semliki Forest virus particles containing only the E1 envelope glycoprotein are infectious and can induce cell-cell fusion

Hydrophobic interaction chromatography (phenyl- and octyl-Sepharose) was performed with Semliki Forest virus to investigate the effect of low pH on its hydrophobicity. At neutral pH, the virus could be bound to the column and completely eluted by the detergent NP-40. Low pH treatment of virus prior to application to the column resulted in stronger binding as reflected by the increased amount of detergent necessary to totally elute the virus. If, however, the low pH treatment was done after binding of the virus to the column, only 15% of the input virus could be eluted by the detergent, indicating a drastic increase in hydrophobicity. Thus binding of the virus to a hydrophobic environment potentiates the effect of low pH on viral hydrophobicity. Trypsin digestion of column-bound virus after low pH treatment resulted in complete digestion of E2 and E3; however, E1 was totally resistant. From this result, we conclude that E1 alone is responsible for the hydrophobic interaction. We have made use of these observations to produce viral particles which were devoid of E2 and E3 by trypsin digestion in the presence of octyl glucoside. These E1 viral particles were infectious and could induce membrane fusion. We conclude that only E1 is necessary and sufficient to mediate membrane fusion. Acid pH induces a drastic increase in the hydrophobicity of E1 which probably facilitates its interaction with the lipid bilayers during the fusion event in endosomes.

Investigation of the role of glycans for the biological activity of Semliki Forest virus grown in Aedes albopictus cells using inhibitors of asparagine-linked oligosaccharides trimming

The effects of N-linked-oligosaccharide-processing inhibitors on the formation of Semliki Forest virus (SFV) in C6/36 Aedes albopictus cells were investigated. The glycosidase inhibitors deoxynojirimycin, deoxymannojirimycin and swainsonine prevented the formation of Endo-H resistant structures, but had little effect on virus formation and on the biological activities of the virus. Tunicamycin greatly inhibited virus formation, but had little effect on cell-cell fusion from within and the cleavage of p 62. These results indicate that correct glycosylation is not a prerequisite for biological activities of SFV, whereas glycosylation per se is needed for virus production.