Interferon-induced enzymatic activities and their role in the antiviral state

Abstract

Department of Biological Sciences State University of New York at Albany Albany, New York 12222 lnterferons are glycoproteins secreted by virus-in- fected cells which promote the establishment of an antiviral state in uninfected cells (Finter, 1973). Treat- ment of animal cells in culture with interferon reduces their ability to support replication of RNA and DNA viruses by mechanisms that are not yet understood. Early investigations of the mechanism of interferon action were carried out with virus-infected cells (re- viewed by Friedman, 1977). These studies showed an inhibition of accumulation and/or translation of viral templates, but failed to reveal the causes of this inhi- bition. Recent work has focused on the enzymatic differences between extracts of interferon-treated and control cells reported by several investigators. The precise relationship between these enzymatic activi- ties and the establishment of the antiviral state has not yet been clarified, but it seems probable that some explanation for the molecular basis of the antiviral state will be forthcoming shortly. Double-stranded RNA (dsRNA) is the most potent inducer of interferon synthesis (reviewed by Colby and Morgan, 1971). Both synthetic and viral dsRNAs induce synthesis and secretion of interferon in animal cells (see Torrence and De Clercq, 1977). These facts led to the hypothesis that double-stranded replicating viral RNA (the “replicative intermediate” formed by RNA viruses) triggers interferon synthesis by an un- known mechanism. Replicative intermediate obtained from mengovirus-infected cells is indeed an effective inducer of interferon synthesis (Falcoff and Falcoff, 1970). The enormous potency of dsRNA in its inter- action with the cell has been elegantly shown by Marcus and Sekellick (1977). A single defective inter- fering particle of vesicular stomatitis virus (VSV) con- taining dsRNA can induce synthesis of interferon. The defective interfering particle is thought to inject into a cell a single molecule of dsRNA, which is sufficient to induce interferon synthesis. Moreover, dsRNA is a potent inhibitor of protein synthesis in extracts of interferon-treated cells (Kerr, Brown and Ball, 1974). Investigations of this enhanced sensitivity to inhibition by dsRNA led to the discovery of two interferon-in- duced dsRNA-dependent enzymatic activities (Figure 1 )-an oligonucleotide polymerase, which synthe- sizes a series of oligonucleotides containing unusual 2’5’phosphodiester bonds from ATP (Kerr and Brown, 1978) and a protein kinase, which phosphorylates the small subunit of initiation factor elF-2 (Farrell et al., 1977). The product of the oligonucleotide polym- erase, pppA(2’p5’A). (designated here 2’5’oligo(A) or 2,5A), is the activator of an endoribonuclease (Cle- mens and Williams, 1978; Eppstein and Samuel, 1978; Baglioni, Minks and Maroney, 1978b; Ratner et al., 1978; Zilberstein et al., 1978). The enzymatic activity which synthesizes the oligonucleotides is des- ignated 2,5A polymerase. This review focuses on recent work on the protein kinase and 2,5A polymerase/endoribonuclease sys- tem and describes studies examining the stability of 2,5A in cell extracts. These studies may help us to understand how the endoribonuclease functions in virus-infected interferon-treated cells. Other studies on the inhibition of viral mRNA methylation (Sen et al., 1975, 1977) are also briefly described. Although mechanism of this inhibition has not yet been char- acterized at the molecular level, it is discussed here because it is an interferon-dependent activity in ceil extracts which does not require addition of dsRNA. Inhibition of viral mRNA methylation may also provide an explanation for impairment of its translation.