Interferon-mediated protein kinase and low-molecular-weight inhibitor of protein synthesis

Abstract

PROTEIN synthesis in cell-free systems from mouse L-cells pretreated with the antiviral agent interferon¹ shows an enhanced sensitivity to inhibition by double-stranded RNA (dsRNA)^2,3. The possible significance of this with respect to events in intact interferon-treated virus-infected cells has already been discussed^2,4. We have already shown that the addition of small amounts of a post-ribosomal supernatant fraction from interferon-treated cells (interferon cell sap) renders protein synthesis in control cell-free systems sensitive to inhibition by dsRNA⁴. Our results are in accord with a two-step model involving an initial 100-fold activation of an inhibitor on incubation of the interferon cell sap with ATP and dsRNA (activation step), the activated inhibitor then interacting with the protein-synthetic system to inhibit translation (inhibitory step)⁵. In view of the dependence of the inhibition of protein synthesis upon incubation with ATP as well as dsRNA⁵ it was of interest to determine if a protein kinase(s) is involved, as seems to be the case in reticulocyte lysates in which phosphorylation of the initiator Met-tRNA binding factor (IF-E2) has been implicated following incubation of the lysates under a variety of inhibitory conditions including the presence of dsRNA (P. Farrell, K. Balkow, T. Hunt and R. J. Jackson, personal communication)⁶. In accord with this, we report here the presence of a dsRNA-dependent protein kinase activity in interferon cell sap. The inhibitor of protein synthesis which is activated on incubation of interferon cell sap with ATP and dsRNA, however, is heat stable and of relatively low molecular weight. It does not correspond either to the major polypeptide product of the interferon-dsRNA specific phosphorylation, or, presumably, to phosphorylated IF-E2.