IN-VITRO ACTIVATION OF THE INTERFERON-INDUCED, DOUBLE-STRANDED RNA-DEPENDENT PROTEIN-KINASE PKR BY RNA, FROM THE 3'-UNTRANSLATED REGIONS OFHUMAN ALPHA-TROPOMYOSIN

The cellular kinase known as PKR (protein kinase RNA-activated) is ind uced by interferon and activated by RNA, PKR is known to have antivira l properties due to its role in translational control, Active PKR phos phorylates eukaryotic initiation factor 2 alpha and leads to inhibitio n of translation, including viral translation, PKR is also known to fu nction as a tumor suppressor, presumably by limiting the rate of tumor -cell translation and growth, Recent research has shown that RNA from the 3' untranslated region (3'UTR) of human alpha-tropomyosin has tumo r-suppressor properties in vivo [Rastinejad, F,, Conboy, M.J., Rando, T,A, & Blau, H.M. (1993) Cell 75, 1107-1117]. Here we report that puri fied RNA from the 3'UTR of human alpha-tropomyosin can inhibit in vitr o translation in a manner consistent with activation of PKR. Inhibitio n of translation by tropomyosin 3'UTR RNA was observed in a rabbit ret iculocyte lysate system, which is known to contain endogenous PKR hut was not seen in wheat germ lysate, which is not responsive to a known activator of PKR. A control RNA purified in the same manner as the 3'U TR RNA did not inhibit translation in either system, The inhibition of translation observed in reticulocyte lysates was prevented by the add ition of adenovirus virus-associated RNA(I) (VA RNA(I)), an inhibitor of PKR activation, Tropomyosin 3'UTR RNA was bound by immunoprecipitat ed PKR and activated the enzyme in an in vitro kinase assay, These dat a suggest that activation of PKR could be the mechanism by which tropo myosin 3'UTR RNA exerts its tumor-suppression activity in vivo.