The Intracellular Proteins Induced by Cricket Paralysis Virus in Drosophila Cells: the Effect of Protease Inhibitors and Amino Acid Analogues

Treatment of Cricket paralysis virus infected Drosophila cells with iodoacetamide before radiolabelling with 35S-methionine results in the appearance of two high molecular weight polypeptides of approximately equal to 200,000 molecular weight, not apparent in untreated infected cells (17). To attempt to differentiate between the effects of iodoacetamide being attributable to either alteration of initial polyprotein or inhibition of the protease (either cellular or viral) the effects of a spectrum of protease inhibitors were examined. These included aprotinin, leupeptin, pepstatin, elevated zinc concentration, phenyl methyl sulphonyl-fluoride, N-tosyl-L-lysine chloromethyl ketone (TLCK) and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK). TLCK and TPCK both inhibited the cleavage of proteins which demonstrates an inhibition of the protease activity. The introduction of amino acid analogues into the infected cells before pulsing also results in the appearance of higher molecular weight proteins. This could be attributed to alternation of the polyprotein making it nonsusceptible to digestion with pre-existing cellular protease or newly synthesized viral protease. The possibility that the presence of the amino acid analogues results in alteration of a viral coded protease cannot be eliminated.