Virus-specified protease in poliovirus-infected HeLa cells

Previous studies have shown that primary cleavages in nascent picornavirus precursors are accomplished by cellular proteases. This study has characterized the enzyme in infected cells that produces the capsid polypeptides by secondary cleavages of viral precursors. The kinetics of the production of protease activity correlate with the time course of virus protein synthesis, and the new enzyme has characteristic pH and temperature optima. Guanidine and cycloheximide, which are inhibitors of virus RNA and protein synthesis, prevent production of the protease. As determined by introduction of amino acid analogs into the protease or inhibition by a leucyl chloromethyl ketone, the enzyme is synthesized at a time of infection when host cell proteins are not produced, and the enzyme copurified with a 40,000-dalton virus polypeptide present in the cytoplasm of infected cells. Wild-type levels of protease activity are produced by viral mutants that are defective in coat protein synthesis. The conclusion is that a non-structural poliovirus gene product participates in protein cleavages that produce the viral coat proteins.