Site‐specific mutagenesis of cDNA clones expressing a poliovirus proteinase

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Abstract
The cleavage of poliovirus precursor polypeptides occurs at specific amino acid pairs that are recognized by viral proteinases. Most of the polio‐specific cleavages occur at glutamine‐glyeine (Q‐G) pairs that are recognized by the viral‐encoded proteinase 3C (formerly called P3–7c). In order to carry out a defined molecular genetic study of the enzymatic activity of protein 3C, we have made cDNA clones of the poliovirus genome. The cDNA region corresponding to protein 3C was inserted into an inducible bacterial expression vector. This recombinant plasmid (called pIN‐III‐C3–7c) utilizes the bacterial lipoprotein promoter to direct the synthesis of a precursor polypeptide that contains the amino acid sequence of protein 3C as well as the amino‐ and carboxy‐terminal Q‐G cleavage signals. These signals have been previously shown to allow autocatalytic production of protein 3C in bacteria transformed with plasmid pIN‐III‐C3–7c. We have taken advantage of the autocatalytic cleavage of 3C in a bacterial expression system to study the effects of site‐specific mutagenesis on its proteolytic activity. One mutation that we have introduced into the cDNA region encoding 3C is a single amino acid insertion near the carboxy‐terminal Q‐G cleavage site. The mutant recombinant plasmid (designated pIN‐III‐C3‐μ10) directs the synthesis of a bacterial‐polio precursor polypeptide that is like the wild‐type construct (pIN‐III‐C3–7c). However, unlike the wild‐type precursor, the mutant precursor cannot undergo autocatalytic cleavage to generate the mature proteinase 3C. Rather, the precursor is able to carry out cleavage at the amino‐terminal Q‐G site but not at the carboxy‐terminal site. Thus, we have generated an altered poliovirus proteinase that is still able to carry out at least part of its cleavage activities but is unable to be a suitable substrate for self‐cleavage at its carboxy‐terminal Q‐G pair.