CLEAVAGE OF THE FELINE CALICIVIRUS CAPSID PRECURSOR IS MEDIATED BY A VIRUS-ENCODED PROTEINASE

Feline calicivirus (FCV), a member of the Caliciviridae, produces its major structural protein as a precursor polyprotein from a subgenomic- sized mRNA, In this study, we show that the proteinase responsible for processing this precursor into the mature capsid protein is encoded b y the viral genome at the 3'-terminal portion of open reading frame 1 (ORF1), Protein expression studies of either the entire or partial ORF 1 indicate that the proteinase is active when expressed either in in v itro translation or in bacterial cells. Site-directed mutagenesis was used to characterize the proteinase Glu-Ala cleavage site in the capsi d precursor, utilizing an in vitro cleavage assay in which mutant prec ursor proteins translated from cDNA clones were used as substrates for trans cleavage by the proteinase, In general, amino acid substitution s in the P1 position (Glu) of the cleavage site were less well tolerat ed by the proteinase than those in the P1' position (Ala). The precurs or cleavage site mutations were introduced into an infectious cDNA clo ne of the FCV genome, and transfection of RNA derived from these clone s into feline kidney cells showed that efficient cleavage of the capsi d precursor by the virus-encoded proteinase is a critical determinant in the growth of the virus.