Antigenic analysis of feline calicivirus capsid precursor protein and its deleted polypeptides produced in a mammalian cDNA expression system.

An entire open reading frame in a cDNA encoding the capsid protein gene of feline calicivirus (FCV) was subcloned into a mammalian expression vector. After transfection of the constructed plasmid (pMCV-II) into COS-7 cells, the expressed protein was detected by indirect immunofluorescence assay (IFA) using a panel of monoclonal antibodies (MAbs) to the capsid protein of FCV. All of the MAbs reacted with the transfected COS-7 cells in IFA. The 76 kDa capsid precursor protein was demonstrated in an immunoblot analysis, indicating that the translated precursor protein was not processed into the matured capsid protein in this expression system. Two in-frame deleted and a frameshift mutated cDNAs were generated by using restriction sites within the capsid protein coding sequence in pMCV-II to analyze the antigenic sites of the protein. The results of IFA using the MAbs and COS-7 cells transfected with the deleted or mutated cDNAs suggested that three neutralizing epitopes had a conformational nature and that the other four linear epitopes were related to 74 amino acid residues between positions 381 and 454 in the protein, in which high variation was known to be present among three strains of FCV.