Identification and genomic mapping of the ORF3 and VPg proteins in feline calicivirus virions

Two minor proteins with molecular masses of 8.5 and 15.5 kDa were identifie d in feline calicivirus (FCV) virions. Direct sequence analysis showed that the N-terminal sequence of the 8.5-kDa protein was identical to that of th e predicted protein encoded by open reading frame 3 (ORF3) of the FCV genom e. The N-terminal sequence of the 15.5-kDa protein corresponded to amino ac ids 961-980 of the FCV ORF1 polyprotein and mapped to the genomic location of the calicivirus VPS. Antisera raised against recombinant ORF3 protein or the N-terminal 20 amino acids of the putative VPS reacted with the corresp onding proteins present in both a Western blot analysis of purified FCV vir ions and an immunofluorescence assay of FCV-infected cells. A comparative a nalysis of radioactivity incorporated into virion proteins during in vivo l abeling experiments indicated that the ORF3 protein is likely present in on e or two copies per virion. The mobility of the ORF3 protein present in vir ions was similar to that of the ORF3 protein found in FCV-infected cells or expressed in bacteria. Direct N- and C-terminal sequence analysis of the p urified ORF3 protein obtained by expression in bacteria demonstrated the pr esence of intact, uncleaved termini, suggesting that the observed differenc e between the calculated and the apparent masses in SDS-PAGE was not due to proteolytic processing of the protein.