Expression and processing of the canine calicivirus capsid precursor.

The ORF2 product of canine calicivirus (CaCV) was identified and its processing in mammalian cells was analysed. Immunoblot analysis revealed the presence of the 75 kDa capsid precursor in addition to a 57 kDa capsid protein and a 22 kDa N-terminal polypeptide in CaCV-infected cells treated at an elevated temperature. When the CaCV ORF2 was expressed in a transient mammalian expression system, only the 75 kDa precursor was detected in immunoblot analysis, suggesting that no post-translational processing occurred in this system. However, the precursor was processed to a 57 kDa protein and a 22 kDa polypeptide by the proteinase of feline calicivirus (FCV) when this was co-expressed with ORF2. Processing was blocked by site-directed mutagenesis of the putative cleavage site in the capsid precursor. The results indicate that the proteinase of FCV can cleave the capsid precursor of CaCV to produce the mature capsid protein and that CaCV may have a similar proteinase.