Canine parvovirus capsid assembly and differences in mammalian and insect cells

We examined the assembly processes of the capsid proteins of canine parvovi rus (CPV) in mammalian and insect cells. In CPV-infected cells empty capsid s assembled within 15 min, and then continued to form over the following 1 h, while full (DNA-containing) capsids were detected only after 60 min, and those accumulated slowly over several hours. In cells expressing VP1 and V P2 or only VP2, empty capsid formation was also efficient, but was slightly slower than that in infected cells. Small amounts of trimer forms of VP2 w ere detected in cells expressing wild type capsid proteins, but were not se en for mutants containing changes that prevented capsid assembly. CPV capsi ds accumulated in the cell nucleus, but mutant VP1 and VP2 proteins that di d not assemble became distributed throughout the nucleus and the cytoplasm, irrespective of whether they were expressed as VPI and VP2, or as VP2 only . Urea or pH treatment of empty capsids released dimer, trimer, or pentamer capsid protein combinations, while treatment of full capsids consistently released trimer and, in some cases, penta mer forms. When wild type or asse mbly-defective VP2 genes were expressed from recombinant baculoviruses in i nsect cells, most of the protein was recovered as noncapsid aggregates, and only a small proportion assembled into capsids. Both the assembled capsids and the noncapsid aggregates were seen primarily in the cytoplasm of the i nsect cells. The VP2 expressed in insect cells that was recovered in aggreg ates had an isoelectric point of about pH 6.3, while that recovered from as sembled capsids had a pi of about 5.2, similar to that seen for the VP2 of capsids recovered from mammalian cells. (C) 2001 Academic Press.