Human papillomavirus type 18 (HPV18)
capsid proteins L1 and L2, synthesised in mammalian cells using recombinant
vaccinia viral expression vectors, are transported to the nucleus and assembled into virus-like particles. When 293T cells, which express SV40
T antigen, were transfected with plasmid DNAs containing an SV40 origin of replication then infected with
vaccinia viral vectors encoding L1 and L2, plasmid
DNA was encapsidated into the particles. The DNAs ranged in size from 5.4 to 7.9 kb. By encapsidating plasmids containing either the
beta-galactosidase gene or the
puromycin-resistance gene, the pseudovirions were shown to be infectious in that they could transfer
beta-galactosidase activity or confer resistance to
puromycin to a number of cell types, indicating that the uptake and decapsidation of HPV particles are not the main determinants of cell type specificity of HPV. Episomal HPV16
DNA in a cervical keratinocyte line could also be encapsidated. Further investigation showed that
DNA encapsidation is independent of HPV DNA sequences and of
T antigen-mediated plasmid DNA replication. Instead, the minor
capsid protein, L2, was found to be attached to plasmid mini-chromosomes extracted from these cells, suggesting a role for L2 in encapsidation. Consistent with this, the L1
protein alone was unable to encapsidate
DNA, although it was able to form virus-like particles. The results suggest that intracellular episomal DNAs of suitable size can be encapsidated by the HPV18 L1 and L2
proteins without the need of any HPV packaging signal, and reintroduced into cells.