Human papillomavirus type 18 (HPV18) capsid proteins L1 and L2, synthe
sised in mammalian cells using recombinant vaccinia viral expression v
ectors, are transported to the nucleus and assembled into virus-like p
articles. When 293T cells, which express SV40 T antigen, were transfec
ted with plasmid DNAs containing an SV40 origin of replication then in
fected 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-galactosi
dase gene or the puromycin-resistance gene, the pseudovirions were sho
wn to be infectious in that they could transfer beta-galactosidase act
ivity or confer resistance to puromycin to a number of cell types, ind
icating 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 i
nvestigation showed that DNA encapsidation is independent of HPV DNA s
equences and of T antigen-mediated plasmid DNA replication. Instead, t
he minor capsid protein, L2, was found to be attached to plasmid mini-
chromosomes extracted from these cells, suggesting a role for L2 in en
capsidation. Consistent with this, the L1 protein alone was unable to
encapsidate DNA, although it was able to form virus-like particles. Th
e results suggest that intracellular episomal DNAs of suitable size ca
n be encapsidated by the HPV18 L1 and L2 proteins without the need of
any HPV packaging signal, and reintroduced into cells. (C) 1998 Academ
ic Press.