Expression of Human Papillomavirus Type 6 L1 and L2 Isolated in China and Self Assembly of Virus-like Particles by the Products

( ¹Key
Laboratory of Silkworm Biotechnology, Ministry of Agriculture, Sericultural
Research Institute,
Chinese Academy of Agricultural Sciences, Zhenjiang 212018, China; 2 State
Key Laboratory of Bioreactor Engineering, East China University of Science
and Technology, Shanghai 200237, China; ³Institute of Biochemistry
and Cell Biology, Shanghai Institutes for Biological Sciences, the Chinese
Academy of Sciences, Shanghai 200031， China )

Abstract
To study variations of genome late region of human papillomavirus
type 6 (HPV-6) isolated in China and assembling capabilities of the encoded
capsid proteins, HPV-6 L1 and L2 sequences were cloned and used
for expression in Bac-to-Bac baculovirus expression systems (Gibco BRL). Based
upon L1 and L2 overlapping sequence two sequences (GenBank accession number
AY015006, AY015008) of HPV-6 late region (2869 bp long) were assembled and
classified into HPV-6b by phylogenetic analysis. Compared with prototype sequence,
nine point mutations were found, including four missense mutations. L1, instead
of L2, could self-assemble into virus-like particles (VLPs) in Sf9 nucleus.
VLPs self-assembled by L1 alone (L1-VLPs) and by L1 plus L2 (L1+L2-VLPs) were
purified and further characterized. Both types of VLPs were spherical particles
with a diameter of approximately 50 nm. L1+L2 VLPs comprising L1 and L2 in
the molar ratio of about 4:1 possessed the HPV-6 L1 VLP surface and conformational
epitopes. In co-expression assay with a series of MOI combination of L1
and L2 recombinant baculoviruses (total MOI=10), existence of L2 of
certain level enhanced L1 production by 0.8 fold and VLP produ ction by 3-4
folds under experimental conditions. In conclusion, variation rate of HPV-6
genome late region is less than 0.28% and the substitutions A to G at position
7081 and G to A at 7099 may represent region characteristics. The cloned HPV-6
L1 and L2 sequences can be expressed efficiently in Sf9 cells,
and the expressed products (L1 or L1+L2) can self-assemble into VLPs that
resemble naturally occurring virions.