Biology Reference
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protein L2 (mol. wt.: 76,000). Seventy-two capsomeres (each is an L1-
pentamer) are arranged on a skew T
7d icosahedral surface lattice of
the capsid. Twelve capsomeres contact five neighbors and the others,
six. Three-dimensional image reconstruction suggests that L2 is located
at the center of a pentavalent capsomere at the virion vertices.
Histological studies of the epithelium infected with HPVs showed
that the HPV genome is maintained as an episome in the nuclei of
basal cells, where the viral gene expression is largely suppressed.
3
When the infected cells initiate terminal differentiation, the HPV
genome starts to be transcribed and replicates.
3
In the upper layers of
the epidermis or mucosa, HPV virions are generated and released.
3
Thus, the HPV lifecycle is closely associated with the epithelial differ-
entiation, and likely accounts for fact that there is no cell culture system
for HPV virion production.
Despite the lack of cell culture systems for HPV propagation, sur-
rogate systems have been developed to produce HPV virion proteins;
capsid proteins L1 and L2; the capsids comprise L1 alone (L1-capsids,
also called L1 virus-like particle or L1-VLPs); the capsids comprised
of L1 and L2 (L1/L2-capsids, also called L2-VLPs); and infectious
pseudovirions that contain reporter DNA. These materials have per-
mitted us to study the biological functions and immunogenicity of the
capsid proteins. Given the role of HPV16 in carcinogenesis, we have
focused on the recent studies of HPV16 capsid proteins in this review.
=
HPV Capsids and Pseudovirions
Surrogate Production of HPV Capsids
Zhou
et al
.
5
introduced a substituting method to produce HPV16
empty virus-like particles in cell cultures. HPV16 L1, when
expressed alone or together with L2 from a recombinant vaccinia
virus vector, self-assembles into L1-capsids or L1/L2-capsids,
respectively, in the nuclei of infected monkey CV-1 cells. Hagensee
et al
.
6
have demonstrated the formation of HPV1 L1- and L1/L2-
capsids by a similar approach.
Kirnbauer
et al
.
7,8
showed that self-assembled BPV1 and HPV16
L1-capsids can be obtained from the expression of recombinant bac-