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binding of L1 to cell receptors without the involvement of L2, because
virions of BPV1,
25
as well as L1/L2- and L1-capsids of BPV1,
HPV11, HPV16, and HPV33,
25-27
are seemingly capable of binding
to cells with a similar efficiency, and because a mouse anti-BPV1 L2
monoclonal antibody, which inhibits the infectivity of BPV1 (focus
formation in mouse C127 cells), allows the binding of BPV1 virions to
C127 cells.
28
Giroglou
et al
.
29
and Shafti-Karamat
et al
.
30
showed that a cellu-
lar receptor for HPV L1 is cell surface heparan sulfates. L1-capsids of
several HPV types bind to cell surface heparan sulfates and heparin,
which is a highly sulfated form of heparan sulfate proteoglycan.
Heparin interferes with the infection of COS-7 cells with HPV16 and
HPV33 pseudovirions and the infection of human primary ker-
atinocytes with authentic HPV11 virions. COS-7 cells whose surface
heparan sulfates have been removed by heparinase I treatment are not
infected with HPV16 and 33 pseudovirions. Erythroleukemia cell line
K562, which does not express syndecans at the cell surface, is not
infected with HPV11 virions, and syndecan-1-positive K562 transfec-
tants produced by an expression plasmid for syndecan-1 are sensitive
to HPV11 infection. The results clearly indicate that heparan sulfate
proteoglycans are a cellular attachment receptor for HPV.
L2 Functions in the Initial Steps of HPV Infection
Yang
et al
.
31
reported the data suggesting that L2 plays essential roles
in the intracellular transport of virions to the nucleus. They compared
the binding, uptake, and intracellular transport of HPV16 L1/L2-
capsids with those of L1-capsids by transmission electron microscopy.
HPV16 L1-capsids and L1/L2-capsids similarly bind to the surface of
BPHE-1 cells. After incubation of the cells at 37C, the L1-capsids are
diffusely distributed within the cytoplasm at 6 h; however, the L1/L2-
capsids are aligned along distinct radial tracks across the cytoplasm and
arrive in the perinuclear region within 2 h. Initially, L1/L2-capsids are
located with cortical actin at the periphery of the cell and then are
located with actin filaments.
Yang
et al
.
31
also showed that HPV16 L2 binds to b-actin.
Bacterially expressed fusion protein comprised of GST/HPV16 L2
