Specific Roles for Lipids in Virus Fusion and Exit Examples from the Alphaviruses - Fusion of Biological Membranes and Related Problems - page 439

be due only to the charge of the lipid because other anionic lipids (phos-
phatidylglycerol or phosphatidic acid) do not substitute. Additionally, PI
from various animal and plant origins is interchangeable, suggesting that
the acyl chain composition is unimportant. While these data suggest that PI
may play a specific role in Ebola spike protein-mediated fusion, the previ-
ously mentioned caveats of work with isolated virus peptides apply here as
well. Further in vitro and in vivo work will be needed to define the impor-
tance of PI in the context of Ebola virus fusion and infection.

6.2.4. African Swine Fever Virus

African swine fever virus (ASFV) is a unique animal DNA virus of
an unnamed family. It has a complex structure with an overall icosahedral
shape, formed by a DNA-containing viral core which is successively
wrapped by two inner lipid bilayers derived from collapsed ER cisterna, a
capsid protein with a hexagonal lattice structure, and an outer lipid mem-
brane derived from the plasma membrane during virus budding (Andres et
al., 1997; Carrascosa et al., 1984). ASFV is thought to infect cells via recep-
tor-mediated endocytosis and low pH-triggered membrane fusion (Valdeira
and Geraldes, 1985; Alcami et al., 1989).
A recent report used several approaches to address the role of cho-
lesterol in ASFV infection (Bernardes et al., 1998). When cellular choles-
terol synthesis is inhibited by cerulenin, W-7 and miconazole, the production
of progeny ASFV is decreased by ~50%.When the cellular cholesterol level
is decreased to ~50% of that in control cells by culturing Vero cells in delip-
idated serum, similar kinetics of endocytic uptake are observed for ASFV,
while the kinetics of virus penetration to the cytoplasm are significantly
slowed. Low pH-triggered fusion of ASFV with the plasma membrane of
cholesterol-depleted cells is inhibited compared to fusion with control cells.
In addition, similar to results previously published for SFV (Phalen and
Kielian, 1991),ASFV fusion with the plasma membrane is inhibited by cho-
lesterol oxidase pretreatment of the target cells. Since this enzyme is known
to oxidize the sterol 3 β -hydroxyl group, this result suggests that, similar to

-hydroxyl group may be important in the ASFV choles-
terol requirement. The role of sphingolipid in ASFV infection and fusion
has not been addressed. These experiments with ASFV, while provocative,
are complicated by our limited ability to deplete cholesterol in mammalian
cell lines and the potential wide-ranging side effects of such sterol deple-
tion (Nes and McKean, 1977a). It will be important to compare the cho-
lesterol-dependence of ASFV fusion and infection in parallel with that of
a virus known to be cholesterol independent, such as VSV, to control for
non-specific effects of inhibitors and depletion. It will also be important to

SFV, the sterol 3 β