Specific Roles for Lipids in Virus Fusion and Exit Examples from the Alphaviruses - Fusion of Biological Membranes and Related Problems - page 432

1 : 1) was analyzed, significantly higher levels of fusion were observed with
srf-3 compared to wt virus. Fusion of both wt and srf-3 showed compara-
ble sphingolipid dependence. Thus, studies of virus interactions with defined
lipid bilayers indicated that both the membrane fusion activity of srf-3 and
the low pH-dependent conformational changes in its El protein are rela-
tively independent of cholesterol while being otherwise similar to those of
wt SFV.

5.3. Mechanism of Cholesterol in Virus Exit

From the data summarized above, it is clear that both SFV and SIN
need cholesterol for virus fusion and exit, and that mutations within the El
226 region can modulate the cholesterol requirements for both viruses.
Both the SFV mutant srf-3 and the SIN mutant SGM which are less cho- _

lesterol dependent for virus entry are also less cholesterol dependent for
virus exit, suggesting that the cholesterol requirement for alphavirus fusion
may be tightly associated with its requirement for exit. Recent studies of
SFV and srf-3 indicate that the cholesterol dependent exit step(s) is late in
the exit pathway, following arrival of the spike protein at the plasma mem-
brane (Lu and Kielian, 1999). The exit pathway does not appear to require
the virus fusion reaction per se, since the final virus assembly and budding
steps involve membrane fission, a pinching-off reaction that initiates at the
cytoplasmic surface of the plasma membrane and occurs at neutral pH
(Marquardt et al., 1993;Vashishtha et al., 1998).Thus, while the data to date
suggest a connection between the cholesterol dependence of alphavirus
fusion and exit, the relationship between the cholesterol-dependent steps
in fusion and exit, and the tightness of the linkage between them, is as yet
unclear. One model is that the cholesterol requirements reflect a role of
cholesterol in the optimal spike protein conformation for both fusion and
exit. In the absence of cholesterol, mutations in the spike protein (e.g.,
within the El 226 region) may act to compensate for the suboptimal spike
conformation and thus allow efficient fusion and exit. The same regions of
the spike protein that require cholesterol to potentiate fusogenic confor-
mational changes may also be involved in interactions between spike pro-
teins during virus assembly and exit. Future studies must address the role
of cholesterol in the spike protein conformation and interactions, and their
importance to the exit pathway.

6. ROLE OF SPECIFIC LIPIDS IN THE ENTRY AND EXIT OF
OTHER PATHOGENS

Specific lipid requirements for membrane fusion and exit were first
described for alphaviruses. Evidence is accumulating that particular lipids