Specific Roles for Lipids in Virus Fusion and Exit Examples from the Alphaviruses - Fusion of Biological Membranes and Related Problems - page 433

may be involved in the entry or exit of other pathogens from cells, or in
other fusion reactions. We will discuss a number of possible examples of
such specific lipid requirements. In cases such as the cholesterol-binding
toxins involved in bacterial entry, the data are compelling and informative
at the molecular level. In other cases, the information available to date is
incomplete and less reliable. The examples presented are not comprehen-
sive and certainly include a wide range of approaches, methods, and some
conflicting results. Nevertheless, they serve to illustrate some of the diverse
ways in which other viruses or pathogens may use specific lipids as critical
features of their lifecycles.

6.1. The Role of Cholesterol in Bacterial Toxin-Membrane Interactions

Clear examples of organisms that exploit cholesterol-protein interac-
tions in their lifecycles are bacteria that produce what are termed thiol-
activated cytolysins. The thiol-activated cytolysins are an antigenically and
structurally related family of more than 20 toxins (including perfringolysin
O, listeriolysin O, pneumolysin, and streptolysin O) that are produced by
Gram-positive bacteria and are important virulence factors in bacterial
pathogenesis (reviewed in Alouf and Geoffroy, 1991; Cossart and Mengaud,
1989). The toxins are protein monomers of 50-80 kDa that bind to choles-
terol-containing membranes and oligomerize to form pores responsible
for the permeabilization of cell membranes. Pores are estimated to contain
-50 monomers and to have a diameter of approximately 150Å. It was
shown many years ago that toxin binding to cell membranes (or artificial
membranes) requires cholesterol or a structurally related sterol with a 3b-
hydroxyl, that binding can be irreversibly inhibited by nM amounts of cho-
lesterol in solution or by cholesterol-binding polyene antibiotics, and that
prokaryotic or mycoplasma cells without cholesterol in their membranes
are not susceptible to the lytic effects of the toxins (Alouf and Geoffroy,
1991; Watson and Kerr, 1974; Rottem et al., 1976). The biological function
of a thiol-activated cytolysin is probably best understood for the example
of the facultative intracellular bacterium Listeria monocytogenes (reviewed
in Portnoy et al., 1992; Falkow et al., 1992). Listeria are internalized by host
cell phagocytosis, escape from the phagocytic vacuole, and replicate in the
cytoplasm. Escape from the phagosome is mediated by bacterial secretion
of the toxin listeriolysin O, which permeabilizes the vacuole by binding
phagosome membrane cholesterol upon exposure to the acid pH of the
phagocytic vacuole (Portnoy et al., 1992). Listeriolysin-cholesterol binding
has a pH optimum of 5.5, and requires the presence of a sterol with a 3 -

hydroxyl group. Thus, both alphaviruses and Listeria use low pH-dependent
lipid interaction as a strategy to breach the plasma membrane barrier. While
other

β

thiol-activated cytolysins are also specific for 3 β -hydroxy sterols,