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of infections, thus contributing to pathogenesis (Alvarez
et al
., 2002; Bounou
et al
.,
2002; Geijtenbeek
., 2001).
Because such binding proteins contribute to infections, it can be difficult to
unambiguously distinguish them from receptors that directly mediate the mem-
brane fusion process, especially for viruses that bind to their authentic receptors
only weakly (e.g., for retroviruses, in the cases of FeLV-T or polytropic MuLVs;
Anderson
et al
., 2000; Jinno-Oue
et al
., 2001; Saphire
et al
, 1999; Temin, 1988). We emphasize this
because pathogenic variants of different animal viruses have often been associated
with abilities to bind to apparently novel cell surface components, and it has
sometimes been inferred that the viruses have switched their receptor specificities.
In these instances, it has generally not been established that the cell surface
binding components are receptors that directly mediate infections. In the
case of Ebola, for example, the receptor(s) that mediates its entry has yet to
be definitively identified. C-type lectins such as DC-SIGN and DC-SIGNR are
thought to serve as adherence factors for Ebola orMarburg virus (Marzi
et al.
, 2000; Marin
et al.
et al.
, 2006;
Matsuno
, 2010). Other plasma membrane-associated proteins have been
implicated in EBOV uptake, including folate receptor alpha and the tyrosine
kinase receptor Axl (Chan
et al.
,
2003), but the physical interaction of EBOV GP and these proteins has not been
demonstrated, and cells that do not express these proteins are permissive for
EBOV GP-mediated virion uptake. Some previous studies have implicated the
actin cytoskeleton in EBOV entry, where agents such as cytochalasin D and
swinholide A that impair microfilament function, inhibited GP-mediated entry
(Yonezawa
et al.
, 2001; Shimojima
et al.
, 2006, 2007; Sinn
et al.
, 2005). Similarly, VSVwas shown to bind ubiquitously to cells via
phosphatidylserine (PS) (Schlegel
et al.
, 1983). However, a more recent study
reports that PS is not a receptor for VSV, as no correlation was found between cell
surface PS levels and VSV infection, and annexin V, which specifically binds PS,
did not inhibit infection of VSV (Coil and Miller, 2004). Therefore, the cell
surface receptors for VSV have not been identified, but it is generally thought that
binding via theG-protein is rather unspecific and involves negative charges on the
plasma membrane (Carneiro
et al.
., 2006; Coil and Miller, 2005).
Adsorption of viruses onto cultured cells from the medium is usually a
very slow and inefficient process, principally because of the slow rates of their
diffusion into contact with the cell surfaces (Allison and Valentine, 1960;
Andreadis
et al
., 2000). In general, the rate of contact cannot be significantly
enhanced by mixing or stirring, because the boundary layer of the relatively
stationary fluid that surrounds walls or other large objects (e.g., cells) in flowing
liquids is substantial compared with the rate of virus diffusion, hence stirring does
not increase the concentration of virus surrounding this boundary zone (Allison
and Valentine, 1960). In the case of retroviruses, it has become especially clear
that adsorption is a severely limiting step in infection of cultured cells. In classic
studies in which virus samples were incubated with cells for several hours before
et al
