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(DAF) in the apical surface of epithelial cells, and subsequently to the Coxsack-
ievirus and adenovirus receptor (CAR), which is localized in the tight junction
region. DAF helps to bring the virus to the tight junctions, and CAR induces a
conformational change and promotes endocytosis (Coyne and Bergelson, 2006).
Almost all animal viruses use receptors exclusively containing a single
TM sequence (see Table 4.1). In striking contrast, cell surface receptors for
-retroviruses have multiple transmembrane (TM) sequences, compatible
with their identification in known instances as transporters for important solutes.
Similarly, hepatitis C virus, in addition to LDL receptor, exclusively uses multi-
transmembrane receptors:the Scavenger receptor class B type 1 (SRB1 with
two transmembrane domains), the tetraspanin receptors CD81 and Claudins
(4 TMs), and another tight junction protein Occludin-1 (4 transmembrane
domains; Ploss and Rice, 2009).
Another surprise is that some viruses, including many
-retroviruses,
use not just one receptor but pairs of closely related receptors as alternatives
(Tailor
., 2003). This appears to have enhanced viral survival by severely
limiting the likelihood of host escape mutations. All of the receptors used by
et al
-retroviruses contain hypervariable regions that are often heavily glycosylated
and that control the viral host range properties, consistent with the idea that
these sequences are battlegrounds of virus-host coevolution. However, in con-
trast to previous assumptions, it is probable that
-retroviruses have adapted to
recognize conserved sites that are important for the receptor's natural function
and that the hypervariable sequences have been elaborated by the hosts as
defense bulwarks surrounding the conserved viral attachment sites.
The fact that all virus groups have been severely limited throughout
evolution in the types of receptors they can employ, may initially appear incon-
sistent with evidence that some viruses can switch their receptor specificities
with apparent ease. This has been most dramatically suggested by shifts of
influenza A viruses between animal reservoirs, which involve single amino acid
changes in the viral hemagglutinin, enabling recognition of different sialic acid
structures (e.g.,
2,3 linkages
to galactose) that predominate in the different host species (Baranowski
N
-acetyl or
N
-glycolyl neuraminic acids in
2,6 or
et al
.,
2001; Gambaryan
., 2005; Skehel and Wiley, 2000). Similarly, slight changes
in specificity for receptors accompanied the emergence, in 1978, of the canine
parvovirus (Parker
et al
., 2001). However, these are small shifts in receptor
specificities rather than global jumps to dissimilar receptors. Similar slight shifts
are involved in the change from the CCR5 to CXCR4 coreceptor usage during
AIDS progression (Scarlatti
et al
., 1997). Small shifts in usages of highly similar
receptors have also been reported during cell culture selections of subgroup B, D,
and E avian leukosis viruses that all use polymorphic variants of the same TVB
receptor (Taplitz and Coffin, 1997) and during cell culture selections of HIV-1
variants (Platt
et al
et al
., 1998). Therefore, despite the rarity of receptor repertoire
