Biology Reference
In-Depth Information
early endosomes to the PNRC is absolutely microtubule dependent.
73
In contrast, bafilomycin does not affect transferrin delivery to the PNRC
and incubation at 20
C only delays transferrin recycling. Taken together,
at least in HeLa cells, the transferrin recycling pathway and the transport
of endocytosed material to lysosomes are differentially affected by
bafilomycin, nocodazole and low temperature (Fig. 4). Consequently,
these treatments can be applied to investigate whether viruses follow a
recycling or degradative pathway and to identify the compartment where
virus penetration/uncoating takes place.
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Entry of Major Group HRVs
ICAM-1, the major group HRV receptor, lacks any known internal-
ization signal in its cytoplasmic domain, and infection by HRV14 also
occurs when the cytoplasmic tail of the receptor is removed or the
transmembrane domain replaced by a GPI-anchor.
80
However, the
virus appears to be internalized via clathrin-mediated endocytosis since
overexpression of the non-functional dynamin 2 mutant K44A in
HeLa cells resulted in inhibition of infection; involvement of caveolae
is unlikely since these cells express little caveolin.
81
Given the absence
of a
bona fide
internalization signal in ICAM-1, it has been speculated
that a co-receptor may be involved. For example, adenovirus requires
α
v
β
3
or
β
5
-integrins for internalization, although it first binds to the
coxsackie-adenovirus receptor (CAR).
82
No evidence for an HRV co-
receptor has been presented so far and
in vivo
data showing co-local-
ization of major group virus with ICAM-1 in a particular compartment
are still lacking. Although the affinity of recombinant ICAM-1 frag-
ments for immobilized HRV3 decreases about 50-fold upon lowering
of the pH from 8.0 to 6.0,
83
it has not been investigated
in vivo
whether the virus dissociates from ICAM-1 at low endosomal pH.
By using immunofluorescence microscopy, HRV14 was found in
typical endosomal compartments (Fig. 5) when internalized at 20
C.
84
Under this condition the conformational modification of the cap-
sid, catalyzed by ICAM-1, is inhibited (see below). However, when
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