Biology Reference
In-Depth Information
washing with fresh medium and subsequently detecting the foci of infection, it
was estimated that only 1/1000 or fewer of the virions in the medium were
infectious. In contrast to previous interpretations, studies suggest that this low
infectivity-to-virion ratio is principally caused by the inefficiency of adsorption
(Andreadis
., 2000). Accordingly, serial incubation of a virus-containing
medium for 2-h periods with sequential cell cultures results in the same titers in
each of the cultures after correction for spontaneous viral decay (Kabat
et al
.,
1994). Furthermore, centrifuging the virus down onto the cultured cells (i.e.,
spinoculation) often increases retroviral titers by 1-2 log orders of magnitude
(Bahnson
et al
., 1995).
Studies aiming to count retrovirions adsorbed onto cell surfaces by
confocal immunofluoresence microscopy or by quantitative PCR methods
(Marechal
et al
., 1999, 2001) have demonstrated that
receptors for viral entry are irrelevant for initial adsorption of retrovirions onto
surfaces of most cells (Pizzato
et al
., 2001; Pizzato
et al
., 1999, 2001). On the contrary, the initial steps
of virus attachment seem to more critically depend on accessory cellular binding
substances, such as heparin sulfates, integrins, or lectins, including DC-SIGN
(Bounou
et al
et al
., 2002; Guibinga
et al
., 2002; Jinno-Oue
et al
., 2001; Mondor
et al
.,
1998; Saphire
., 2001). By forming multivalent weak reversible bonds with
such abundant cell surface components, a virus would become efficiently bound
in a manner that would allow it to “graze” until it makes appropriate contact with
a true receptor (Haywood, 1994; Park
et al
et al
., 2000).
2. S-S shuffling
It is well known that for several viruses (Rubella togavirus, BVDV pestivirus,
Newcastle disease paramyxovirus, HIV lentivirus, mouse hepatitis coronavirus
(MHV)), rearrangements of the thiol content and of the disulfide bridges,
induced by thioredoxine or protein disulfide isomerase (PDI), are essential to
induce some big conformational changes necessary for the membrane fusion
(Fenouillet
., 2007).
Interestingly, the MLV and HTLV retroviruses developed an “internal”
oxydoreduction activity by adapting a catalytic motif involved in disulfure bridge
isomerization. The two SU and TM subunits can be linked in either a covalent or
noncovalent manner. For HIV-1, the existence of the soluble gp120 protein
indicates a noncovalent link between SU and TM (Kowalski
et al
., 1987).
However, for most other retroviruses, a covalent link was described at one
stage. In all cases, with the exception of MMTV and JSRV, a disulfide bond
between the SU and the TM is formed between the highly conserved CX6CC
motif of the TM and the CXXC of the SU (Pinter
et al
et al
., 1997; Schulz
et al
., 1992;
Sitbon
et al
., 1991). This CXXC motif is extremely rare in cellular proteins and is
