Chemistry Reference
In-Depth Information
Binding of RC-1 to gp120 and CD4 was greatly reduced once the glycoproteins
were enzymatically deglycosylated. The converse was also true, since
- defensins
bound neoglycoproteins (for information on neoglycoconjugates, please see Chap-
ters 4 and 27) more than unmodifi ed albumin. Cyclic analogs of RC-1 lacking its
internal tridisulfi de ladder had little ability to bind glycoproteins or neoglycopro-
teins. Noncyclic RC-1 analogs with the tridisulfi de ladder and a
θ
- hairpin structure
were also relatively ineffective. From these observations, we concluded that RCs
had lectin-like activity, and that both their cyclic backbone and their disulfi de
ladder contributed to this property [5]. Since other lectins, most notably cyanovirin,
had been shown to afford protection against HIV-1 (and other viruses) by prevent-
ing viral entry, we felt confi dent that we were on the right path to showing how
β
θ
- defensins prevented HIV - 1 entry.
We tested many RC-1 analogs, including its
enantio
,
retro
and
retroenantio
coun-
terparts, and variants in which noncysteine residues of RC-1 were replaced one-
by-one or in groups by other residues. The ability of these analogs to inhibit entry
of HIV-1 correlated well with their ability to bind gp120 and/or CD4 [5] .
26.7
Studies with Infl uenza A Virus
RC-2 potently inhibits infection by infl uenza A virus (IAV), again acting as an
entry inhibitor [6]. Unlike HIV-1, the entry of IAV is initiated by binding interac-
tions between the hemagglutinin of the virus and cell-surface sialic acid receptors
of the target cell, and viral internalization takes place by subsequent endocytosis
(please see also Chapter 17.2.1). Fusion between the viral envelope and the endo-
somal membrane allows IAV to enter the cell. RC-2 did not change viral binding,
inhibit endocytosis or directly inactivate the virions. Intact disulfi de bonds were
necessary for activity against IAV, as they were for activity against HIV-1. How
then was fusion inhibited?
Fusion mediated by viral glycoproteins is a multistep process that depends on
close contact between the viral and cell membranes. Normally, the virus- cell
contact zone is crowded with membrane proteins that cover both of the mem-
branes. To consummate the fusion process, these membrane proteins must be
displaced from the future fusion site. Binding to and then cross-linking the surface
glycoproteins by RC-2 frustrates this process by preventing the displacement of
proteins from the fusion site. A similar mechanism allows RC-2 to inhibit fusion
mediated by the glycoproteins of Sindbis virus and baculovirus, suggesting that
the antiviral effects of RCs do not arise from interactions specifi c to any one class
of viral or cellular (co)receptors or viral fusion proteins. This mechanism is not
unique to RC-2, since both HBD-3 and MBL also use this mechanism to block viral
fusion (for further information on the collectin MBL, please see Chapter 19 ).
Any reader who has remained with this chapter has no doubt noticed the
author's circumforaneous (you can look it up) tendencies and will not be surprised
that he is about to take you on another side trip. There is a plaque on his offi ce
