Chemistry Reference
In-Depth Information
door that states 'Not all who wander are lost'. If this observation is correct, the
excursion may be relevant. If not, at least the excursion will be short.
26.8
A Toxic Side- Trip
In addition to their antimicrobial, antiviral and lectin-like properties,
θ
- defensins
(and
-defensins) inhibit several bacterial exotoxins, none of which is glycosyl-
ated. For example, anthrax lethal factor and botulinum neurotoxin A are zinc-
dependent proteases. The
α
-defensins bind them, and neutralize their
toxicity by inhibiting their enzymatic activity in a noncompetitive manner [7] .
They also inactivate listeriolysin-O and pneumolysin, which are nonenzymatic,
pore-forming toxins. Low micromolar concentrations of RC-1 protect cells by
binding and cross-linking the toxin monomers, and preventing their assembly
into pores (unpublished data). Since none of the aforementioned toxins are gly-
cosylated, the protective effects of
α
- and
θ
θ
-defensins cannot be attributed to their lectin-
like behavior.
26.9
And Now, the Surprise
As mentioned above, the ability of RCs to prevent HIV-1 entry and their lectin-like
properties were strongly correlated. One might assume (as we did) that the protec-
tive mechanism delineated in our studies with IAV would apply to HIV- 1. However,
RC-1 inhibited HIV-1 envelope-mediated fusion without either cross-linking
membrane glycoproteins, as in IAV, or inhibiting gp120- CD4 interactions [8] .
Instead, RC-1 acted late in the HIV-1 Env fusion cascade, just prior to 6-helix
bundle formation, by binding to the C-terminal heptad repeat of gp41 and prevent-
ing 6 - helix bundle formation - an integral component of the entry process. Binding
of RC-1 to the heptad repeat was selective, showed high affi nity and (here comes
the surprise) it was glycan independent.
Why were the carbohydrate-dependent binding of RCs and their ability to prevent
viral entry strongly correlated? Perhaps the molecular features of RCs that promote
high affi nity binding to anionic residues in heptad repeat-2 also contribute sub-
stantially to its carbohydrate-binding properties. The
- defensins contain only 18
residues. Of these, 10 (six cysteines and the four arginines common to RC- 1 and
RTD-1) were also conserved in the
DEFT
genes found in other primate species
[2]. The importance of the cysteine disulfi de bonds for carbohydrate binding and
activity against HIV-1 has already been mentioned [3, 5]. Molecular modeling and
studies with RC analogs indicated that the four conserved arginines were strategi-
cally positioned to interact with highly conserved anionic residues found in HIV- 1
[8]. In HIV-2, these anionic residues are not conserved and this retrovirus is, in
fact, considerably more resistant to RCs. The highly mutable genome of HIV-1
θ
