HIV
Capsid Protein
Kelly Wahl '12 and Kiersten Bell '13
Contents:
I.
Introduction
The HIV genome
and its core proteins are protected by an outer cell
membrane-derived envelope
and an inner viral protein shell, or capsid. HIV's fullerene or
cone-shaped capsid offers both structure and support. The entire
multimeric capsid is composed of the HIV capsid protein (CA). 12
pentamers and 250 hexamers of CA
link together in a higly ordered
manner to form the capsid.
Model
of HIV fullerene and CA pentamer and hexamer
The capsid is a potential drug target due to its vital role in the HIV
life cycle. If capsid assembly or disassembly is disrupted, viral
replication, and consequently
transmission, can be stopped. A new class of HIV inhibitors
has recently been
discovered that bind to the N-terminus
domain
(NTD)
of
CA interrupting both
uncoating of the viral genome after entry into the cell and prevents
assembly of the capsid during viral replication. One of these new
potential HIV capsid inhibitors is PF-3450074
.
Presented here are two HIV-1
CA NTDs
bound
to the
potential HIV inhibitor PF-3450074
.
II.
CA Structure
CA
is comprosed of a c-terminus domain (CTD) and a NTD briged by a short
linker region.
The NTD is comprised of seven
alpha helices
,
with
a flexible
linker between
helix 4 and 5
.
The
helices pack together to give the
entire monomer a flat, roughly
triangular shape. The NTD also contains a short,
antiparallel 13
residue beta sheet
.
This beta sheet packs against helix 6 through hydrophobic contacts
.
The amino group of
proline 1 of the beta sheet and the carbonyl group of asparagine 51
form a salt bridge that stabilizes the structure
.
III.
Capsid
Structure
CA
monomers are oriented in the same way and have the protein interactions
wherther they are forming a hexamer or pentamer. The linker domain
appears to be the key feature that allows both hexameric and pentameric
faces to form out of CA. This linker is found between the CTD and NTD
domains of a monomer. The NTDs act like “spokes on a
wheel” and the outward facing CTDS interact with each
other to form the “rim” of the wheel and hold the
NTD
"spokes" in place so CTD-CTD interactions with other faces can occur.
NTD
ring formation is
mediated by three a-helices: helix
1 and 3
of one CA's NTD bind to helix
2 of another CA's NTD
.
The
side chains methionine
55; valine
59, 27, 26, and 24; leucine 56, 52, 20, and 52; alanine 31, 22, and 65;
and tryptophan 23
form a
hydrophobic region, stabilizing helix interactions to help create a
rigid, wheel like hexameric or pentameric face
CTD-CTD binding
differs slightly depending on
which face, pentamer or hexamer, is formed.
No interamolecular interactions occur between the CTD and the NTD of a
monomer.
IV.
PF-3450074
The
drug sits in a preformed pocket of the NTD, made up of helices 3, 4, 5,
and 7
.
The hydrophobic interactions between the two
benzene substituents of
PF-3450074
and the binding pocket anchor the drug to the NTD of CA
.
The
other ringed
structure of the drug
interacts with lysine
70
of
helix 4 further stabilizing the drug-protein interactions. PF-3450074
binding at the hydrophobic pocket limits the flexibility of the linker
domain connecting the CTD and the NTD. When the
drug binds at the hydrophobic
pocket, tyrosine
145 changes orientation
.
This
prevednts NTD and CTD interactions between CA monomers resulting in the
premature dissolution of the capsid after viral entry. It also prevents
pentameric and hexameric face formation during capsid assembly.
Mutant
CA's
with amino acid substitutions at in the NTD region were constructed to
determine drug-protein interactions.
When threonine107
is
exchanged for an
asparagine, there is a 6-fold
reduction in viral
susceptibility to the drug.
Additional mutations Q67H, K7OR,
and L111I
resulted in a >60-fold reduction in susceptibility, highlighting
the importance of the NTD binding pocket in CA’s interaction
with
PF-3450074
.
PF-3450074
represents a new class of HIV inhibitors that prevent HIV replication
by interfering with capsid formation. Other classes of HIV inhibitors
work by blocking viral reverse transcriptase and protease activity.
Although
select substitutions of amino acids in the binding pocket
resulted in
HIV strains resistant to PF-3450074, this drug still has the potential
to be a powerful drug in the treatment of HIV. PF-345007 would most
likely be used in a cocktail with other classes of HIV inhibitors,
decreasing the odds of
widespread
resistance developing quickly and therefore prolonging its
use.
VI.
References
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