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Journal of Virology, December 2001, p. 12266-12278, Vol. 75, No. 24
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.24.12266-12278.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Adaptive Mutations in the V3 Loop of gp120 Enhance
Fusogenicity of Human Immunodeficiency Virus Type 1 and Enable Use of a
CCR5 Coreceptor That Lacks the Amino-Terminal Sulfated Region
Emily J.
Platt,
Shawn E.
Kuhmann,
Patrick P.
Rose, and
David
Kabat*
Department of Biochemistry and Molecular
Biology, Oregon Health Sciences University, Portland, Oregon 97201-3098
Received 3 July 2001/Accepted 24 September 2001
To identify sites in gp120 that interact with the CCR5 coreceptor
and to analyze the mechanisms of infection, we selected variants of the
CCR5-dependent JRCSF molecular clone of human immunodeficiency virus
type 1 (HIV-1) that adapted to replicate in HeLa-CD4 cells that express
the mutant coreceptor CCR5(Y14N) or CCR5(G163R), which were previously
shown to bind purified gp120-CD4 complexes only weakly.
Correspondingly, these mutant CCR5s mediate infections of wild-type
virus only at relatively high cell surface concentrations,
demonstrating a concentration-dependent assembly requirement for
infection. The plots of viral infectivity versus concentration of
coreceptors had sigmoidal shapes, implying involvement of multiple
coreceptors, with an estimated stoichiometry of four to six CCR5s in
the active complexes. All of the adapted viruses had mutations in the
V3 loops of their gp120s. The titers of recombinant HIV-1 virions with
these V3 mutations were determined in previously described panels of
HeLa-CD4 cell clones that express discrete amounts of CCR5(Y14N) or
CCR5(G163R). The V3 loop mutations did not alter viral utilization of
wild-type CCR5, but they specifically enhanced utilization of the
mutant CCR5s by two distinct mechanisms. Several mutant envelope
glycoproteins were highly fusogenic in syncytium assays, and these all
increased the efficiency of infection of the CCR5(Y14N) or CCR5(G163R)
clonal panels without enhancing virus adsorption onto the cells or
viral affinity for the coreceptor. In contrast, V3 loop mutation N300Y
was selected during virus replication in cells that contained only a
trace of CCR5(Y14N) and this mutation increased the apparent affinity
of the virus for this coreceptor, as indicated by a shift in the
sigmoid-shaped infectivity curve toward lower concentrations.
Surprisingly, N300Y increased viral affinity for the second
extracellular loop of CCR5(Y14N) rather than for the mutated amino
terminus. Indeed, the resulting virus was able to use a mutant CCR5
that lacks 16 amino acids at its amino terminus, a region previously
considered essential for CCR5 coreceptor function. Our results
demonstrate that the role of CCR5 in infection involves at least two
steps that can be strongly and differentially altered by mutations in either CCR5 or the V3 loop of gp120: a concentration-dependent binding
step that assembles a critical multivalent virus-coreceptor complex and
a postassembly step that likely involves a structural rearrangement of
the complex. The postassembly step can severely limit HIV-1 infections
and is not an automatic consequence of virus-coreceptor binding, as was
previously assumed. These results have important implications for our
understanding of the mechanism of HIV-1 infection and the factors that
may select for fusogenic gp120 variants during AIDS progression.
*
Corresponding author. Mailing address: Department of
Biochemistry and Molecular Biology, Oregon Health Sciences University, Portland, OR 97201-3098. Phone: (503) 494-8442. Fax: (503) 494-8393. E-mail: kabat{at}ohsu.edu.
Journal of Virology, December 2001, p. 12266-12278, Vol. 75, No. 24
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.24.12266-12278.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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