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Allosteric Modulation of the HIV-1 gp120-gp41 Association Site by Adjacent gp120 Variable Region 1 (V1) N-Glycans Linked to Neutralization Sensitivity

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      Abstract

      The HIV-1 gp120-gp41 complex, which mediates viral fusion and cellular entry, undergoes rapid evolution within its external glycan shield to enable escape from neutralizing antibody (NAb). Understanding how conserved protein determinants retain functionality in the context of such evolution is important for their evaluation and exploitation as potential drug and/or vaccine targets. In this study, we examined how the conserved gp120-gp41 association site, formed by the N- and C-terminal segments of gp120 and the disulfide-bonded region (DSR) of gp41, adapts to glycan changes that are linked to neutralization sensitivity. To this end, a DSR mutant virus (K601D) with defective gp120-association was sequentially passaged in peripheral blood mononuclear cells to select suppressor mutations. We reasoned that the locations of suppressors point to structural elements that are functionally linked to the gp120-gp41 association site. In culture 1, gp120 association and viral replication was restored by loss of the conserved glycan at Asn 136 in V1 (T138N mutation) in conjunction with the L494I substitution in C5 within the association site. In culture 2, replication was restored with deletion of the N 139INN sequence, which ablates the overlapping Asn 141-Asn 142-Ser-Ser potential N-linked glycosylation sequons in V1, in conjunction with D601N in the DSR. The 136 and 142 glycan mutations appeared to exert their suppressive effects by altering the dependence of gp120-gp41 interactions on the DSR residues, Leu 593, Trp 596 and Lys 601. The 136 and/or 142 glycan mutations increased the sensitivity of HIV-1 pseudovirions to the glycan-dependent NAbs 2G12 and PG16, and also pooled IgG obtained from HIV-1-infected individuals. Thus adjacent V1 glycans allosterically modulate the distal gp120-gp41 association site. We propose that this represents a mechanism for functional adaptation of the gp120-gp41 association site to an evolving glycan shield in a setting of NAb selection.

      Author Summary

      The envelope glycoprotein gp120-gp41 complex of HIV-1 mediates receptor attachment and virus-cell membrane fusion, leading to cellular entry. A shield of asparagine-linked oligosaccharides occludes the gp120-gp41 protein surface and evolution of this glycan shield provides a means for evading circulating neutralizing antibody. Here we examined how conserved structural elements of the glycoprotein complex, in particular the gp120-gp41 association site, retain functionality in the context of glycan shield evolution. This information is important for the evaluation and exploitation of such conserved functional determinants as potential drug and/or vaccine targets. Our data indicate that the loss of either of 2 adjacent glycans in variable region 1 of gp120 leads to changes in local and remote glycan-dependent epitopes and that this is linked to a remodelling of gp120-gp41 interactions in order to maintain a functional gp120-gp41 complex. We propose that this represents a mechanism for the functional adaptation of the gp120-gp41 association site to an evolving glycan shield in a setting of neutralizing antibody selection.

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      Most cited references 116

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      UCSF Chimera--a visualization system for exploratory research and analysis.

      The design, implementation, and capabilities of an extensible visualization system, UCSF Chimera, are discussed. Chimera is segmented into a core that provides basic services and visualization, and extensions that provide most higher level functionality. This architecture ensures that the extension mechanism satisfies the demands of outside developers who wish to incorporate new features. Two unusual extensions are presented: Multiscale, which adds the ability to visualize large-scale molecular assemblies such as viral coats, and Collaboratory, which allows researchers to share a Chimera session interactively despite being at separate locales. Other extensions include Multalign Viewer, for showing multiple sequence alignments and associated structures; ViewDock, for screening docked ligand orientations; Movie, for replaying molecular dynamics trajectories; and Volume Viewer, for display and analysis of volumetric data. A discussion of the usage of Chimera in real-world situations is given, along with anticipated future directions. Chimera includes full user documentation, is free to academic and nonprofit users, and is available for Microsoft Windows, Linux, Apple Mac OS X, SGI IRIX, and HP Tru64 Unix from http://www.cgl.ucsf.edu/chimera/. Copyright 2004 Wiley Periodicals, Inc.
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        We describe a comparative protein modelling method designed to find the most probable structure for a sequence given its alignment with related structures. The three-dimensional (3D) model is obtained by optimally satisfying spatial restraints derived from the alignment and expressed as probability density functions (pdfs) for the features restrained. For example, the probabilities for main-chain conformations of a modelled residue may be restrained by its residue type, main-chain conformation of an equivalent residue in a related protein, and the local similarity between the two sequences. Several such pdfs are obtained from the correlations between structural features in 17 families of homologous proteins which have been aligned on the basis of their 3D structures. The pdfs restrain C alpha-C alpha distances, main-chain N-O distances, main-chain and side-chain dihedral angles. A smoothing procedure is used in the derivation of these relationships to minimize the problem of a sparse database. The 3D model of a protein is obtained by optimization of the molecular pdf such that the model violates the input restraints as little as possible. The molecular pdf is derived as a combination of pdfs restraining individual spatial features of the whole molecule. The optimization procedure is a variable target function method that applies the conjugate gradients algorithm to positions of all non-hydrogen atoms. The method is automated and is illustrated by the modelling of trypsin from two other serine proteinases.
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          Antibody neutralization and escape by HIV-1.

          Neutralizing antibodies (Nab) are a principal component of an effective human immune response to many pathogens, yet their role in HIV-1 infection is unclear. To gain a better understanding of this role, we examined plasma from patients with acute HIV infection. Here we report the detection of autologous Nab as early as 52 days after detection of HIV-specific antibodies. The viral inhibitory activity of Nab resulted in complete replacement of neutralization-sensitive virus by successive populations of resistant virus. Escape virus contained mutations in the env gene that were unexpectedly sparse, did not map generally to known neutralization epitopes, and involved primarily changes in N-linked glycosylation. This pattern of escape, and the exceptional density of HIV-1 envelope glycosylation generally, led us to postulate an evolving 'glycan shield' mechanism of neutralization escape whereby selected changes in glycan packing prevent Nab binding but not receptor binding. Direct support for this model was obtained by mutational substitution showing that Nab-selected alterations in glycosylation conferred escape from both autologous antibody and epitope-specific monoclonal antibodies. The evolving glycan shield thus represents a new mechanism contributing to HIV-1 persistence in the face of an evolving antibody repertoire.
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            Author and article information

            Affiliations
            [1 ]Macfarlane Burnet Institute for Medical Research and Public Health, Melbourne, Victoria, Australia
            [2 ]Department of Microbiology, Monash University, Clayton, Victoria, Australia
            [3 ]Department of Microbiology and Immunology, The University of Melbourne, Melbourne, Victoria, Australia
            [4 ]Department of Immunology, Monash University, Alfred Medical Research and Education Precinct, Melbourne, Victoria, Australia
            [5 ]Department of Surgery, Austin Health, The University of Melbourne, Heidelberg, Victoria, Australia
            [6 ]Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria, Australia
            [7 ]Deakin University School of Medicine, Geelong, Victoria, Australia
            Harvard University, United States of America
            Author notes

            The authors have declared that no competing interests exist.

            Performed the molecular modelling: PAR PP. Conceived and designed the experiments: HED PP. Performed the experiments: HED MKH ALM SW JM PP. Analyzed the data: HED MKH ALM SW PAR JM PP. Contributed reagents/materials/analysis tools: HED MKH ALM SW PAR JM PP. Wrote the paper: PP HED.

            Contributors
            Role: Editor
            Journal
            PLoS Pathog
            PLoS Pathog
            plos
            plospath
            PLoS Pathogens
            Public Library of Science (San Francisco, USA )
            1553-7366
            1553-7374
            April 2013
            April 2013
            4 April 2013
            : 9
            : 4
            23592978
            3616969
            PPATHOGENS-D-12-01453
            10.1371/journal.ppat.1003218
            (Editor)

            This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

            Counts
            Pages: 18
            Funding
            This work was supported by The National Health and Medical Research Council of Australia (Grants 1009809 and 603735) and its Infrastructure Support Scheme, and the Australian Centre for HIV and Hepatitis Virology Research. HED is a recipient of an R. D. Wright Award (433929). JM is an Australian Research Council Future Fellow. The authors gratefully acknowledge the contribution to this work of the Victorian Operational Infrastructure Support Program received by the Burnet Institute. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
            Categories
            Research Article
            Biology
            Microbiology
            Host-Pathogen Interaction
            Medical Microbiology
            Microbial Pathogens
            Virology
            Medicine
            Infectious Diseases
            Viral Diseases
            HIV
            Retrovirology and HIV immunopathogenesis
            Sexually Transmitted Diseases

            Infectious disease & Microbiology

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