The HIV-1 Envelope Glycoprotein gp120 Features Four Heparan Sulfate Binding Domains, Including the Co-receptor Binding Site

Elodie Crublet,Jean-Pierre Andrieu,Romain R Vivès,Hugues Lortat-Jacob

doi:10.1074/jbc.m800066200

Abstract

It is well established that the human immunodeficiency virus-1 envelope glycoprotein surface unit, gp120, binds to cell-associated heparan sulfate (HS). Virus infectivity is increased by such interaction, and a variety of soluble polyanions efficiently neutralize immunodeficiency virus-1 in vitro. This interaction has been mainly attributed to the gp120 V3 loop. However, although evidence suggested that this particular domain does not fully recapitulate the binding activity of the protein, the ability of HS to bind to other regions of gp120 has not been completely addressed, and the exact localizations of the polysaccharide binding sites are not known. To investigate in more detail the structural basis of the HS-gp120 interaction, we used a mapping strategy and compared the heparin binding activity of wild type and mutant gp120 using surface plasmon resonance-based binding assays. Four heparin binding domains (1-4) were identified in the V2 and V3 loops, in the C-terminal domain, and within the CD4-induced bridging sheet. Interestingly, three of them were found in domains of the protein that undergo structural changes upon binding to CD4 and are involved in co-receptor recognition. In particular, Arg(419), Lys(421), and Lys(432), which directly interact with the co-receptor, are targeted by heparin. This study provides a complete account of the gp120 residues involved in heparin binding and identified several binding surfaces that constitute potential target for viral entry inhibition.

Highlights

In addition to mediating viral attachment and entry, gp120 targets a number of uninfected or non-permissive cells. It is involved in some aspects of the AIDS-associated pathologies, including apoptosis and oxidative stress, and this has motivated the development of heparin-mimetics that inhibit gp120-heparan sulfate (HS) interactions [27]
In agreement with these observations, it has been previously reported that gp120 in its CD4-bound state had a substantially increased binding activity toward heparin, compared with free gp120, and molecular modeling further suggested that the bridging sheet includes a possible HS binding domain (HBD) [33]
Similar amounts were produced using an expression vector, including a leader sequence derived from the baculovirus ecdysteroid UDP glucosyltransferase

Summary

Introduction

The observation that a relatively small oligosaccharide (dp12) targets the same sequences and exactly returns the same results than full-length heparin indicates that having a large number of anionic charge does not give rise to additional (nonspecific) binding and supports the specificity of the assay (see Ref. 37). Mutation of Arg419, Lys421, and Lys432 but not Lys121 Decrease gp120 Binding to Heparin in the Presence of sCD4—Molecular modeling analysis, performed on a sCD41⁄7gp120 complex revealed that a number of residues, located between the stems of the V1/V2 and V3 loops, form a cluster of positively charged residues organized as a possible HBD [33].

Results

Conclusion