HIV Entry and Envelope Glycoprotein-mediated Fusion

Abstract

HIV entry involves binding of the trimeric viral envelope glycoprotein (Env) gp120/gp41 to cell surface receptors, which triggers conformational changes in Env that drive the membrane fusion reaction. The conformational landscape that the lipids and Env navigate en route to fusion has been examined by biophysical measurements on the microscale, whereas electron tomography, x-rays, and NMR have provided insights into the process on the nanoscale and atomic scale. However, the coupling between the lipid and protein pathways that give rise to fusion has not been resolved. Here, we discuss the known and unknown about the overall HIV Env-mediated fusion process. The Virus and Its TargetHIV virions exist as roughly spherical nanoparticles ϳ100 nm in diameter and are coated by the viral envelope membrane (1). The viral membrane is a lipid bilayer ϳ4 nm thick, interspersed with membrane-embedded glycoproteins. The HIV matrix protein Gag Pr55 drives viral assembly by recruiting all the building blocks, which include both viral and cellular components required for the formation of a fully infectious virion (2). A typical HIV viral membrane contains ϳ300,000 lipids, with a rather unique distribution of lipids compared with the lipid composition of cells from which it is derived (3,4). In addition to the virally encoded envelope glycoprotein gp120/ gp41, the HIV membrane also incorporates a plethora of cell membrane proteins in the process of assembly and budding (5). The HIV envelope proteins are expressed in the endoplasmic reticulum as a precursor protein, gp160, which transits the Golgi apparatus, where glycosylation is completed (6, 7). The precursor is cleaved in the trans-Golgi by the cellular protease furin into two proteins, gp120 and gp41 (8). These proteins are present on the cell surface as the envelope complex, a mushroomshaped trimer of heterodimers of gp120 and gp41, incorporated into the viral envelope through the transmembrane region of gp41 as virus particles bud from the cell surface (9).The number of gp120/gp41 trimers per virion ranges between 10 and 100 depending on the isolate (10). HIV gp120 is ϳ50% carbohydrate and is one of the most glycosylated proteins known (11). The number of accessory HIV membrane proteins has, in general, not been determined, with the exception of HLA class II in HIV-1 MN derived from H9 cells, which has ϳ50 native HLA class II complexes (12). The lipids and proteins are glycoconjugated, providing an additional shield for HIV against immune and environmental challenges. The viral genome is thus comfortably ensconced in a wrapper of lipid and protein covered by carbohydrate. However, mannose-type carbohydrates on HIV are recognized by DC-SIGN (dendritic cellspecific intercellular adhesion molecule-3-grabbing non-integrin), which is a C-type lectin receptor present on dendritic cells (13). The dendritic cells internalize HIV-DC-SIGN complexes and migrate to the lymph nodes, where they interact with T cells. The HIV-DC-SIGN complexes are then recycled to the cell periphery, and ...

High-Throughput HIV–Cell Fusion Assay for Discovery of Virus Entry Inhibitors

“…5C). In control experiments performed in the presence of NH 4 Cl, which raises endosomal pH, VSVpp fusion was nearly completely blocked (data not shown). These results demonstrate that the inhibitory effect of P2X1 antagonists is specific to HIV-1 Env-mediated fusion.…”

“…[1][2][3][4] The formation of ternary complexes between the gp120 subunit of Env, CD4, and coreceptors triggers the refolding of the transmembrane gp41 subunit, which promotes membrane merger. 5,6 This refolding progresses through prehairpin intermediates, characterized by the formation/exposure of the N-proximal heptad repeat (HR1) regions and the membraneproximal heptad repeat (HR2) regions (reviewed in Ref.…”

Dissociation of the trimeric gp41 ectodomain at the lipid–water interface suggests an active role in HIV-1 Env-mediated membrane fusion

“…Binding of gp120 to the cell surface receptors CD4 and chemokine receptors CXCR4 or CCR5 triggers a cascade of conformational changes that disrupt the interactions between gp41 and gp120 and result in an extended gp41 conformation (1, 6). In this extended prefusion state, the highly hydrophobic N-terminal fusion peptide (FP) of gp41 anchors in the host cell membrane, while being spatially remote from its transmembrane domain (TM), which traverses the viral membrane (7,8). After the host cell and viral membranes have fused, the gp41 ectodomain, which links the FP and TM domains, has transitioned into a C3-symmetric six-helix bundle (6HB), with the FP in physical proximity to the TM domain (9).…”

Unique Functional Properties of Conserved Arginine Residues in the Lentivirus Lytic Peptide Domains of the C-terminal Tail of HIV-1 gp41

“…In the case of the lentivirus HIV-1, the gp160 precursor is processed into two subunits, gp120 and gp41, that remain non-covalently associated. The gp120 protein interacts with the CD4 receptor of the target cells, mediating the first step of virus entry (1,2). Because of its exposure at the surface of the virus particle, gp120 is the target of most broadly neutralizing antibodies identified so far and thus represents a key protein in vaccine development (3,4).…”

“…Retrovirus envelope (Env) 2 proteins are important mediators of virus replication and immunogenicity. In the case of the lentivirus HIV-1, the gp160 precursor is processed into two subunits, gp120 and gp41, that remain non-covalently associated.…”