| Structural highlights
Function
[Q2N0S5_9HIV1] The envelope glyprotein gp160 precursor down-modulates cell surface CD4 antigen by interacting with it in the endoplasmic reticulum and blocking its transport to the cell surface.[RuleBase:RU004292][SAAS:SAAS00139820] The gp120-gp41 heterodimer allows rapid transcytosis of the virus through CD4 negative cells such as simple epithelial monolayers of the intestinal, rectal and endocervical epithelial barriers. Both gp120 and gp41 specifically recognize glycosphingolipids galactosyl-ceramide (GalCer) or 3' sulfo-galactosyl-ceramide (GalS) present in the lipid rafts structures of epithelial cells. Binding to these alternative receptors allows the rapid transcytosis of the virus through the epithelial cells. This transcytotic vesicle-mediated transport of virions from the apical side to the basolateral side of the epithelial cells does not involve infection of the cells themselves.[SAAS:SAAS00140087] [Q2N0S7_9HIV1] Envelope glycoprotein gp160: Oligomerizes in the host endoplasmic reticulum into predominantly trimers. In a second time, gp160 transits in the host Golgi, where glycosylation is completed. The precursor is then proteolytically cleaved in the trans-Golgi and thereby activated by cellular furin or furin-like proteases to produce gp120 and gp41.[HAMAP-Rule:MF_04083] Surface protein gp120: Attaches the virus to the host lymphoid cell by binding to the primary receptor CD4. This interaction induces a structural rearrangement creating a high affinity binding site for a chemokine coreceptor like CXCR4 and/or CCR5. Acts as a ligand for CD209/DC-SIGN and CLEC4M/DC-SIGNR, which are respectively found on dendritic cells (DCs), and on endothelial cells of liver sinusoids and lymph node sinuses. These interactions allow capture of viral particles at mucosal surfaces by these cells and subsequent transmission to permissive cells. HIV subverts the migration properties of dendritic cells to gain access to CD4+ T-cells in lymph nodes. Virus transmission to permissive T-cells occurs either in trans (without DCs infection, through viral capture and transmission), or in cis (following DCs productive infection, through the usual CD4-gp120 interaction), thereby inducing a robust infection. In trans infection, bound virions remain infectious over days and it is proposed that they are not degraded, but protected in non-lysosomal acidic organelles within the DCs close to the cell membrane thus contributing to the viral infectious potential during DCs' migration from the periphery to the lymphoid tissues. On arrival at lymphoid tissues, intact virions recycle back to DCs' cell surface allowing virus transmission to CD4+ T-cells.[HAMAP-Rule:MF_04083] Transmembrane protein gp41: Acts as a class I viral fusion protein. Under the current model, the protein has at least 3 conformational states: pre-fusion native state, pre-hairpin intermediate state, and post-fusion hairpin state. During fusion of viral and target intracellular membranes, the coiled coil regions (heptad repeats) assume a trimer-of-hairpins structure, positioning the fusion peptide in close proximity to the C-terminal region of the ectodomain. The formation of this structure appears to drive apposition and subsequent fusion of viral and target cell membranes. Complete fusion occurs in host cell endosomes and is dynamin-dependent, however some lipid transfer might occur at the plasma membrane. The virus undergoes clathrin-dependent internalization long before endosomal fusion, thus minimizing the surface exposure of conserved viral epitopes during fusion and reducing the efficacy of inhibitors targeting these epitopes. Membranes fusion leads to delivery of the nucleocapsid into the cytoplasm.[HAMAP-Rule:MF_04083]
Publication Abstract from PubMed
Inducing broad spectrum neutralizing antibodies against challenging pathogens such as HIV-1 is a major vaccine design goal, but may be hindered by conformational instability within viral envelope glycoproteins (Env). Chemical cross-linking is widely used for vaccine antigen stabilization, but how this process affects structure, antigenicity and immunogenicity is poorly understood and its use remains entirely empirical. We have solved the first cryo-EM structure of a cross-linked vaccine antigen. The 4.2 A structure of HIV-1 BG505 SOSIP soluble recombinant Env in complex with a CD4 binding site-specific broadly neutralizing antibody (bNAb) Fab fragment reveals how cross-linking affects key properties of the trimer. We observed density corresponding to highly specific glutaraldehyde (GLA) cross-links between gp120 monomers at the trimer apex and between gp120 and gp41 at the trimer interface that had strikingly little impact on overall trimer conformation, but critically enhanced trimer stability and improved Env antigenicity. Cross-links were also observed within gp120 at sites associated with the N241/N289 glycan hole that locally modified trimer antigenicity. In immunogenicity studies, the neutralizing antibody response to cross-linked trimers showed modest but significantly greater breadth against a global panel of difficult-to-neutralize Tier-2 heterologous viruses. Moreover, the specificity of autologous Tier-2 neutralization was modified away from the N241/N289 glycan hole, implying a novel specificity. Finally, we have investigated for the first time T helper cell responses to next-generation soluble trimers, and report on vaccine-relevant immunodominant responses to epitopes within BG505 that are modified by cross-linking. Elucidation of the structural correlates of a cross-linked viral glycoprotein will allow more rational use of this methodology for vaccine design, and reveals a strategy with promise for eliciting neutralizing antibodies needed for an effective HIV-1 vaccine.
Structural and immunologic correlates of chemically stabilized HIV-1 envelope glycoproteins.,Schiffner T, Pallesen J, Russell RA, Dodd J, de Val N, LaBranche CC, Montefiori D, Tomaras GD, Shen X, Harris SL, Moghaddam AE, Kalyuzhniy O, Sanders RW, McCoy LE, Moore JP, Ward AB, Sattentau QJ PLoS Pathog. 2018 May 10;14(5):e1006986. doi: 10.1371/journal.ppat.1006986., eCollection 2018 May. PMID:29746590[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
- ↑ Schiffner T, Pallesen J, Russell RA, Dodd J, de Val N, LaBranche CC, Montefiori D, Tomaras GD, Shen X, Harris SL, Moghaddam AE, Kalyuzhniy O, Sanders RW, McCoy LE, Moore JP, Ward AB, Sattentau QJ. Structural and immunologic correlates of chemically stabilized HIV-1 envelope glycoproteins. PLoS Pathog. 2018 May 10;14(5):e1006986. doi: 10.1371/journal.ppat.1006986., eCollection 2018 May. PMID:29746590 doi:http://dx.doi.org/10.1371/journal.ppat.1006986
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