Fusion Peptides: The Claws of the Viral Fusion Glycoproteins

Abstract

Enveloped viruses, comprising highly relevant human pathogens such as human immunodeficiency (HIV), influenza (IFV) or Ebola (EBOV) viruses, have evolved a common ‘membrane fusion’ strategy to gain access to the biosynthetic resources of their host cells. The viral fusion peptides (VFPs) constitute conserved hydrophobic domains that are required for the virus–cell fusion process mediated by the viral membrane glycoproteins. A generally accepted model postulates that VFPs can exist in three states: (1) stably folded within the native glycoprotein ectodomain, (2) inserted into the membrane of the target cell, and (3) coassembled with transmembrane domains (TMDs) into membrane integral complexes. Upon fusion activation, sequential access to states (2) and (3) would enable the glycoprotein ectodomain for docking and dragging of the target cell membrane and thus promote merger. Our current understanding of the structure and function of the VFPs has potential implications for clinical intervention.

Key Concepts

  • For replication, viruses must enter into living cells and access their synthetic machinery.
  • Enveloped viruses fuse their membrane with that of the host cell during the entry process.
  • Envelope glycoproteins identify the host cell and induce virus–cell membrane fusion.
  • Envelope glycoproteins access at least 3 different states: (1) pre‐fusion, in the native virion; (2) activated, when inserted both in the viral and target cell membranes; and (3) post‐fusion, the conformation adopted in the final fused membranes.
  • Envelope glycoproteins utilise two tools to perform their function: viral fusion peptides (VFPs) that insert into the target cell membrane, and helical hairpins that bring viral and cell membranes close together.
  • The sequences of the VFPs are overall hydrophobic, a feature conferring the capacity of inserting into membranes, and enriched in conserved Ala/Gly residues, which provides the required flexibility for function.
  • In the pre‐fusion state of envelope glycoproteins, VFPs have been observed exposed to solvent or occluded within the trimeric complex.
  • Synthetic VFPs are conformationally diverse in the membrane milieu.
  • Insertion of VFPs into membranes can help fusion to evolve by restructuring the bilayer architecture locally during the process.
  • Conserved and functional VFPs may exist in a native state accessible to drugs and biologics.

Keywords: membrane fusion; viral fusion; viral entry; fusion glycoprotein; viral fusion peptide; HIV‐1 Env; influenza hemagglutinin; Ebola GP

Figure 1. Model for viral glycoprotein‐induced membrane fusion and proposed functions for the VFP sequence in the process. (a) Schematic displaying of the general organisation of the HIV‐1 Env glycoprotein sequence. HIV‐1 Env is synthesised as a single polypeptide (gp160), which is posttranslationally cleaved into two chains, gp120 (SU subunit) and gp41 (TM subunit). Functional domains designated in gp41 include FP, fusion peptide; NHR and CHR, amino‐ and carboxyterminal helical regions, respectively; MPER, membrane‐proximal external region; TMD, transmembrane domain; CD, cytoplasmic domain. (b) and (c) General models of cell–virus membrane fusion promoted by Class I and Class II viral fusion glycoproteins (top and bottom, respectively). The inset in panel C displays the crystallographic structure of the pocket formed by the VFLs of Rift Valley fever virus Class II fusion protein Gc (in green) together with the interacting lipids in sticks representation (PDB: 6EGU).
Figure 2. VFP structures in the pre‐fusion state of Class I glycoproteins. (a) VFP structures in compact and open conformations of HIV‐1 Env glycoprotein. It is proposed that the exposed and accessible version of the VFP in the compact state (structures on the left; PDB accession number: 5I8H) can transition into a version occluded within a hydrophobic pocket in the open conformation (structures on the right, PDB accession number: 5VN3). (b) and (c) Organisation of the VFP sequences in the IFV Hemagglutinin and EBOV‐GP (PDB accession numbers: 2HMG and 5JQ3, respectively). In all panels, trimeric and monomeric structures of each protein are shown. SU and TM subunits are coloured in blue and grey, respectively. The CHR region and the VFP are depicted in red and green, respectively. TMDs are displayed as blue elongated‐rounded rectangles.
Figure 3. Membrane‐inserted structures of VFPs and lipid bilayer architecture destabilisation. (a) NMR structures of synthetic VFPs derived from HIV‐1, IFV and EBOV obtained in membrane mimetics and proposed modes of interaction with the lipid bilayer (top views are shown below). Conformations on the left are proposed to represent initial insertion, and those on the right be implicated in promoting membrane destabilisation. PDB accession numbers used to render the figure include 2P5V (HIV VFP monomer), 2KXA (IFV VFP hairpin), 1IBN (IFV VFP ‘boomerang’), 2LCZ and 2LCY (EBOV VFP at pHs 7.0 and 5.5, respectively). (b) Examples of the restructuring effects exerted by VFPs on the target cell membrane to prime it for fusion. The β‐sheet oligomers established by the HIV‐1 VFP would create poorly solvated spots to facilitate initial interbilayer contacts. The inserted IFV VFP helical hairpin would induce the membrane monolayer to curve, whereas its open conformation could promote extraction of phospholipid acyl chains, thereby generating a lipid bridge between the merging membranes. Accession to the hydrophobic core by the EBOV VFP might soften the core of the lipid bilayer making it more prone to bend.
Figure 4. Fusion pore expansion and the role of the 6‐HB clamp in the process. (a) Completion of the 6‐HB by bringing together VFPs and TMDs within the fused bilayer is proposed to favour a flat wall of the fusion pore, hence promoting its expansion and the widening of the aqueous channel that connects the previously separated cell and virus compartments. (b) The X‐ray structures of the 6‐HBs formed by HIV‐1 gp41 (built as a chimeric model of PDB structures 2X7R and 2EZO), IFV HA2 (PDB: 1QU1) and EBOV GP2 (PDB: 2EBO), displaying VFPs (green) and TMDs (blue) on the same side of the molecule, and inserted into the expanded fusion pore wall. The CHR regions are shown in red. Trimeric and monomeric hairpins are shown for each structure.
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Further Reading

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Sánchez‐Eugenia, Rubén, and Nieva, José L(Aug 2019) Fusion Peptides: The Claws of the Viral Fusion Glycoproteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0028400]