Paramyxovirus Glycoproteins and the Membrane Fusion Process

Abstract

The family Paramyxoviridae includes many viruses that significantly affect human and animal health. An essential step in the paramyxovirus life cycle is viral entry into host cells, mediated by virus-cell membrane fusion. Upon viral entry, infection results in expression of the paramyxoviral glycoproteins on the infected cell surface. This can lead to cell-cell fusion (syncytia formation), often linked to pathogenesis. Thus membrane fusion is essential for both viral entry and cell-cell fusion and an attractive target for therapeutic development. While there are important differences between viral-cell and cell-cell membrane fusion, many aspects are conserved. The paramyxoviruses generally utilize two envelope glycoproteins to orchestrate membrane fusion. Here, we discuss the roles of these glycoproteins in distinct steps of the membrane fusion process. These findings can offer insights into evolutionary relationships among Paramyxoviridae genera and offer future targets for prophylactic and therapeutic development.

Keywords: F-triggering; Hemifusion; Hendra; Measles; Mumps; NDV; Nipah; Paramyxoviridae; Paramyxovirus; RSV; association model; attachment; attachment glycoprotein; dissociation model; fusion; fusion cascade; fusion glycoprotein; fusion model; fusion pore formation; hMPV; hexamer of trimers; membrane fusion; postfusion; prefusion; prehairpin intermediate; syncytia; viral entry; viral receptors.

Fig. 1. Paramyxoviridae polymerase protein phylogeny tree

Polymerase/Large protein sequences of selected viruses were acquired from the NCBI Protein Database and Virus Pathogen Resource (ViPR). L protein sequences were aligned using the COBALT Multiple Alignment Tool-NCBI. Aligned sequences were then used to generate a phylogenic tree using COBALT NCBI TreeView1.8. The generated tree was visualized and modified using the FigTree Program. Abbreviations: APIV-2, avian parainfluenza virus 2; ASPV, Atlantic salmon paramyxovirus; BeV, Beilong virus; CDV, canine distemper virus; CeV, Cedar Virus; FDLV, Fer-de-Lance virus; GhV, Ghana virus; HeV, Hendra virus; hMPV, Human metapneumovirus; hPIV1, Human parainfluenza virus 1; hPIV2, Human parainfluenza virus 2; hPIV3, Human parainfluenza virus 3; JPV, J Paramyxovirus; PIV5, Parainfluenza virus 5; RSV, respiratory syncytial virus; MeV, Measles virus; MuV, Mumps virus; NDV, Newcastle disease virus; NiV, Nipah Virus; PPRV, Peste-des-petits ruminants virus; TaV, Tailam Virus. Paramyxoviridae family (black); sub-families (red); genus (blue); unclassified (yellow).

Fig. 2. Paramyxovirus attachment and fusion glycoproteins

A) Diagram depicting the attachment glycoprotein. B) Diagram depicting the precursor of the paramyxovirus fusion glycoprotein (F₀, top) and the cleaved, biologically active and disulfide linked paramyxovirus protein (F₁-F₂, bottom). The fusion peptide (FP), heptad repeats HR1, HR2, and HR3, transmembrane (TM); and cytoplasmic tail (CT) domains are shown, and the N- and C-termini are indicated.

Fig. 3. Receptor binding with attachment glycoproteins occurs at the head domain

The attachment protein binds the cellular receptor within a binding pocket(s) in the globular head. As illustrated, the glycoprotein monomer heads have separate β-blades distinguished by different colors for structures of the PIV5 HN (A) or NiV G (B). In both of these ribbon models sialic acid (A; orange cluster) or ephrinB2 (B; colored grey) binds the attachment protein towards the center of the β-propeller. This figure was adapted from (123). The PDB code for the NiV G-ephrin B2 complex is 2VSM (52).

Fig. 4. The pre- and post-fusion conformations of paramyxovirus F glycoproteins

Substantial structural rearrangements occur in the progression from pre-fusion to post-fusion F glycoprotein conformations. Several domains are distinguished by color including the large HR1-containing region (magenta) and the HR2 domain (blue). In the pre-fusion structure from PIV5 (A), a trimeric stabilization domain was added to support crystallization (grey). The six-helix bundle conformation is apparent in the hPIV3 structure (B). This figure was adapted from (111). The PDB codes for A and B are 2B9B and 1ZTM, respectively (111, 119).

Fig. 5. Five models of F activation (A–E)

Attachment glycoprotein tetramers are colored in purple (heads) and green (stalks, transmembrane, and cytoplasmic tail domains). F is highlighted in red (heads) and blue (stalks, transmembrane, and cytoplasmic tail domains). In the bi-dentate model, the conformational changes of the head are shown as slightly distinct shapes. F-triggering regions in the stalk are shown as black lines (B–D).

Fig. 6. Model of the late steps in paramyxovirus membrane fusion

Inner and outer membrane leaflets of viral and cellular membranes are shown as green and brown, respectively. The attachment protein is shown with a green stalk and purple head, while yellow depicts its bound receptor. For F, the cytoplasmic tail (CT), transmembrane (TM) domain, and HR2 are shown in blue. The head region through HR1 is shown in red and the fusion peptide in orange. A) The attachment glycoprotein binds its receptor and triggers prefusion F. B) To reach the prehairpin intermediate (PHI), HR2 melts and forms an open stalk. HR1 extends and projects the fusion peptide into the target membrane. (C) The head continues to refold, bringing HR1 and HR2 into close proximity, and pulls the membranes in. (D) HR1 and HR2 “zipper” together and the outer membranes fuse to form the hemifusion intermediate. (E) Inner membranes fuse as zippering continues through the transmembrane domain, forming the 6-helix bundle. F’s CT is now exposed to intracellular proteins and the fusion pore expands. The actin cytoskeleton (shown in light green) is likely involved in fusion pore expansion.