Human parainfluenza virus fusion complex glycoproteins imaged in action on authentic viral surfaces

Abstract

Infection by human parainfluenza viruses (HPIVs) causes widespread lower respiratory diseases, including croup, bronchiolitis, and pneumonia, and there are no vaccines or effective treatments for these viruses. HPIV3 is a member of the Respirovirus species of the Paramyxoviridae family. These viruses are pleomorphic, enveloped viruses with genomes composed of single-stranded negative-sense RNA. During viral entry, the first step of infection, the viral fusion complex, comprised of the receptor-binding glycoprotein hemagglutininneuraminidase (HN) and the fusion glycoprotein (F), mediates fusion upon receptor binding. The HPIV3 transmembrane protein HN, like the receptor-binding proteins of other related viruses that enter host cells using membrane fusion, binds to a receptor molecule on the host cell plasma membrane, which triggers the F glycoprotein to undergo major conformational rearrangements, promoting viral entry. Subsequent fusion of the viral and host membranes allows delivery of the viral genetic material into the host cell. The intermediate states in viral entry are transient and thermodynamically unstable, making it impossible to understand these transitions using standard methods, yet understanding these transition states is important for expanding our knowledge of the viral entry process. In this study, we use cryoelectron tomography (cryo-ET) to dissect the stepwise process by which the receptor-binding protein triggers F-mediated fusion, when forming a complex with receptor-bearing membranes. Using an on-grid antibody capture method that facilitates examination of fresh, biologically active strains of virus directly from supernatant fluids and a series of biological tools that permit the capture of intermediate states in the fusion process, we visualize the series of events that occur when a pristine, authentic viral particle interacts with target receptors and proceeds from the viral entry steps of receptor engagement to membrane fusion.

Host cell membrane capture by the SARS CoV-2 spike protein fusion intermediate

“…However, the structure of the fusion intermediate, the fusogenic form of the spike protein that facilitates entry, has not been determined for CoV-2 or any coronavirus. Among class I fusion proteins, the fusion intermediates of HIV-1, parainfluenza and influenza were only recently visualized [20][21][22] .…”

“…Flexibility may also help multiple FIs simultaneously capture target membrane at different distances. Indeed, influenza, parainfluenza and HIV-1 appear to use several FIs for virus-host attachment 20,22,43 . Second, by allowing the ~25 nm long FI to bend significantly, the extreme FI flexibility may facilitate the prefusion-to-FI transition even in the confined circumstances of a nearby host cell membrane (Fig.…”

“…Such peptides might also stabilize FI for visualization of this critical intermediate. Indeed, the fusion intermediates of HIV gp41 and parainfluenza F were recently visualized by stabilizing the intermediate with HR2-derived fusion inhibitors 20,22 .…”

“…The FI has proved experimentally elusive, likely because FIs have sec-min lifetimes 7,19 , far shorter than the pre-or postfusion lifetimes, and membrane fusion is thought a fast step in the virus life cycle 20 . Only very recently were the first class I virus FI structures visualized with cryo-EM, the HIV-1 gp41 intermediate, the parainfluenza F intermediate and the influenza HA2 intermediate [20][21][22] .…”

Viral Membrane Fusion Proteins and RNA Sorting Mechanisms for the Molecular Delivery by Exosomes

“…Ciliated epithelial cells [101][102][103] Alteration in sialic acid processing that leads to an upregulation of sialylated glycans and its receptors in many tumors [104] Hepatitis B virus Sodium Taurocholate Cotransporting Polypeptide (NTCP) Hepatocytes [105] Use in target therapy for liver fibrosis and cancer [106] Hepatitis C virus CD81 tetraspanin, scavenger receptor class B type I (SR-B1)…”