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 hemagglutinin-neuraminidase (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 cryo-electron 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.
Highlights
Human parainfluenza virus (HPIV) entry into human airway epithelial cells, as the initial step of infection, is mediated by fusion of viral and host cell membranes at the cell surface
F is only activated to mediate membrane fusion during entry when it is triggered by a signal from the separate receptor-binding molecule, HN–a mechanism first elucidated for HPIV3 and later extended to this entire group of human pathogenic viruses
These two distinct membrane glycoproteins work in synchrony as a molecular machine, exquisitely tuned to enter the right cells at the right time
Summary
Human parainfluenza virus (HPIV) entry into human airway epithelial cells, as the initial step of infection, is mediated by fusion of viral and host cell membranes at the cell surface. Virus-cell fusion for parainfluenza, as well as for most other enveloped RNA viruses of the Paramyxovirus family [2,3], results from the coordinated action of the two envelope glycoproteins that comprise the viral entry complex—the receptor binding protein (hemagglutinin neuraminidase (HN) for HPIV3) and a separate membrane fusion protein (F). This well-timed cooperation between two separate surface glycoproteins is different than the fusion machinery of influenza virus, wherein just the hemagglutinin protein contains both the receptor-binding and the fusion domains. While the exact receptor is unknown, the HPIV3 transmembrane protein HN binds preferentially to a α2,3-linked sialic acid-containing receptor [4,5] on the host cell plasma membrane, and the F protein, once activated by the receptor-binding protein after receptor engagement [6,7,8], mediates the fusion of viral and host membranes, in order to deliver the viral genetic material into the host cell
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
