IFITM3 Restricts Influenza A Virus Entry by Blocking the Formation of Fusion Pores following Virus-Endosome Hemifusion

• Type of Material: Article
• Authors: Tanay M. Desai, Emory University; Mariana Marin, Emory University; Christopher R. Chin, University of Massachusetts; George Savidis, University of Massachusetts; Abraham L. Brass, University of Massachusetts; Gregory Melikian, Emory University
• Language: English
• Date: 2014-04
• Publisher: Public Library of Science
• Publication Version: Final Published Version
• Copyright Statement: © 2014 Desai et al.
• Final Published Version (URL): http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1004048
• Title of Journal or Parent Work: PLoS Pathogens
• ISSN: 1553-7366
• Volume: 10
• Issue: 4
• Start Page: e1004048
• End Page: e1004048
• Grant/Funding Information: This work was supported by the NIH R01 GM054787 grant (to GBM) and 1R01AI091786 grant (to ALB).; ALB is grateful to the Charles H. Hood Foundation, the Burroughs Wellcome Fund, the Phillip T. and Susan M. Ragon Foundation, the Bill and Melinda Gates Foundation and the Harvard and UMMS Centers for AIDS Research for their generous support.
• Research Categories: Health Sciences, Pathology; Biology, Virology

Abstract

nterferon-induced transmembrane proteins (IFITMs) inhibit infection of diverse enveloped viruses, including the influenza A virus (IAV) which is thought to enter from late endosomes. Recent evidence suggests that IFITMs block virus hemifusion (lipid mixing in the absence of viral content release) by altering the properties of cell membranes. Consistent with this mechanism, excess cholesterol in late endosomes of IFITM-expressing cells has been reported to inhibit IAV entry. Here, we examined IAV restriction by IFITM3 protein using direct virus-cell fusion assay and single virus imaging in live cells. IFITM3 over-expression did not inhibit lipid mixing, but abrogated the release of viral content into the cytoplasm. Although late endosomes of IFITM3-expressing cells accumulated cholesterol, other interventions leading to aberrantly high levels of this lipid did not inhibit virus fusion. These results imply that excess cholesterol in late endosomes is not the mechanism by which IFITM3 inhibits the transition from hemifusion to full fusion. The IFITM3's ability to block fusion pore formation at a post-hemifusion stage shows that this protein stabilizes the cytoplasmic leaflet of endosomal membranes without adversely affecting the lumenal leaflet. We propose that IFITM3 interferes with pore formation either directly, through partitioning into the cytoplasmic leaflet of a hemifusion intermediate, or indirectly, by modulating the lipid/protein composition of this leaflet. Alternatively, IFITM3 may redirect IAV fusion to a non-productive pathway, perhaps by promoting fusion with intralumenal vesicles within multivesicular bodies/late endosomes.