Interferon-induced transmembrane protein 3 blocks fusion of sensitive but not resistant viruses by partitioning into virus-carrying endosomes

Suddala, Krishna C.; Lee, Christine C.; Meraner, Paul; Marin, Mariana; Markosyan, Ruben M.; Desai, Tanay M.; Cohen, Fredric S.; Brass, Abraham L.; Melikyan, Gregory B.

The exact mechanism by which IFITM3 inhibits the transition from hemifusion to fusion is not clear. A large body of work demonstrates a critical role of lipid composition, and specifically of mechanical properties of lipid membranes, in protein-mediated membrane fusion (reviewed in [69] ). Bending energies of highly curved lipid intermediates that form and resolve during merger of lipid bilayers are key determinants of the fusion process (reviewed in [69] [70] [71] ). In addition, hemifusion and the formation of a fusion pore within a hemifusion diaphragm are associated with changes in areas of contacting and distal monolayers, respectively. Thus, viral fusion pore opening could be blocked by: (1) increased membrane bending modulus; (2) increased negative curvature of the cytoplasmic leaflet that disfavors the formation of a net positive curvature fusion pore [69, 72] ; (3) expansion of the hemifusion diaphragm to a size beyond that permissible for fusion pore formation [73] ; or (4) reduced "fluidity" (lateral diffusion) of the cytoplasmic leaflet, which can be caused by IFITM homo/hetero-oligomerization [21] . The latter effect is expected to disfavor the fusion pore opening due to inability to quickly remove excess lipid from the hemifusion site. IFITMs have been reported to alter membrane fluidity [21, 35] , and to increase the lipid order and confer positive spontaneous curvature [22, 33] . It is thus possible that individual effects of IFITMs on lipid membranes or their combination are responsible for the fusion block. Importantly, a recent study demonstrated that mutations in distinct regions of IFITM3 regulate its inhibitory vs enhancing activity against infection by different coronaviruses [74] . The ability to switch between inhibition and promotion of coronavirus fusion by introducing point mutations in IFITM3 further supports the proximity-based mechanism of virus restriction.