Amphotericin B Increases Influenza A Virus Infection by Preventing IFITM3-Mediated Restriction

Lin, Tsai-Yu; Chin, Christopher R.; Everitt, Aaron R.; Clare, Simon; Perreira, Jill M.; Savidis, George; Aker, Aaron M.; John, Sinu P.; Sarlah, David; Carreira, Erick M.; Elledge, Stephen J.; Kellam, Paul; Brass, Abraham L.

We also used AmphoB as a molecular probe to investigate the mechanism of IFITM3-mediated restriction and determined that while IFITM3 increases both endosomal acidity and salinity, these events are unlikely to play major roles in restriction. Instead, these properties may all arise from the same mechanism, specifically that IFITMs alter the physical properties of membranes. In support of this idea, we determined that IFITM1 decreases membrane fluidity and that this event is associated with inhibition of IAV-induced cell-to-cell fusion; these data are consistent with those recently generated using orthologous approaches . Collectively, these findings have several implications. First, by decreasing membrane fluidity, IFITMs may impede the sequential assembly of viral-host receptor complexes, thereby blocking viral entry. Viruses that rely on multiple receptors for entry would be susceptible to this effect (e.g., HCV and HIV-1). Consistent with this, IFITM1 inhibits both HCV and HIV-1 (Schoggins et al., 2011; Wilkins et al., 2013) , and a separate study has reported that the restricted mobility of one of the two HIV-1 host receptors, CD4, can inhibit viral fusion (Rawat et al., 2008) . Similarly, recent work has elegantly shown that the clustering of multiple viral envelopes is needed for IAV entry, suggesting that this activity may be hindered by IFITMs residing in the host-derived viral and/or host membranes (Ivanovic et al., 2013) . Second, IFITM homo-and heteromers within the membrane may stabilize membrane complexes by inhibiting dissociation, i.e., stabilizing the vacuolar ATPase may increase endosomal acidity and potentially salinity (Wee et al., 2012) . Third, the insertions of each IFITM's IM domains into the cytosolic-facing leaflet of the membrane may produce a positive curvature in the bilayer as well described for other membrane-associated proteins (Callan-Jones and Bassereau, 2012; Nikolaus and Herrmann, 2012; Voeltz et al., 2006; Yount et al., 2012) . Therefore, by producing a concave and rigid membrane, the IFITM proteins may create a barrier for the viral fusion machinery. Notably, using such a general strategy to protect oneself from many viruses would produce multiple fitness advantages.