IFITM3 Clusters on Virus Containing Endosomes and Lysosomes Early in the Influenza A Infection of Human Airway Epithelial Cells

Kummer, Susann; Avinoam, Ori; Kräusslich, Hans-Georg

IFITMs have been suggested to disrupt viral membrane fusion [50, 51] by altering cellular membrane properties such as fluidity and curvature [38, 52, 53] . It has also been discussed that IFITMs alter the lipid/protein composition of acidic intracellular compartments such as endosomes and lysosome [37, 54] . The lipid composition-based models of the IFITM3 function cannot easily explain the lack of antiviral effects on viruses like amphotropic MLV or arenaviruses [55, 56] , which also enter by endocytosis and endosomal fusion. In the case of IAV, it was recently reported that IFITM3 elevates the level of cholesterol on late endosomes and lysosomes, thereby restricting early IAV infection [57] . Other hypotheses suggest that IFITM3 directly interferes with the IAV fusion pore formation or redirects IAV containing endosomes to a non-productive pathway [41] . It was further demonstrated that the inhibition of the hemagglutinin-mediated membrane fusion required the amphipathic helix of IFITM3 [58] . Most findings about the antiviral action and localization of IFITM3 are based on studies using expression plasmids creating an over-expression and/or focusing at the late stages of infection (>12 h post-infection) [37, 38, 51] . Due to the enormous protein load in the overexpression situation, it is far more difficult to determine the subtle differences in the localization of individual molecules, and thus it is possible that these changes may be overlooked. In addition, proteins whose expression is enhanced artificially are more prone to be degraded via the lysosomal pathway. In this regard, it is difficult to judge whether this is a natural re-localization as part of the antiviral defense or just a degradation via lysosomes, particularly in the late stages of infection. We therefore investigated the role of endogenous IFITM3 in the viral uptake at an early stage.