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.

Inhibition of IAVpp fusion after sustained co-trafficking with IFITM3-imTFP1-enriched endosomes may occur through a direct block of viral fusion by the restriction factor. Alternatively, IFITM3 may indirectly interfere with IAV fusion by altering the properties of endosomes, such as the luminal pH. The overall acidity of IFITM3+ endosomes was assessed by loading A549-IFITM3-imTFP1 cells with the acidic compartment marker, LysoTracker Red (S4 Fig) . Imaging of fixed cells shows strong cytoplasmic colocalization between IFITM3 + endosomes and LysoTracker Red positive compartments, with only a small fraction of peripheral IFITM3+ endosomes lacking a detectable LysoTracker Red signal (S4A Fig, Inset) . Analysis of multiple fields of view confirms that most IFITM3+ endosomes, with the exception of a small number of peripheral endosomes, accumulate the lysosomal marker (S4B Fig) . This finding is consistent with progressive acidification of early IFITM3-containing endosomes through a maturation process and thus supports the notion that virus-carrying late IFITM3 + endosomes are acidic.