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.

Single particle tracking revealed that IAV fusion was inhibited in compartments that accumulated substantial amounts of fluorescent IFITM3. Due to the relatively high and variable background fluorescence in cells expressing fluorescent IFITM3, it is difficult to quantitatively assess whether there is a threshold density of this protein below which viruses are not restricted. In other words, it is unclear whether inhibition of IAV fusion by IFITM3 occurs through an all-or-none mechanism or there is an inhibition "gradient" whereby the probability of fusion is inversely proportional to the IFITM3 signal. Future studies using improved fluorescence labeling techniques and controlled IFITM3 expression levels will help distinguish between these modes of action. The existence of distinct domains within the highly dynamic endosomal membranes (e.g., [62] [63] [64] [65] [66] ) adds an additional layer of complexity when interpreting the IFITM3 restriction results. It is possible that the extremely rare single LASVpp fusion events that appear to colocalize with IFITM3 occur with IFITM3-free domains within the limiting membrane of an endosome.