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.

Fusion of enveloped viruses with the host cell membrane is a key step leading to infection. Viral fusion is initiated upon interactions between virus surface glycoproteins and cellular receptor(s) and/or upon the reduction in pH that follows endocytosis (reviewed in [1] [2] [3] ). The extensive conformational changes that ensue in the viral glycoproteins promote fusion between viral and cellular membranes [4] [5] [6] . There is strong evidence that viral fusion-and membrane fusion in general-proceeds through a hemifusion intermediate defined as a merger of two contacting membrane leaflets without additional merger of distal leaflets that results in the formation of a fusion pore [6] [7] [8] . Accordingly, hemifusion is manifested as lipid mixing between viral and host membranes without viral content release, while full membrane fusion entails mixing of distinct aqueous contents delimited by the two membranes [4] [5] [6] . It has been demonstrated that sub-optimal conditions for membrane fusion including low density of viral glycoproteins, reduced temperature, and-where applicable-insufficiently acidic pH, favor dead-end hemifusion that does not progress to full fusion [9] [10] [11] [12] . Thus, the progression to full viral fusion that culminates in the release of nucleocapsid into the cytoplasm is largely dependent on local conditions. The viral envelope glycoproteins responsible for mediating membrane fusion are targets for neutralizing antibodies and virus entry inhibitors. In addition, new innate restriction factors inhibiting virus fusion have been discovered in recent years [13] [14] [15] . Among these factors is the family of small interferon-induced transmembrane proteins (IFITMs) that exhibits broadrange of antiviral activity [15] [16] [17] . This family includes IFITM1, which localizes predominantly at the plasma membrane, as well as IFITM2 and IFITM3, which contain an endocytic signal in their cytoplasmic N-terminal domain and thus localize to late endosomal and lysosomal membranes [18] [19] [20] [21] . IFITMs effectively block entry of many unrelated enveloped viruses, including orthomyxoviruses (influenza A virus, IAV), paramyxoviruses (Respiratory Syncytial Virus, RSV), flaviviruses (Dengue, West Nile), filoviruses (Marburg, Ebola), and coronaviruses (SARS) [15-17, 20, 22-25] . IFITM3 alone is responsible for the bulk of antiviral effects of interferon in cell culture [15] . Importantly, mice lacking the ifitm3 gene more readily succumb to IAV and RSV infection than control mice [26, 27] . There are, however, viruses that are resistant to IFITM-mediated restriction. Murine Leukemia Virus (MLV), Old and New World arenaviruses (Lassa Virus and Junin Virus, respectively), as well as several enveloped DNA viruses, are not affected by IFITMs [15, 28, 29] .