Direct Visualization of Ebola Virus Fusion Triggering in the Endocytic Pathway

Spence, Jennifer S.; Krause, Tyler B.; Mittler, Eva; Jangra, Rohit K.; Chandran, Kartik

Our findings suggest that, following GP-NPC1 binding, cysteine cathepsins are dispensable for GP fusion triggering and lipid mixing but play a critical role in the formation and/or expansion of fusion pores that allow cytoplasmic release of viral genomes. We advance two mechanistic scenarios to account for our observations. First, the fusion trigger (comprising NPC1 and possibly an additional signal, e.g., acid pH, but not cysteine cathepsins) releases GP from its prefusion conformation, allowing formation of a membrane-bound prehairpin intermediate and driving hemifusion of viral and cellular membranes. A final cysteine cathepsin cleavage event then enables GP refolding to a 6HB, which mediates fusion pore formation and genome release. A similar two-step mechanism, with sequential roles for receptor binding and low pH, has been proposed for fusion by the avian sarcoma/leukosis retrovirus (61) . Alternatively, it is conceivable that proteolytic cleavage of NPC1-bound GP is indeed the fusion trigger but that GP fusion triggering and 6HB formation can nevertheless occur inefficiently in the absence of cysteine cathepsin activity. Rearrangement of a few GP trimers may permit lipid mixing but not energetically costly fusion pore formation and expansion.