Direct Visualization of Ebola Virus Fusion Triggering in the Endocytic Pathway

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

GP2 fusion loop mutations decouple lipid mixing and infection. To examine the effect of fusion loop mutation on lipid mixing, we tested two previously published GP2 mutants (Fig. 7) (54, 55) . Internalization and late endosomal delivery of VSV particles bearing these mutants did not differ significantly from VSV-EBOV GP⌬Muc (see Fig. S1C in the supplemental material), indicating that their defects are specific to aspects of fusion. The GP (F535R) mutant, characterized as having impaired membrane binding and insertion capability (54) , exhibited no defect in lipid mixing while being unable to support infection. Another fusion loop mutant, GP (L529A/I544A) (54), showed substantially de- creased lipid mixing. The L529A/I544A mutation is believed to prevent formation of the hydrophobic fusion loop tip at low pH, with L529 and I544 creating a scaffold along with F535 (54, 55) . While nonviable, the double mutant did effect low-level lipid mixing (Fig. 7) , suggesting that even severely disrupted GP fusion loops retain some capacity for membrane insertion in cells.