GYPC and infection: DF-1 cells were chosen over A549 cells for the subsequent imaging experiments for the following reasons: (1) low permissiveness to LASV fusion/infection in the absence of ectopically expressed hLAMP1; (2) consistent effects of hLAMP1 expression on fusion and infection across different virus platforms (LASVpp, LASV-VLP and LCMV-LASV GPC) (Figs 2 and 3 and S5 Fig); and (3) tolerance to prolonged exposure to low pH, which is essential for the 1 hour-long forced fusion imaging experiments described below.