Cell culture.

HEK293T cells were maintained in Dulbecco modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS; Gibco), 100 U/ml penicillin, and 100 μg/ml streptomycin at 37°C in an incubator with 5% CO₂. The 293T-NoDice cell line was kindly provided by Bryan R. Cullen (Durham, NC). Drosophila S2 or Aag2 cells were cultured in Schneider insect medium with 10% FBS at 27°C. Before transfection with FuGene HD reagent (Roche, Basel, Switzerland), the medium was changed to DMEM or Schneider insect medium containing 2% FBS without any antibiotic. Cells were harvested at 48 hpt.