GST pull-down assay GST pull-down was performed as described previously (Tu et al., 1999) . In brief, GST±hnRNP A1 fusion proteins on glutathione beads (Pierce, Rockford, IL) were incubated with the in vitro translated, 35 S-labeled N protein in 0.3 ml of GST-binding buffer containing 0.1% NP-40 for 2 h at 4°C. The beads were washed ®ve times with the GST-binding buffer containing 0.3% NP-40. Proteins bound to beads were eluted by boiling in Laemmli buffer for 5 min and separated on a 10% polyacrylamide gel.