GST-pulldown assays. Assays were performed using GST and GST-UL44 and in vitro-translated UL54, PB1, or Ubc9 as previously described [29] , with modifications (Supplementary Material and Methods in Text S1). In vitro transcription-translation of proteins was performed from the appropriate plasmid by using the TNT T7 coupled transcription-translation system (Promega). The translation products were labeled with [ 35 S]methionine (Amersham Pharmacia Biotech).