Endogenous Tax is ubiquitylated. (A) HuT-102 or Jurkat cells were either directly lysed in Laemli buffer or processed for immunoprecipitation with anti-Tax mab and blotting with anti-Tax sera. Lysates were also blotted with anti-GAPDH antibody. (B) C8166 cells were cotransfected with the NF-κB-LacZ (▪) or the HTLV-LTR-LacZ (□) reporter plasmids, and increasing doses of either Tax plasmid. The amount of transfected DNA was maintained at 3 μg using the PSG5 plasmid. Results correspond to β-gal activity after subtraction of the signal obtained in absence of LacZ plasmid and normalization to the amount of total proteins. Activity of the wild-type Tax protein was set to 100%.