The E4orf6/E1B55K complex cooperates with E1A in inducing E2F. (A) H1299 cells were transfected with plasmid DNAs expressing E4orf6, E1B55K, and HA-13S E1A proteins as indicated, and at 48 h, whole-cell extracts were immunoblotted as indicated using the appropriate antibodies. The numbers listed at the top for HA-E1A indicate the amount of plasmid DNA used in micrograms. (B and C) H1299 cells were transfected with plasmid DNAs expressing E4orf6, E1B55K, E2F1, and 13S E1A (B) or 12S E1A (C) proteins as indicated or infected with AdE4orf6 and AdE1B55K at an MOI of 35 each (last lane). At 24 h, immunoprecipitates (IP) obtained using E2F1 antibodies as well as whole-cell extracts were immunoblotted as indicated using the appropriate antibodies.