C/EBPϵ and E2F1 interact in vivo and in vitro. (A) COS-1 cells were transfected with either an empty vector or a C/EBPϵ expression vector. Lysates were immunoprecipitated with an E2F1 antibody and analyzed by immunoblotting with C/EBPϵ antibody. (B) Protein lysates from NB4 and U937 cells treated with ATRA as indicated were immunoprecipitated with an E2F1 antibody followed by Western blot with a C/EBPϵ antibody. The blots were stripped and rehybridized with an E2F1 antibody. (C) NB4 and U937 lysates were incubated with either various GST-C/EBPϵ fusion proteins or GST alone as indicated. Bound E2F1 was detected by Western blot analysis. Protein lysate from COS-1 cells transfected with an E2F1 expression vector was used as control for E2F1 expression. (D) In vitro–translated E2F1 (input) was incubated with either GST or GST-C/EBPϵ, followed by Western blot analysis with E2F1 antibody.