CHOP heterodimerizes with C/EBPϵ. (A) GST pull-down assay was performed by incubating either GST or GST-CHOP fusion protein with C/EBPϵ in vitro-translated in the presence of ³⁵S-methionine. Autoradiograph of the SDS-PAGE gel shows that GST-CHOP binds to C/EBPϵ, but GST does not. (B) Protein lysates for immunoprecipitation experiments were made from COS-1 cells transfected with an empty vector (EV) or with C/EBPϵ and CHOP expression vectors and from NB4 cells treated with ATRA (10^-8 M, 48 hours). Lysates were immunoprecipitated with preimmune serum or anti-C/EBPϵ antibody, followed by Western blot analysis with CHOP antibody. As a control for CHOP migration, one tenth of the COS-1 lysate, which was used for immunoprecipitation, was resolved by SDS-PAGE (input).