ECSIT induces the processing of MEKK-1, and ECSITΔ inhibits this processing. (A) 293 cells were transfected with MEKK-1 and vector DNA (lane 3), increasing amounts of ECSITΔ (lanes 1,2), or increasing amounts of ECSIT wild type (lanes 4–6). Twenty-four hours after transfection, lysates were prepared and analyzed by SDS-PAGE. Gel was immunoblotted (IB) with an antibody to the carboxyl terminus of MEKK-1. The full-length and processed forms of MEKK-1 are indicated by asterisks (*). (B) Differential inhibition of NF-κB activity induced by MEKK-1 and MEKK-1Δ. 293 κB–LUC cells were transfected with either MEKK-1 wild type (full length) or MEKK-1Δ in the presence of increasing amounts of ECSITΔ or vector control as indicated. Twenty-four hours after transfection, lysates were prepared and assayed for luciferase activity. (C) Differential inhibition of AP-1 activity induced by MEKK-1 and MEKK-1Δ. 293 cells were transfected with 0.4 μg of AP-1–LUC reporter gene and MEKK-1 wild type (wt) or MEKK-1Δ, or vector alone as indicated. Twenty-four hours after transfection, lysates were prepared and assayed for luciferase activity.