The N-terminus of Cul1 binds alpha subunits of the 20S proteasome. 293T cells were transfected with empty vector (lanes 1 and 7) or FLAG-tagged versions of full-length Cul1 aa 1-776 (lanes 2 and 8), Cul1 aa 1-300 (lanes 3 and 9), Cul1 aa 1-452 (lanes 4 and 10), Cul1 aa 1-645 (lanes 5 and 11) or Cul1 aa 324-776 (lanes 6 and 12). Extracts were immunoprecipitated with a monoclonal antibody to the FLAG epitope (lanes 1–6). Lanes 7–12 contain whole cell lysates from transfected cells. Immunoprecipitates and whole cell lysates were analyzed by immunoblotting with antibodies to the indicated proteins.