Fig. 7.

(A) Immunoprecipitation of MEF cell lysate with anti-Pa28α, anti-Pa28γ, anti-Pa200 or anti-β1 antibodies causes co-immunoprecipitation of both 20S and 26S proteasome subunits, however while anti-β1 and anti-Pa200 pull out a comparable amount of S4, anti-Pa28α and anti-Pa28γ antibodies are relatively poor at co- immunoprecipitation of S4. Immunoprecipitation protocols were as reported in the legends to Figs. 3 and 4. Non-specific binding was measured through immunoprecipitation of anti-porin. (B) Depletion of S4 through immunoprecipitation causes a small loss of Pa28α and Pa28γ but a very large loss of Pa200. MEF cell lysates were subjected to three cycles of immunoprecipitation with either anti-S4 or anti-porin. Non-immunoprecipitated supernatants were then taken and run on Western blots.