Co-IP experiments identify procaspase-3, procaspase-8, procaspase-9, and procaspase-10 as PCNA partners. Co-IP experiments were performed using neutrophil cytosol. Unbound material (UB) and bound (B) immunoprecipitated proteins were analyzed by Western blot analysis. (A–D) In IP, PCNA pAb (IP PCNA) or empty beads (IP control) were used, whereas in Western blot analysis, PC10 anti-PCNA was used (to ascertain the presence of PCNA) together with one of the following: anti–procaspase-3 mAb (A), anti–procaspase-8 mAb (B), anti–procaspase-9 mAb (C), or anti–procaspase-10 mAb (D). (E) Co-IP experiments performed as in C using PCNA containing neutrophil cytosol incubated with 100 µM carboxyp21 or the mutated p21 peptide (used as a control). Both unbound material and bound immunoprecipitated proteins were analyzed by Western blot analysis using anti–procaspase-9 and anti-PCNA (PC10). A–E show data from representative experiments that were performed at least four times, yielding identical results. (F) Caspase-9 activity in PLB985 cells overexpressing wild-type or mutated PCNA compared with controls (CT). CD11b⁺–caspase-9⁺ cells were measured by flow cytometry before and after DMF-induced differentiation. Data are means ± SEM of four independent experiments (**, P < 0.01; Student’s t test).