Procaspase-9 is autoprocessed through a CARD-displacement mechanism. (A) T7-tagged C9-p35/p12 and ProC9-TM were incubated with T7-tagged ProC3* in the presence of Apaf-1 and Cc/dATP. Reactions were carried out, as described in the experimental procedures, resolved by SDS–PAGE, and immunoblotted using an anti-T7 antibody. As C9-p35/p12 and ProC3* co-migrated (top panel), the membranes in these experiments were also stripped and re-blotted for caspase-9 (anti-C9) and caspase-3 (anti-C3) using caspase-specific antibodies. (B) The initial velocities (V0) of apoptosome-bound C9-p35/p12, ProC9-TM, and C9-p35/p12-R56A on the substrate ProC3 were assessed by measuring caspase-3 DEVDase activity, as described in the Methods section. Each bar represents the mean of three separate experiments±s.e.m. (C) Recombinant wild-type C9-p35/p12 or C9-p35/p12-R56A were incubated with Apaf-1 and Cc/dATP. The samples were subsequently fractionated using Superose-6 gel-filtration chromatography, resolved by SDS–PAGE, and immunoblotted using Apaf-1 and caspase-9-specific antibodies (). (D) T7-tagged C9-p35/p12 and ProC9-TM were incubated with Flag-tagged ProC9*-R56A in the presence of Apaf-1 and Cc/dATP. Reactions were carried out, as described in the Methods section, and immunoblotted for the processing of Flag–ProC9*-R56A using an anti-Flag antibody. In lanes 7 and 14, active caspase-3 (1 μM) was added as a positive control to confirm that ProC9*-R56A could be cleaved within its activation loop. (E, F) Apoptosome-bound T7-tagged C9-p35/p12 or ProC9-TM were incubated with T7-tagged ProC9*-R56A and either T7-tagged ProC3* or ProC3. Samples were then analysed by SDS–PAGE and immunoblotted using an anti-T7 antibody or assayed for active caspase-3 DEVDase activity. The membranes in panel (E) were also stripped and re-blotted for caspase-9 (using anti-C9) and caspase-3 (using anti-C3) using caspase-specific antibodies. Each bar represents the mean of three separate experiments±s.e.m.