Fig. 4. Cdk1 phosphorylation increases APC activity and Cdc20 binding. (A) Xenopus interphase APC was phosphorylated in vitro by Cdk1 and/or Plk1 and incubated with recombinant Cdc20. Binding of Cdc20 to the APC was monitored by immunoblotting. As a control, beads coupled to non-specific antibodies were treated in the same way (lane 5) and one sample was incubated with buffer instead of Cdc20 (lane 6). (B) Interphase APC purified and treated as in (A) was analysed for the ability to ubiquitinate ¹²⁵I-cyclin B. (C) The amount of ¹²⁵I-cyclin B conjugated to ubiquitin is shown as the percentage of the total amount of ¹²⁵I-cyclin B per reaction.