page 23). They show that Plo1-mediated activation is essential—a metaphase arrest results when Plo1 can no longer bind avidly to the Cdc23 subunit of the APC.
The authors came to this conclusion after identifying mutants that were dependent on high Plo1 expression for their survival. One of the mutants made a Cdc23 protein that interacted poorly with Plo1, a deficit that May et al. suggest is overcome by excess Plo1. Following up on this clue, May et al. mapped the Plo1–Cdc23 interaction to the noncatalytic domain of Plo1 and the TPR domain of Cdc23.
The APC subunit or residues that act as the Plo1 target remain unknown. In tracking down this target, May et al. plan to continue their analysis of the APC at the level of individual subunits or residues rather than as a whole complex. ▪