Mes1 controls the meiosis I to meiosis II transition by distinctly regulating the anaphase-promoting complex/cyclosome coactivators Fzr1/Mfr1 and Slp1 in fission yeast.

Meiosis is a specialized form of cell division generating haploid gametes and is dependent upon protein ubiquitylation by the anaphase-promoting complex/cyclosome (APC/C). Accurate control of the APC/C during meiosis is important in all eukaryotic cells and is in part regulated by the association of coactivators and inhibitors. We previously showed that the fission yeast meiosis-specific protein Mes1 binds to a coactivator and inhibits APC/C; however, regulation of the Mes1-mediated APC/C inhibition remains elusive. Here we show how Mes1 distinctively regulates different forms of the APC/C. We study all the coactivators present in the yeast genome and find that only Slp1/Cdc20 is essential for meiosis I progression. However, Fzr1/Mfr1 is a critical target for Mes1 inhibition because fzr1Δ completely rescues the defect on the meiosis II entry in mes1Δ cells. Furthermore, cell-free studies suggest that Mes1 behaves as a pseudosubstrate for Fzr1/Mfr1 but works as a competitive substrate for Slp1. Intriguingly, mutations in the D-box or KEN-box of Mes1 increase its recognition as a substrate by Fzr1, but not by Slp1. Thus Mes1 interacts with two coactivators in a different way to control the activity of the APC/C required for the meiosis I/meiosis II transition.