Fission yeast Mes1p ensures the onset of meiosis II by blocking degradation of cyclin Cdc13p.

Meiosis is a special form of nuclear division to generate eggs, sperm and spores in eukaryotes. Meiosis consists of the first (MI) and the second (MII) meiotic divisions, which occur consecutively. MI is reductional, in which homologous chromosomes derived from parents segregate. MI is supported by an elaborate mechanism involving meiosis-specific cohesin and its protector. MII is equational, in which replicated sister-chromatids separate as in mitosis. MII is generally considered to mimic mitosis in mechanism. However, fission yeast Mes1p is essential for MII but dispensable for mitosis. The mes1-B44 mutant arrests before MII. Transcription of mes1 is low in vegetative cells and boosted in a narrow window between late MI and late MII. The mes1 mRNA undergoes meiosis-specific splicing. Here we show that Mes1p is a factor that suppresses the degradation of cyclin Cdc13p at anaphase I. Mes1p binds to Slp1p, an activator of APC/C (anaphase promoting complex/cyclosome), and counteracts its function to engage Cdc13p in proteolysis. Inhibition of APC/C-dependent degradation of Cdc13p by Mes1p was reproduced in a Xenopus egg extract. We therefore propose that Mes1p has a key function in saving a sufficient level of MPF (M-phase-promoting factor) activity required for the execution of MII.