Multi-signal regulation of the GSK-3β homolog Rim11 governs meiosis entry in yeast

Starvation in diploid budding yeast cells triggers a cell-fate program culminating in meiosis and spore formation. Transcription activation of early meiotic genes (EMGs) hinges on the transcription activator Ime1, its DNA-binding partner Ume6, and GSK-3 kinase Rim11. Phosphorylation of Ume6 by Rim11 is key for EMG activation. We report that Rim11 functions as the central signal integrator for controlling Ume6 phosphorylation and EMG transcription. In nutrient-rich conditions, PKA suppresses Rim11 levels, while TORC1 retains Rim11 in the cytoplasm. Inhibiting PKA and TORC1 induces Rim11 expression and nuclear localization. Remarkably, nuclear Rim11 is required, but not sufficient, for Rim11-dependent Ume6 phosphorylation. Additionally, Ime1 is an essential anchor protein for phosphorylating Ume6. Subsequently, Ume6-Ime1 coactivator complexes form, which drive EMG transcription. Our results demonstrate how varied signalling inputs (PKA/TORC1/Ime1) converge through Rim11 to regulate EMG expression and meiosis initiation. We posit that the signalling-regulatory network elucidated here generates robustness in cell-fate control.