RESUMEN
In addition to the degradation of cell-cycle proteins, short-lived, damaged, or unfolded proteins are constantly cleared from cells by the proteasome. During proliferation, the proteasome localizes to the nucleus and cytoplasm; however, the functional relevance of this compartmentalization remains unclear. Here, we show that folding stress increases 26S/30S proteasome activity, which correlates with the upregulation of Ump1, a chaperone involved in 20S assembly. Conversely, ump1 inactivation results in a drop of 20S and 26S/30S proteasomes. Limited 26S/30S proteasomes in ump1-deficient cells accumulate in the nucleus where they degrade mitotic substrates, allowing cells to proceed through mitosis; however, these cells present cytoplasmic aggregates and constitutive activation of the heat shock response. Thus, our data suggest that an increase in proteasome assembly induced by folding stress functions as an additional layer to proteasome regulation and highlight the importance of balanced proteasome compartmentalization to sustain cell proliferation while maintaining proper cytoplasmic proteostasis.
RESUMEN
BACKGROUND: Mitogen-activated protein kinases (MAPKs) preserve cell homeostasis by transducing physicochemical fluctuations of the environment into multiple adaptive responses. These responses involve transcriptional rewiring and the regulation of cell cycle transitions, among others. However, how stress conditions impinge mitotic progression is largely unknown. The mitotic checkpoint is a surveillance mechanism that inhibits mitotic exit in situations of defective chromosome capture, thus preventing the generation of aneuploidies. In this study, we investigate the role of MAPK Pmk1 in the regulation of mitotic exit upon stress. RESULTS: We show that Schizosaccharomyces pombe cells lacking Pmk1, the MAP kinase effector of the cell integrity pathway (CIP), are hypersensitive to microtubule damage and defective in maintaining a metaphase arrest. Epistasis analysis suggests that Pmk1 is involved in maintaining spindle assembly checkpoint (SAC) signaling, and its deletion is additive to the lack of core SAC components such as Mad2 and Mad3. Strikingly, pmk1Δ cells show up to twofold increased levels of the anaphase-promoting complex (APC/C) activator Cdc20Slp1 during unperturbed growth. We demonstrate that Pmk1 physically interacts with Cdc20Slp1 N-terminus through a canonical MAPK docking site. Most important, the Cdc20Slp1 pool is rapidly degraded in stressed cells undergoing mitosis through a mechanism that requires MAPK activity, Mad3, and the proteasome, thus resulting in a delayed mitotic exit. CONCLUSIONS: Our data reveal a novel function of MAPK in preventing mitotic exit and activation of cytokinesis in response to stress. The regulation of Cdc20Slp1 turnover by MAPK Pmk1 provides a key mechanism by which the timing of mitotic exit can be adjusted relative to environmental conditions.
