The Yeast Polo Kinase Cdc5 Regulates the Shape of the Mitotic Nucleus

Abstract

Summary Abnormal nuclear size and shape are hallmarks of aging and cancer [1, 2]. However, the mechanisms regulating nuclear morphology and nuclear envelope (NE) expansion are poorly understood. In metazoans, the NE disassembles prior to chromosome segregation and reassembles at the end of mitosis [3]. In budding yeast, the NE remains intact. The nucleus elongates as chromosomes segregate and then divides at the end of mitosis to form two daughter nuclei without NE disassembly. The budding yeast nucleus also undergoes remodeling during a mitotic arrest; the NE continues to expand despite the pause in chromosome segregation, forming a nuclear extension, or “flare”, that encompasses the nucleolus [4]. The distinct nucleolar localization of the mitotic flare indicates that the NE is compartmentalized and that there is a mechanism by which NE expansion is confined to the region adjacent to the nucleolus. Here we show that mitotic flare formation is dependent on the yeast polo kinase, Cdc5. This function of Cdc5 is independent of its known mitotic roles, including rDNA condensation. High-resolution imaging revealed that following Cdc5 inactivation, nuclei expand isometrically rather than forming a flare, indicating that Cdc5 is needed for NE compartmentalization. Even in an uninterrupted cell cycle, a small NE expansion occurs adjacent to the nucleolus prior to anaphase in a Cdc5-dependent manner. Our data provides the first evidence that polo kinase, a key regulator of mitosis [5], plays a role in regulating nuclear morphology and NE expansion.

High resolution microscopy reveals the nuclear shape of budding yeast during cell cycle and in various biological states

“…Previous studies have suggested a continuous increase of the nuclear volume during the cell cycle (Jorgensen et al, 2007). By contrast, we observed a discontinuous increase of the nuclear envelope surface; at mitotic onset (60 min), we detected a fast and significant nuclear envelope expansion, whereas the nuclear volume slightly increased from G1 to M. The measured timing of this rapid nuclear envelope surface increase is fully compatible with the activation timing of polo kinase Cdc5 described to be required for nuclear envelope expansion at mitotic onset (Walters et al, 2014). …”

Recent advances in understanding nuclear size and shape

“…131 In yeast, formation of NE flares adjacent to the nucleolus in response to excess membrane production or mitotic arrest is dependent on polo kinase Cdc5. 132,133 A question for future research is precisely how kinases with well-established roles in mitosis, including cyclin-dependent kinases, polo kinases, and PKC, contribute to the maintenance of proper nuclear morphology. Given the potential impact of the cell cycle on nuclear morphology, it is worth considering the interplay between size scaling of mitotic structures and the nucleus.…”

Ribosome profiling the cell cycle: lessons and challenges

“…Membrane rearrangements late in the cell cycle (e.g., for cytokinesis and those involving the nuclear membrane) are hallmarks of the eukaryotic cell cycle. In fact, acetyl-CoA carboxylase and fatty acid synthase mutants in yeast fail to appropriately increase the area of their nuclear membrane (Walters et al 2014). The nuclear membrane displays a massive reorganization during mitosis, and this phenomenon is observed across a wide range of mitotic strategies.…”