Yeast cell division ‘pacemaker’ found inside nucleus, not outside as once believed

Researchers at the Francis Crick Institute have shown that the “pacemaker” controlling yeast cell division lies inside the nucleus rather than outside it, as previously thought. Having the pacemaker in the same compartment as the cell’s DNA helps keep the genome stable.

Cyclin-dependent kinase (CDK) is the master regulator of the cell cycle—the process by which a cell copies its contents and then divides to form two daughter cells. CDK is only active when bound to a second protein, cyclin, and it is this cyclin-CDK duo that sets in motion a cascade of signals instructing the cell to divide.

When and where CDK is active is tightly regulated. The complex can be thought of as the pacemaker of cell division, because if it ‘beats’ at the wrong time, cell division goes catastrophically wrong.

Researchers have generally thought CDK was first activated in the structure that marshals components of the cell division machinery. This structure, the centrosome, sits in the cytoplasm, the part of the cell outside the nucleus. However, in research published today in Nature, Nitin Kapadia, a postdoctoral researcher in the Cell Cycle Laboratory at the Crick, has overturned this long-held view.

Setting the pace of cell division

To do this, Nitin developed sensors that allowed him to look inside single live yeast cells, and to monitor CDK activity simultaneously in both the nucleus and the cytoplasm. He saw that the sensor in the nucleus reported a peak in activity before the sensor in the cytoplasm, meaning that the first place CDK was activated was the nucleus.

Next, Nitin fluorescently tagged cyclin molecules to map their movements once bound to CDK. The amount of cyclin in the nucleus dropped at the same time as the amount in the cytoplasm increased, suggesting that some active cyclin-CDK complexes were being exported from the nucleus to drive the next steps of the signaling cascade.

Adding the CDK activity sensors to this experiment showed that nuclear activation occurred before cyclin-CDK was sent to the cytoplasm, where it initiated CDK activity immediately. Just a small amount of cyclin was needed to activate CDK in the cytoplasm.

Maintaining mitosis and amplifying the signal

Mitosis, the process of duplicating the genome to give one copy to each daughter cell, is central to accurate cell division. Nitin’s next question was to ask how the nucleus stayed in a mitotic state even though some of its cyclin-CDK had been exported.

He mapped CDK activity in response to different amounts of cyclin in the nucleus and cytoplasm, and found that lots of cyclin needed to accumulate in the nucleus before CDK is activated. However, once CDK was activated, the nucleus could tolerate decreases in cyclin without slipping out of mitosis. In contrast, a much smaller amount of cyclin could activate CDK in the cytoplasm.

This is likely because the higher CDK activation threshold in the nucleus couples the cell division process to the mechanism for monitoring DNA replication and damage, thereby preventing mitosis from happening when the DNA is not “ready.”

To determine whether nuclear activation of CDK was sufficient to drive a cell to divide, Nitin blocked cyclin from leaving the nucleus and going to the centrosome, and found that this stopped the cytoplasm from entering mitosis even if the nucleus was in a mitotic state. This suggests that some cyclin-CDK needs to reach the centrosome, which relays the signal to the rest of the cell.

Nitin said, “We’ve shown, inside live cells, that the nucleus is the pacemaker for cell division, allowing DNA replication to be precisely coordinated with division. Now that we know where the process begins, we can take a closer look at what’s happening in the nucleus and whether DNA also plays a role in kicking off the entry to mitosis. And hopefully we can begin to unravel whether this process also happens in other animals and humans.”

Paul Nurse, Director of the Crick and head of the Cell Cycle Laboratory, said, “There’s conflicting evidence about where mitosis and cell division starts, and it’s tricky to observe this process in human cells as there’s more complexity and conflicting factors to consider. Using a model system like yeast has allowed us to unpick this process, observing signals and changes in real time, and understand how the cell coordinates mitosis across the different parts of the cell.”

In 1975, Nurse published his first paper in Nature exploring how fission yeast controlled the onset of mitosis. Fifty years on, almost to the month, his lab has published another paper in the same journal, still exploring how fission yeast controls onset of mitosis.