What inspires yeast cells to divide?

Rockefeller researchers discover unexpected trigger

Often in science a novel set of experiments comes along that forces researchers to abandon old models in exchange for new ones that better fit their observations. This is the case in a new Nature report by Rockefeller University researchers, which finds that past models of cellular division in the simple yeast organism were focused on the wrong protein.

Until now, scientists thought that yeast cells began dividing into two separate cells upon the destruction of a “cyclin” protein called Clb5. But the new research shows that a related protein called Clb2 is in fact the real trigger.

“To our surprise, the current model of cyclins and cellular division in yeast does not appear to hold true,” says Ralph Wäsch, M.D., a postdoctoral researcher at Rockefeller and first author of the paper. “We found that replicating cells do divide in the presence of Clb5, which means that its destruction cannot be the signal for division. What’s more, we show that replicating cells cannot divide in the presence of Clb2.”

In addition to providing fundamental insight into the “cell cycle,” the process by which all cells from yeast to human create exact duplicates of themselves, the findings have implications for treating cancer – which is characterized by a cell cycle gone awry.

“Yeast and human cells share many of the same cell cycle mechanisms,” says Frederick R. Cross, Ph.D., head of the Laboratory of Yeast Molecular Genetics and principal author of the paper. “Because of this and because they are easier to work with, yeast organisms are ideal models for studying how the cell cycle may normally work in humans, as well as how it might malfunction in cancer.”

How cells reproduce

All eukaryotic cells (cells that contain a nucleus) must undergo some form of a cell cycle in order to grow and reproduce. During this process, two crucial events must occur within a cell’s nucleus: replication of the DNA, called S-phase, and separation of the resulting chromosomes into two groups, called mitosis or M-phase. Completing the cell cycle are two periods of rest, which take place just before both S- and M-phase, and are called G1 and G2, respectively.

Only when the cell senses that these events have transpired without error will it exit mitosis and divide into two daughter cells. At this point, the process either begins anew, or a cell enters a state of dormancy, called G0.

How a cell moves from one phase to the next depends on periodic waves of cyclins: low levels prepare DNA for replication, higher levels trigger S-phase and mitosis, and a drastic drop in cyclin number signals the cell to begin dividing. Equally important to this process are the proteins that cyclins bind to and activate, called cyclin-dependent kinases (CDKs). Once activated, CDKs carry out the specific cellular tasks required for growth and division.

Cancer arises when the body fails to properly regulate this process. For example, healthy cells respond to DNA-damaging agents, such as sunlight or cigarette smoke, by halting their cell cycle while the damage is repaired, or by committing a type of cell suicide called apoptosis. But cancerous cells have lost this system of checks and balances, resulting in uncontrolled cell growth, DNA damage and eventually tumors. This breakdown in the cell cycle is caused by genetic mutations that lead to abnormal quantities of cell cycle proteins, such as the cyclins.

Cellular oscillators

The latest findings also suggest a new way of thinking about a yeast cell’s “oscillators.” Oscillators are protein complexes that control the ebb and flow of cyclins within a cell’s nucleus, thereby ensuring an orderly progression through the cell cycle. During mitosis, they signal the cell to destroy certain cyclins, which then forces it to exit mitosis and begin division. In both human and yeast cells, there are two oscillators: the Cdc20 oscillator and the Cdh1 oscillator.

Previously, scientists thought that the Cdc20 oscillator controlled chromosome separation as well as mitotic exit via elimination of Clb5, while the Cdh1 oscillator was thought to complete exit from mitosis by destroying Clb2.

But the new Nature report tells a different story. It shows that the Cdc20 oscillator dictates exit from mitosis via elimination of Clb2, not Clb5. “Previous experiments showing the destruction of Clb5 to be the primary trigger for cell division were not flawed,” says Wäsch. “Rather, the conclusions drawn from them were incorrect. We can now go back and reinterpret those experiments as meaning only that the elimination of Clb5 can act as a trigger for mitotic exit under experimental conditions. But we now know that the essential trigger is the direct destruction of Clb2 by Cdc20.”

The researchers say that the destruction of Clb5 may instead be required for proper chromosome maintenance.

Interestingly, the results also suggest how the two oscillators may have evolved. According to the researchers, the first oscillator appears to control both chromosome separation and mitotic exit, while the second mainly oversees the break between cycles of growth and division, G1. Because G1 provides higher organisms with the ability to create different types of cells, the researchers speculate that this second oscillator may represent a necessary step in the evolution of both yeast and humans.

This research is funded by Deutsche Krebshilfe, a German cancer research foundation, and the National Institutes of Health.

John D. Rockefeller founded Rockefeller University in 1901 as The Rockefeller Institute for Medical Research. Rockefeller scientists have made significant achievements, including the discovery that DNA is the carrier of genetic information. The University has ties to 21 Nobel laureates, six of which are on campus. Rockefeller University scientists have received this award for two consecutive years: neurobiologist Paul Greengard, Ph.D., in 2000 and cell biologist Günter Blobel, M.D., Ph.D., in 1999, both in Physiology or Medicine. At present, 33 faculty are elected members of the U.S. National Academy of Sciences. Celebrating its Centennial anniversary in 2001, Rockefeller – the nation’s first biomedical research center – continues to lead the field in both scientific inquiry and the development of tomorrow’s scientists.