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Stanford Report, August 6, 2003

Medical center researchers pick apart mystery behind asymmetric cell division


During the normal course of cell division one cell multiplies into two nearly identical cells. But occasionally something unusual happens — the one cell splits and two entirely different progeny result. The question for researchers is how a cell divvies up internal components so that the two cells receive different instructions.

In a report published in the July 8 issue of Proceedings of the National Academy of Sciences, Harley McAdams, PhD, associate professor of developmental biology, and his colleagues teased apart one aspect of how the bacterium Caulobacter crescentus splits into two cells with completely different fates. This work could help researchers understand asymmetric divisions that take place during our own development, according to McAdams.

Caulobacter lives part of its life fastened to a stationary object using a stalk at one end of the cell. When the bacteria divides, it produces one offspring, called the stalk cell, which stays in place while it’s sibling, called the swarmer cell, swims off in search of a good spot to settle down, grow a stalk and produce swarmer cells of its own.

Of these two cells, only the stalk cell can reproduce. The migrant swarmer cell contains a protein called CtrA that puts the brakes on cell growth and division until it transforms into a stalked cell. The new report shows how CtrA ends up only in the swarmer end as the cell divides.

According to the work of McAdams and graduate student Ellen Judd, 18 minutes before the bacteria divides the constriction between the forming progeny becomes too tight for molecules to slip back and forth. At this time, CtrA molecules trapped in the swarmer end remain active while those same division-preventing molecules trapped in the stalk end are destroyed, taking the brakes off cell division and clearing the way for the new stalk cell to reproduce again.

Figuring out when the molecules get trapped behind the constricting gap required some optics expertise supplied by Judd, who is a student in applied physics, and by W. E. Moerner, the Harry S. Mosher professor of chemistry. Judd used Caulobacter containing a gene whose protein product glows green under blue-green light. Using a well-aimed laser in Moerner’s laboratory, Judd could zap the protein, called GFP, and wipe out its green color.

Early in the division process, when the two cells are still connected by a wide opening, the GFP moves rapidly throughout both halves of the cell. With this unrestricted movement, all of the GFP molecules in the cell would have time to float through a five-second pulse from the laser beam and lose their green color. This is exactly what Judd saw, right up until roughly 18 minutes before division.

At that time the GFP, which is about the same size as CtrA, can no longer slip through the narrowing constriction between the cells. When Judd zapped the spot on the eventual stalk cell, only the GFP in that cell meandered through the laser beam and lost color. In contrast, the GFP in the eventual swarmer cell remained stuck behind the shrinking opening and retained its color.

"Diffusion of molecules inside the cell is too fast to maintain an asymmetry until the hole is extremely small," McAdams said.

Judd said this experiment doesn’t address how CtrA trapped in the stalk cell is destroyed. Caulobacter researchers suspect that a protein lodged in the stalk end of the bacteria demolishes the CtrA.

Once the constriction narrows between two forming cells, CtrA being produced in the swarmer end can’t make it through the opening and therefore doesn’t encounter the protein that breaks it down. Active CtrA remains in the swarmer cell and prevents that cell from dividing before it has settled down and become a stalk cell.

Judd said that by starting to destroy CtrA before it breaks free, the stalked cell has a jump start on its future. "When the two cells separate, the stalked cell is fully geared-up to begin dividing again," Judd said, while the swarmer cell can swim away and forage without reproducing prematurely.

Other authors on the paper are Kathleen Ryan, PhD, a postdoctoral fellow, and Lucy Shapiro, the Virginia and D. K. Ludwig professor of developmental biology.

Researchers discover protein that helps control timing of cell division process (7/25/01)