Volume 2, Issue 9
Ring Around the Collar for Chromosomes
Cells have amazing quality control records when it comes to passing on genetic material during cell division. Only once every 100,000 times or so do the chromosomes containing the DNA misalign during division. Even then, the cell usually catches the problem and halts the process. UC Berkeley professor David Drubin and his colleagues hope to understand the secret to this precision by taking a very close look at the microscopic machines in the cellular factory.
The research team: (from left) Eva Nogales, Stefan Westermann, David Drubin and Georjana Barnes (courtesy Berkeley Lab)
"When a chromosome becomes misaligned, it can lead to birth defects or cancer," says Drubin, a professor of molecular and cell biology. "But it doesn't happen very often. One of my research thrusts is to try to understand the cellular mechanisms that ensure this high fidelity."
For a cell to divide properly, it must first duplicate its chromosomes. The pairs of chromosomes remain connected at the center in a structure called a kinetochore. Then, the cytoskeleton that gives shape to the cell breaks down and the microtubule fibers that formed that cytoskeleton transform into a spindle. Next, the chromosomes line up along the ends of the microtubule fibers and a biomolecular process literally pulls the chromosome pairs apart. As they travel to opposite ends of the spindle, the microtubule fibers disperse before eventually forming the cytoskeleton for the daughter cells.
"The process is almost like the videogame Pac Man," Drubin says. "As the chromosomes move down the microtubule, the fiber segments just fall off."
The question though is how the kinetochore binds to the microtubule spindle to keep the chromosomes segregated before they split apart. Recently, Drubin—collaborating with his wife Georgjana Barnes, also a professor of genetics and development, Eva Nogales, a professor of molecular and cell biology and a staff scientist at Lawrence Berkeley National Laboratory, and postdoctoral researcher Stefan Westermann—found the answer. It's a case of ring around the collar.
In this electron microscope image, rings are visibly bound to the microtubules. (courtesy the researchers)
"It's common in scientific papers for researchers to draw cartoons of the kinetochore structure so that it looks like a collar around the microtubule," Drubin says. "A collar structure would mean that segments of the microtubule could fall away but the chromosome could hold on as it moves down the fiber to the pole."
The researchers were surprised when experiments seemed to show that the cartoon art really does imitate life. In a recent study, Drubin and his colleagues expressed the kinetochore proteins using a genetically-engineered E. coli bacteria. They then mixed the proteins with microtubules in a test tube. Nogales, an electron microscopy expert, and her research group photographed the reaction.
"They immediately saw beautiful rings decorating the microtubules," Drubin says. "When they showed us the images, we almost fell on the floor."
Kinetochore proteins bind to a microtubule spindle to keep chromosomes segregated during cell division. This segregation is critical for preventing mistakes that can lead to cancer and birth defects. (courtesy the researchers)
Through those experiments, Drubin explains, Westermann and Nogales's team also demonstrated that the rings remain at the end of the microtubule, even as the fiber "peels back like a banana." Eventually, the ring, with the chromosome attached, makes its way to the appropriate daughter cell.
The next step is to seek out the rings in their natural habitat, using a high-voltage electron microscope to peer inside a living cell. Of course, the ring formation is just one of the key mechanisms in cell division. Drubin's dream is to understand the finely-tuned operation of the entire system, including how the ring grabs onto the chromosome and what cellular signal kicks off the process.
"So far, we have purely biochemical results," he says. "But when we're studying self-organizing systems like this, we'd really like to watch these processes as they happen inside living cells."