Stephan Grill, Joe Howard, Erik Schäffer, Ernst Stelzer and Tony Hyman - in a collaboration between the Max-Planck Institute of Molecular Cell Biology and Genetics in Dresden and EMBL in Heidelberg - have done something few scientists have managed: they have counted the number of proteins that help an egg cell divide. This initial division happens in a special way in the roundworm C. elegans, one of biology's most important model organisms.
"The fertilized egg splits into one large and one smaller cell," Grill says. "That difference in size is crucial to the development of the whole roundworm body. Normally people think of cells as dividing into two identical daughters; if they don't, there must be forces at work that create an imbalance. We wanted to map them."
As a PhD student at EMBL, working between the research groups of Tony Hyman and biophysicist/microscopist Ernst Stelzer, Grill pursued an intriguing lead. A cable-like network of proteins called microtubules tows freshly-copied DNA off to opposing sides of the cell. The identical sets of genetic material are then sealed off in their own cells. Normally the anchors that the tow-lines are attached to, called centrosomes, remain near the center of the cell. But in the roundworm egg, one centrosome wanders off towards the outer rim of the cell. Either it was being pulled there or pushed there, Grill reasoned, so he began zapping parts of the cell with a laser, trying to disrupt the mechanism.
Grill followed Hyman – and the laser microscope – to Dresden, maintaining the collaboration with Stelzer. In the latest round of experiments, he used the laser to punch a hole in the core of the centrosome. As the structure disintegrated, he tracked what happened to the fragments. By measuring the rate at which they drifted apart, he could put exact numbers on the forces pulling them.
"The 'force-generators' are molecules called motors; their job is to pull cargoes down microtubules," Grill says. "Here they pull on the centrosome to position it. We thought that there might be more motors on one side, or stronger motors, which would create a stronger pull. But we couldn't distinguish whether that was the case."."
At this point, Joe Howard came into play, Grill says. "He just looked at the data, and suggested that we should look at the variance in the speed of the fragments from experiment to experiment. This was possible because we had performed a large enough number of experiments for a thorough statistical analysis." The differences that they observed displayed an intriguing feature that the scientists could submit to a mathematical analysis. They learned that there are more motors pulling on the posterior centrosome: about 25, compared to roughly 15 on the other side. Even though a small number of motors are involved, it is sufficient to to pull the centrosome off-center. This has dramatic consequences – it permits the proper development of the body of the embryo.
The measurements will now permit Grill and his colleagues to understand how other molecules change cellular forces and influence cell division. They have already shown that a signal passed along by the protein G-alpha is necessary to activate motors and pull the centrosome off-center.
"Cell division is a very complex process, whether the result is identical daughters or asymmetric ones," Grill says. "Having precise numbers will let us fine-tune the mathematical models and use them to look for molecules that help orchestrate this process in many other types of cells."
Stephan W. Grill, Jonathon Howard, Erik Schäffer, Ernst H. K. Stelzer, Anthony A. Hyman. The Distribution of Active Force Generators Controls Mitotic Spindle Position. Science, 301, 25. Juli 2003
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Science