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Like oil in water, the contents of cells can separate into droplets. Finding out why is one of biology's hottest questions.
When David Courson and Lindsay Moore arrived for a summer research placement in Woods Hole, Massachusetts, they expected to try some new techniques and play with high-end microscopes. As graduate students, they never imagined that they would help to solve a biological problem that had baffled researchers for more than 25 years.
Their instructors at the Marine Biological Laboratory asked them to decipher how pellets of RNA and protein called P granules form in worm embryos - a tall order given how long the structures had flummoxed biologists. Yet as soon as Courson and Moore started making movies ofthe process, they and their instructors could see something unusual happening under the microscope: the P granules were colliding and coalescing like blobs in a lava lamp.
Solid structures don't do that; only liquids can. The P granules, they realized, were not hard kernels, as most researchers thought. Rather, they behaved like oil droplets in a bottle of vigorously shaken vinaigrette, first dispersing, then quickly fusing and blending into larger liquid blobs.
This process is a bread-and-butter concept in engineering, chemistry and physics, called liquid-liquid phase separation. It occurs whenever there's a force pushing two liquids apart, as when oil floats on top ofwater. Phase separation is common in nature and crucial in many industrial processes. Still, it wasn't an idea that Courson, a cell biologist now at Old Dominion University in Norfolk, Virginia, had come across. When he saw the P granules fuse like liquids, "it was a really neat moment', he says, "but I didn't understand the scope or the scale of it". There was no more time to examine the process on the short summer course. But when the instructors, cell biologist Tony Hyman and his postdoc, biophysicist Cliff Brangwynne, returned to their lab at the Max Planck Institute of Molecular Cell Biology and Genetics (MPICBG) in Dresden, Germany, they ran some more experiments: they stuck worm gonads filled with P granules between two thin plates of glass and slid the plates past each other. Under the shear stress ofthe sliding plates, solids would smear out, but the granules merged, dripped and...