Inside cells, where DNA is packed tightly in the nucleus and rigid proteins keep intricate transport systems on track, some molecules have a simpler way of establishing order. They can self-organize, find one another in crowded spaces, and quickly coalesce into droplets­­ - like oil in water.

Now, researchers show in the journal Science demonstrate that these droplets do more than keep cells' interiors tidy. In one study, they have shown how liquid droplets inside neurons keep signals racing through the brain. In the other, a team has discovered that droplets of a danger-sensing enzyme generate signals that launch an immune response.

The formation of these droplets is a phenomenon known as phase separation. In the last decade, biologists have watched proteins and RNA molecules rapidly organize themselves into droplets inside test tubes and spotted liquid-like droplets inside cells.

But it hasn't always been clear what, if any, advantages these droplets provide. The new discoveries offer an answer - a clear link between phase separation and biological function.

Researchers dropped a solution of fluorescent synapsin molecules onto a cover slip and watched them quickly coalesce into droplets. Occasionally, two droplets merged into one, just like oil droplets finding one another in water. In other experiments, they observed individual synapsin molecules moving freely between droplets. Just as the scientists had guessed, synapsin was behaving like a fluid.

Authors went on to show that synapsin can even organize vesicle-like structures - like those inside nerve cells - into droplets. What's more, the droplets rapidly break up when exposed to a signal that triggers neurotransmitter release. "You go from beautiful droplets to the complete disassembly of droplets," senior author says, explaining that this mimics the natural dispersion of synaptic vesicles that occurs when nerve cells communicate.

In nerve cells, droplets of synaptic vesicles offer a clear advantage: a ready supply of neurotransmitter messengers. The finding explains how neurons can keep up when the demand for neurotransmitter release is high.

https://www.hhmi.org/news/protein-droplets-keep-neurons-at-the-ready-and-immune-system-in-balance

http://science.sciencemag.org/content/361/6402/604