If you have an ACS member number, please enter it here so we can link this account to your membership. (optional)

ACS values your privacy. By submitting your information, you are gaining access to C&EN and subscribing to our weekly newsletter. We use the information you provide to make your reading experience better, and we will never sell your data to third party members.



Protein droplets help cells respond to osmotic stress

Researchers find physiological role for liquid-liquid phase separation

by Laurel Oldach
November 9, 2022

Liquid-liquid phase separation can occur when drops or layers of one liquid, like oil, form alongside another liquid, like water. It has been widely observed within cells, where proteins can condense out of the cytoplasm despite being evenly mixed at first. Whether these protein globules have a function has been hotly debated.

Credit: Arohan Subramanya/University of Pittsburgh
Full-length fluorescent WNK expressed in a cell forms phase-separated particles after a solute is added, and the cell recovers its volume.
Credit: Arohan Subramanya/University of Pittsburgh
A cell expressing a truncated version of WNK that still can function as a kinase but cannot phase separate shrivels up under hyperosmotic stress.

“At this point, there’s no aspect of biology, I think it’s fair to say, that has not been suggested to be impacted by or controlled by or facilitated by phase separation,” says Clifford Brangwynne, a Princeton University biophysicist who recently received the 2023 Breakthrough Prize in Life Sciences for work on phase separation in biology. But it has been hard to demonstrate conclusively that phase separation drives cell processes.

Now, Arohan Subramanya of the University of Pittsburgh and colleagues have shown that phase separation is key for cells to sense an excess of salt or other solutes in their surroundings and start damage-control measures (Cell 2022, DOI: 10.1016/j.cell.2022.09.042).

When a cell’s environment has a higher solute concentration than the cytoplasm, water rushes out of the cell. That’s bad news for the cell, which shrivels and begins to malfunction as molecules crowd together. Healthy cells, however, will rebound using ion transporters that pump in solutes until balance is restored.

Kinases known as with-no-lysines, or WNKs, coordinate these ion transporters by sensing cell shrinkage and molecular crowding. Researchers in Subramanya’s lab found that WNKs handle these tasks by forming phase-separated droplets moments after a salt or molecular crowding stress is applied and that those droplets orchestrate the ion transporter signal. They observe phase separation in WNKs from many species.

At this point, there’s no aspect of biology, I think it’s fair to say, that has not been suggested to be impacted by or controlled by or facilitated by phase separation.
Clifford Brangwynne, biophysicist, Princeton University

The phase separation, and not just the WNKs’ enzymatic activity, is key. The authors generated several truncated versions of a WNK that still had enzymatic activity but could not phase separate. Cells with these versions “have a very sluggish response and volume recovery,” Subramanya says. However, they do eventually begin to recover, apparently using other mechanisms that are slower to kick in.

Researchers have observed these phase-separated droplets, which they call condensates, in response to many stimuli, but this study is “a direct illustration of how a condensate that forms via phase separation contributes to cellular physiology,” says Rohit Pappu, a biophysicist at Washington University in St. Louis, who was not involved with the work.

Brangwynne says that the paper reflects a general transition in the field from describing the properties of phase-separated condensates to beginning to explore their physiology. “We’re only beginning to understand all the ways in which condensates facilitate biological function,” he says.



This article has been sent to the following recipient:

Chemistry matters. Join us to get the news you need.