Biological complexity: cellular semi-liquid organelles (Introduction)

by David Turell @, Thursday, October 10, 2019, 22:53 (7 days ago) @ dhw

This research snows how it works in one instance:

"New research may help to explain an intriguing phenomenon inside human cells: how wall-less liquid organelles are able to coexist as separate entities instead of just merging together.

"These structures, called membrane-less organelles (MLOs), are liquid droplets made from proteins and RNA, with each droplet holding both materials. The organelles play a crucial role in organizing the internal contents of cells, and can serve as a center of biochemical activity, recruiting molecules needed to carry out essential cellular reactions.

"But how different droplets stay apart from each other remains a mystery. Why don't they always just combine to form bigger droplets?

"'These organelles don't have any membrane, and hence, common intuition would tell you that they are free to mix," says Priya Banerjee, Ph.D., assistant professor of physics in the University at Buffalo College of Arts and Sciences.


"The team found that certain types of RNA and proteins are "stickier" than others, enabling them to form gelatinous droplets that don't fuse easily with other droplets in the same viscoelastic state. Specifically, droplets are more likely to be gel-like when they contain RNA molecules rich in a building block called purine, and proteins rich in an amino acid called arginine.


"In addition to providing insight into why MLOs resist mixing (due to enhanced viscoelasticity), the study probed the role of RNA in the formation and dissolution of liquid organelles containing FUS. The research found that for the type of droplet being studied, adding low concentrations of RNA to a solution containing the proteins caused droplets to form. But as more RNA was added, the droplets then dissolved.

"'There's usually a very small window where these droplets exist, but the window is significantly wider for arginine-rich proteins," Banerjee says.


"Though the team uses model systems to examine individual properties of the droplets, it's likely that many forces work together in a cell to determine the behavior and function of the organelles, he says. There may be multiple other mechanisms, for example, that cause MLOs to take on a gelatinous state or otherwise refuse to mix.

"'Cells are enormously complex, with many different molecules undergoing different processes that come together at the same time to influence what goes on inside MLOs," Banerjee says. "By using model systems, we are able to better understand how one particular variable may impact the formation and dissolution of these organelles. And we do expect to see these same forces at play in nature, inside cells."

Comment: Single cells are hard working factories, and too many membranes would take up too much space, so this is a convenient designed solution that could not have developed in a step-wise way.

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