New research could help explain an intriguing phenomenon within human cells: how liquid organelles without walls can coexist as separate entities, instead of just merging together.
These structures, called membrane-free organelles (MLOs), are fluid droplets made of protein and RNA, with each droplet holding both materials. Organelles play a key role in organizing the internal content of cells and can serve as a hub for biochemical activity, the recruitment of molecules required to carry out essential cellular responses.
But how different droplets stand apart from each other remains a mystery. Why don't they just combine into larger droplets?
"These organelles do not have a membrane and therefore a common intuition might tell you that they are free to mix," says Dr. Priya Banerjee, an assistant professor of physics at the University of Buffalo College of Arts and Sciences.
Banerjee is the lead investigator for a new study, which investigates why this is not happening.
Co-authors of the study include first author and PhD candidate in physics, Ibraheem Alshareedah; Taranpreet Kaur, PhD; undergraduate Jason Ngo; Hannah Seppal's undergraduate physics and mathematics degree; undergraduate biomedical engineering Liz-Audrey Djomnang Kounatse; and physics by postdoctoral researchers Wei Wang and Mahdi Moosa. They're all from UB.
The droplets will not mix easily if they take on a gel-like state
The results were released on August 22 at Journal of the American Chemical Society – point to the chemical structure of proteins and RNA molecules within droplets as one of the key factors that can prevent MLOs from mixing.
The team found that certain types of RNAs and proteins were "nicer" than the others, allowing them to form gelatin droplets that do not blend 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.
The experiments did not take place in the cells. Instead, the findings were based on tests performed on model systems consisting of RNA and a droplet-forming protein called fused sarcoma (FUS) floating in buffer solution.
One of the reasons that FUS is of interest to researchers is its potential association with neurotrogenerative disease amyotrophic lateral sclerosis (ALS). As Banerjee explains, arginine-rich protein molecules are associated with a predominant form of the disease, known as c9orf72-mediated AL9.
"Our discovery points to the special role of arginine-rich proteins in determining the material state – membrane-free organelles," says Banerjee. "This study may be important in understanding how ALS-linked arginine-rich proteins can alter the viscoelastic state of RNA-rich MLOs."
In addition to providing insight into why MLOs resist mixing (due to their increased viscoelasticity), the study examined the role of RNA in the creation and dissolution of liquid FUS-containing organelles. The study found that for the type of droplets being studied, the addition of low concentrations of RNA to the protein-containing solution caused the formation of droplets. But as more RNA was added, the droplets then dissolved.
"There is usually a very small window where these droplets exist, but the window is significantly wider because of arginine-rich proteins," says Banerjee.
The complicated life of liquid organelles
The new work is the latest in a series of studies conducted by Banerjee's group to investigate the forces that drive the creation, maintenance and dissolution of MLOs.
Although the team uses systems models to test individual droplet properties, it is likely that many forces work together in the cell to determine the behavior and function of the organelles, he says. There may be several other mechanisms, for example, that cause MLOs to become gelatinous or otherwise refuse to interfere.
"The cells are extremely complex. Many different molecules go through different processes that come together at the same time to influence what's happening inside the MLO," says Banerjee. "Using model models, it's easier to understand how one particular variable can affect the formation and dissolution of these organelles. And we expect to see these same forces in nature, within cells."