Organizing Organelles

How do cells organize proteins without membranes? St. Jude scientists unlock the mysteries of liquid-liquid phase separation.

By Erin Podolak; Photos by Peter Barta and Seth Dixon

The first step of organization involves sorting — picture shirts on one side of your closet, pants on the other. This process also applies to cells. They need to stay organized to efficiently accomplish various tasks.

Cells find ways to sort and separate proteins and other cell components. They do that through liquid-liquid phase separation (LLPS). It’s the same process that governs the way oil forms droplets in water.

Organelles are bodies within a cell that are surrounded by membranes and serve specific functions. Cell biologists have long known that alongside traditional organelles there were cellular bodies without membranes. No one had an explanation for these membraneless organelles. Then, a decade ago, things changed. Membraneless organelles were found to behave like liquid droplets. LLPS was proposed to explain how a cell organizes certain proteins without enclosing them in a membrane.

 
 

“This field went unappreciated for years,” says J. Paul Taylor, PhD, St. Jude Cell and Molecular Biology chair. “But once we started to look at liquid-liquid phase separation, just about every scientist studying any biological process realized this is relevant to their work.”

Now, researchers are studying LLPS to better understand membraneless organelles, the functions they serve and how errors in this process play a role in disease.

 
 
J. Paul Taylor, PhD

J. Paul Taylor, PhD
 
Richard Kriwacki, PhD

Richard Kriwacki, PhD
 
Tanja Mittag and Amandine Molliex

Tanja Mittag, PhD (at left)
 
 

Big Ideas Shape New Field

Richard Kriwacki, PhD, of St. Jude Structural Biology studies a membraneless organelle called the nucleolus. He discovered how the nucleolus produces molecular machines called ribosomes using LLPS.

The protein and RNA building blocks of the ribosomes are held in a liquid phase by a protein called nucleophosmin through phase separation. This enables the ribosomes’ component proteins and RNA to bind each other properly, allowing ribosome subunits to assemble.

“We revealed the mechanism for how phase separation contributes to a critical biological process,” Kriwacki explains.

Order in Disorder

Tanja Mittag, PhD, of St. Jude Structural Biology seeks to understand how interactions between intrinsically disordered proteins, notable for their lack of structure, can drive LLPS. Mittag and Taylor were among the first to show that an intrinsically disordered region in a protein is enough for it to phase separate. One-third of all proteins are intrinsically disordered. While not all of these easily phase separate, this suggests LLPS may guide the interactions of many more proteins than initially thought.

“This highlights the scale at which liquid-liquid phase separation underlies the inner working of a cell, as well as the importance of intrinsically disordered proteins,” Mittag says. “But we’re only scratching the surface of understanding the driving forces for phase separation and what can happen when these processes go awry.”

 
 

We’re only scratching the surface of understanding the driving forces for phase separation and what can happen when these processes go awry.

Tanja Mittag, PhD

 

Revelations

Taylor studies neurodegenerative disorders, which have only recently been linked to abnormal phase separation. Mittag and Taylor found that some mutations that underlie neurodegenerative diseases can also drive LLPS. Taylor then showed that some of these mutations can also drive pediatric cancers.

“Mutations can modify the concentration threshold at which phase separations occur,” Taylor says. “Lowering the threshold can make phase separation more likely to happen, disturbing biological processes and at times leading to disease.”

Kriwacki is also studying LLPS in neurodegenerative disorders, providing insight into how the process can contribute to amyotrophic lateral sclerosis (ALS). He is studying abnormal proteins found in abundance in ALS and how undergoing LLPS disturbs the nucleolus and contributes to cell death.

Genes that control cell growth can be hijacked by cancer to allow for uncontrolled development. Mittag recently showed that mutations in the tumor suppressor gene SPOP contribute to cancer by disrupting LLPS.

 
 

The phase separation work at St. Jude and elsewhere is rewriting how we understand cell biology. We’re going to need to revise the textbooks.

Richard Kriwacki, PhD

 

A New Framework

This research provides a framework for scientists to explore how drugs could modify LLPS to achieve therapeutic results.

“It would be like targeting the cell’s abnormal equilibrium to prod it back into balance,” Taylor says.

There is great potential for treatments to target LLPS to modify protein interactions in a cell. But there are also challenges to surmount — like the fleeting nature of phase separation. Still, researchers say they relish the opportunity to work in an up-and-coming field.

 “The phase separation work at St. Jude and elsewhere is rewriting how we understand cell biology,” Kriwacki says. “We’re going to need to revise the textbooks.” 

From Promise, Autumn 2019

 
 

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