Liquid-liquid phase separation (LLPS), analogous to the demixing of oil and vinegar in a vinaigrette salad dressing, is now one of the hottest topics in cell biology, according to Science magazine.
In their announcement of the 2018 Science Breakthrough of the Year and the nine runners-up, Science magazine states: “Beginning in 2009, researchers discovered that many proteins separate, or condense, into discrete droplets, concentrating their contents, especially when the cell is responding to stress.”
Recent research shows that LLPS promotes critical biochemical reactions and appears to be a basic organizing principle of the cell, Science editors wrote.
This emerging field in cell biology — “How Cells Marshall Their Contents” — was named one of the nine runners-up to the 2018 Science Breakthrough of the Year.
Recent work by Miami researcher Andor Kiss and colleagues from the University of California, Irvine, is shedding more light on LLPS. In a study published last month in the Journal of Molecular Biology, they demonstrate a model system which they can control, and/or tune, to either prevent or encourage LLPS droplet formation.
“We now understand that within cells there are areas where certain components are brought together in regions of very high concentrations, called LLPS droplets,” Kiss said.
“Within these LLPS droplets are very high concentration of components needed for a specific cellular process or needed to prevent a cellular process. The control of the droplet formation is dictated by specific proteins and their specific sequences,” he said.
But, “when the process goes awry, what should be a liquid can become a gel, and a gel can solidify, forming the kinds of aggregates seen in neurodegenerative diseases such as amyotrophic lateral sclerosis,” Science editors wrote.
Predicting the factors associated with protein-protein interactions is difficult, mainly because we lack the fundamental models to understand these events, according to Kiss.
“Often we only observe the aftermath of the problem — the Alzheimer’s plaques, the lens cataract,” he said.
Antarctic toothfish as a model
Kiss and his colleagues study these protein-protein interactions in the eye lens of the Antarctic toothfish — a vertebrate that is adapted to extreme environments.
The eye lenses of vertebrates are made of high density proteins called lens crystallins. Human lenses (and those of other warm-blooded vertebrates) can develop a “cold cataract” at temperatures below 20 degrees C (68 degrees F). This cold-cataract process has been used to model age-related cataracts and other protein condensation diseases such as Alzheimer’s and sickle cell disease.
But the lenses of the Antarctic toothfish, which lives year-round in -1.9 degrees C (27 degrees F), can resist cold cataract even in subzero waters, Kiss said. This indicates that “evolution has solved the lens clouding problem and can alter globular protein stability,” Kiss said.
Looking at eye lens crystallins
In their study, Kiss and his research team investigated six types of eye lens crystallins in the toothfish.
They identified specific amino acid differences and specific key locations in the protein structure that had controlling influence on their aggregation (how they accumulate and clump together) and stability (the specific three-dimensional structure that determines the activity of the protein).
They found that by simply changing three amino acids — from lysine to arginine and vice versa — they could control the temperature at which the protein aggregated and formed cold cataracts.
Now, Kiss said, by using animals adapted to extreme environments, we have models to understand protein-protein stability and models to test potential modifications (corrections) to those proteins to alter their stability.
A “new phase” in cell biology
According to a review in the June 2018 issue of Trends in Cell Biology, a combination of techniques “are starting to help establish the molecular principles of an emerging field, thus paving the way for exciting discoveries,” such as novel therapeutic approaches for the treatment of age-related disorders and human diseases associated with protein aggregation.
The work of Kiss and his research team is directly related to this new understanding of cellular physiology.
Their study, “Controlling Liquid-Liquid Phase Separation of Cold-Adapted Crystallin Proteins from the Antarctic Toothfish,” was published in the Dec. 7, 2018 issue of the Journal of Molecular Biology.
Study authors include senior author Rachel Martin, professor of molecular biology and biochemistry at University of California, Irvine, and first author Jan Bierma, doctoral student in biochemistry, UC Irvine.
Written by Susan Meikle, University News and ommunications, with Andor Kiss, Director, Miami’s Center for Bioinformatics and Functional Genomics, and adjunct assistant professor of biology and microbiology. Originally appeared as a “Top Story” on Miami University’s News and Events website.