AsianScientist (Nov. 20, 2018) – In a study published in the Biophysical Journal, researchers in South Korea have explained how liquid-like droplets made of proteins and DNA form in vitro. These findings have implications for human diseases such as amyotrophic lateral sclerosis.
Often compared to tiny factories, cells have devised sophisticated ways to organize their subcellular spaces. This is important as most biological processes require cells to bring together biomolecules such as proteins and nucleic acids in a precisely orchestrated fashion.
In the same way walls divide a factory into departments, the cell has lipid membranes to divide its space into organelles. However, in the past ten years, scientists have realized that some cellular compartments are not enclosed by membranes. Called membrane-less organelles, such organelles behave like dense liquid droplets.
In this study, scientists at the Center for Soft and Living Matter of the Institute for Basic Science (IBS) in South Korea have found the molecular mechanism behind the creation of droplets made of protein and DNA. They first tested how different sequences of DNA form droplets with a simple protein made from a single repeating amino-acid, lysine (poly-L-lysine). DNA is negatively charged while poly-L-lysine carries a positive charge, so they attract each other but are still able to remain in solution.
“As the rigidity of DNA molecules can be slightly tuned depending on its nucleotide sequence, we compared DNA molecules with the same charge density, but with different sequences,” explained Dr. John T. King of IBS.
The researchers found that when the overall electric charge between two DNA molecules is the same, the DNA sequence ultimately determines the stability and appearance of the liquid-like droplets. For example, single-stranded DNA with only T’s was able to form droplets more readily than single-stranded DNA with only A’s, primarily because poly(T) is more flexible than poly(A). Double-stranded DNA rich in A’s and T’s were more rigid than double-stranded DNA rich in G’s and C’s, requiring the addition of more salts to obtain droplets, said the scientists.
The team also demonstrated that adenosine triphosphate (ATP), which typically acts as a fuel source in cells, facilitates the formation of liquid-like droplets. Mixtures of poly-L-lysine and double-stranded DNA, which would typically precipitate at low salt concentrations, readily formed stable liquid-like droplets in the presence of ATP.
“The most fascinating part is to imagine how cells may take advantage of this sequence-dependent information to guide and regulate liquid-liquid phase separation in vivo,” said first author Dr. Anisha Shakya of IBS.
The article can be found at: Shakya & King (2018) DNA Local-flexibility-dependent Assembly of Phase-separated Liquid Droplets.
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Source: Institute for Basic Science.
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