Poster number 132 submitted by Gable Wadsworth
Entropy-driven Phase Transition of RNA
Gable M. Wadsworth (Department of Physics, The State University of New York at Buffalo), Xiangze Zeng (Department of Biomedical Engineering and Center for Biomolecular Condensates CBC, Washington University in St. Louis), Walter J. Zahurancik (Department of Chemistry & Biochemistry, Center for RNA Biology, The Ohio State University, ), Lien B. Lai, Vaishnavi Sidharthan, Venkat Gopalan (Department of Chemistry & Biochemistry, Center for RNA Biology, The Ohio State University, ), Rohit V. Pappu (Department of Biomedical Engineering and Center for Biomolecular Condensates CBC, Washington University in St. Louis), Paul Pullara, Priya R. Banerjee (Department of Physics, The State University of New York at Buffalo)
RNAs are ubiquitously found alongside protein cofactors as components of ribonucleoprotein (RNPs) granules, which are thought to form and dissolve under the influence of reversible phase transitions. Recently, RNAs, specifically sequences with different repeat expansions have been observed to form independent foci in cells. The formation of protein-free, RNA foci is thought to be driven primarily by multivalent base pairing and stacking interactions. Utilizing temperature-controlled titrations, we uncover the surprising result that RNA molecules can undergo reversible phase transitions upon heating above RNA-specific threshold temperatures. This type of thermo-responsive phase transition, known as phase behavior with a Lower Critical Solution Temperature (LCST), is an entropy-driven transition. An assortment of systems including RNA homopolymers, inorganic polyphosphate, RNAs with trinucleotide repeats, catalytic RNase P RNA, and enhancer RNA all demonstrate LCST-type phase behavior in the presence of Mg2+. These observations suggest that the coupling of associative and segregative transitions known as phase separation coupled to percolation (PSCP) is driven, at least in part, by entropic factors. Such transitions cannot be ascribed to base pairing and base-stacking, which diminish as temperature increases. Instead, using molecular simulations we find that the LCST-type phase behavior in the simplest case of polyphosphate chains, which lack base pairing and stacking interactions, is driven by a combination of desolvation to minimize the entropic penalty associated with solvation, and Mg2+ ion-mediated bridging interactions among phosphate moieties within and across chains. Further, we find that the heat-induced RNA phase transition is tuned by the nucleobase composition and sequence patterning, giving rise to a set of generalizable rules to encode and manipulate thermo-responsive PSCP transitions of RNA molecules.
Keywords: phase separation, LCST transition, RNA condensates