Since the last decade, an increasing number of proteins have been shown to be capable of undergoing reversible liquid-liquid phase separation (LLPS) in response to an external stimulus, and the resulting protein-rich phase (coacervate) is considered as one of the main components of membrane-less organelles. Most of these proteins are intrinsically disordered proteins (IDPs) or contain intrinsically disordered regions. More importantly, LLPS often plays an important role in cellular signaling and development of cells and tissues. However, the molecular mechanisms underlying LLPS of proteins remain poorly understood. Elastin-like proteins (ELPs), a class of IDPs derived from the hydrophobic domains of tropoelastin, are known to undergo LLPS reversibly above a concentration-dependent transition temperature (TT), allowing ELPs to be a promising thermo-responsive drug delivery vector for treating cancer. Previous studies have suggested that, as temperature increases, ELPs experience an increased propensity for type II beta-turns. Our hypothesis is that the interaction is initiated at the beta-turn positions. In this work, integrative approaches including experimental and computational methods were employed to study the early stages of ELP phase separation. Using nuclear magnetic resonance spectroscopy (NMR), and paramagnetic relaxation enhancement (PRE), we have characterized structural properties of self-association in several ELPs. NMR chemical shifts suggest that ELPs adopt a beta-turn conformation even at temperatures below the TT. The intermolecular PRE reveals there is a stronger interaction between the higher beta-turn propensity regions. Building on this observation, a series of structural ensembles were generated for ELP incorporating differing amounts of beta-turn bias, from 1% to 90%. To mimic the early stages of the phase change, two monomers were paired, assuming preferential interaction at beta-turn regions. Following dimerization, the ensemble-averaged hydrodynamic properties were calculated for each degree of beta-turn bias, and results were compared with analytical ultracentrifugation (AUC) experiments at various temperatures. The ensemble calculation reveals that accessible surface area changes dramatically as oligomers are formed from monomers with a high beta-turn content. Together, these observations suggest a model where ELP self-association is initiated at beta-turn positions, where the driving force of phase separation is solvent exclusion due to changes in the hydrophobic accessible surface area.
| Identifer | oai:union.ndltd.org:MSSTATE/oai:scholarsjunction.msstate.edu:td-5748 |
| Date | 08 December 2017 |
| Creators | Zhang, Yue |
| Publisher | Scholars Junction |
| Source Sets | Mississippi State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
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