Physical Principles that Govern the Sequence-Encoded Phase Behavior of Intrinsically Disordered Block-Copolymeric Proteins

Abstract

Many intrinsically disordered proteins (IDPs) and sequences with long disordered regions undergo reversible phase transitions such as liquid-liquid demixing or sol-gel transitions to form dense liquids or gel-like phases. Such phases underlie the formation of membraneless compartments within cells. Block-copolymeric disordered regions appear to be necessary and sufficient to drive phase separation in many systems. The interfacial tension between well-mixed and dense phases can be modulated in different ways: It can be diminished at high salt concentrations, regulated by post-translational modifications such as Ser / Thr phosphorylation, and altered by interactions with RNA molecules. These observations suggest that sequence-encoded electrostatic interactions provide at least part of the driving force for phase separation of IDPs with blocks that are enriched in charged residues.Our goal is to uncover the physical principles that govern the sequence-encoded driving forces for reversible phase transitions of block copolymeric IDPs. Here, we focus specifically on charge-mediated interactions by considering the phase behavior of block-copolymeric polyampholytes and polyelectrolytes. The physics of electrostatically driven phase separation, known as complex coacervation, has been explored in polymer chemistry for synthetic polyelectrolytes. Through a novel combination of lattice-based coarse grain simulations, off-lattice coarse-grain simulations driven by machine learning, atomistic simulations, and theoretical insights, we are uncovering a physical framework for sequence-encoded complex coacervation of polyampholytic and polyelectrolytic IDPs. Our focus is on the collective interplay among charge patterning, charge density, chain-length, and the influence of charged versus uncharged residues on the phase behavior, fluidity, and structures adopted by disordered proteins in dense phases. Our findings should enable the systematic design of IDPs with desired phase behavior and proteome-level identification and understanding of how specific types of sequences drive intracellular phase transitions.