In nature, biopolymers partition into dynamic compartments to facilitate and regulate their interactions. These dynamic compartments are referred to as membraneless organelles (e.g., nucleolus, P bodies, stress and germ granules) and consist of biopolymer rich interiors that rapidly assemble and disassemble to form liquid droplets, hydrogels or fibril structures. The physical interactions that affect the formation, dissolution, and regulation of these assemblages are poorly understood, yet vital in determining their function in normal and disease states. Therefore, there is a strong need to investigate the underlying mechanisms that drive dynamic biopolymer complexation and develop an understanding of their biochemical function. Recent studies have identified several significant interactions that drive phase-separation, including specific multivalent protein interactions, RNA-protein interactions and non-specific interactions between intrinsically disordered regions (IDRs). IDRs are disordered protein domains with low amino acid sequence diversity that can self-associate into oligomeric structures via non-specific weak interactions. These sequences are repetitive and rich in charged, polar and aromatic side-chains and highlight the importance of the interplay between electrostatic, dipole-dipole and short-range aromatic interactions.
Membraneless organelles formed in vivo and in vitro are strikingly similar to polyelectrolyte complexes formed using much simpler synthetic polyelectrolytes and charged homopolymer polypeptides (e.g., (poly)-lysine and (poly)-glutamic acid). In light of these observations, we designed a series of synthetic polypeptide sequences that mimic IDRs to elucidate the forces that govern membraneless organelle formation. Using solid-phase synthesis we are able to control the polypeptide sequence precisely to create alternating, block and gradient polypeptide copolymers to systematically investigate the influence of charged, polar and aromatic groups on phase separation using a variety of techniques. Our work holds the potential to develop a fundamental understanding of complex biopolymer complexation, impacting design of novel functional biomimetic materials for various applications.