Role of Liquid-Liquid Phase Separation in Assembly of Elastin and Other Extracellular Matrix Proteins

Liquid-liquid phase separation resulting in formation of colloidal droplets has recently attracted attention as a mechanism for rapid and transient assembly of intracellular macromolecules into functional units. Phase separation also appears to be a widespread and evolutionarily ancient mechanism for organization of proteins of the extracellular matrix into fibrillar, polymeric assemblies. Elastin, which provides the physical properties of extensibility and elastic recoil to large arteries, lungs and other tissues, is the best-characterized extracellular matrix protein whose polymeric assembly is initiated by phase separation. Recent studies have provided an atomistic description of the conformational ensemble of elastin-like proteins, and have begun to uncover how the interplay of local secondary structure, hydrophobicity and conformational disorder govern the structure, assembly and function of elastin. Monomeric elastin is a non-polar, glycine-rich, low-complexity, modular protein that remains predominantly disordered even in the crosslinked polymeric state, consistent with its function as an entropic elastomer. Unlike intracellular phase separation, which is reversible, phase separation of elastin and other matrix proteins proceeds to stabilization and clustering of condensed phase droplets and subsequent molecular organization into fibrillar, supramolecular structures. Short β-sheets appear to mediate the interaction and organization of these phase-separated droplets and modulate the ultimate material properties of the matrix. Whether phase separation is intracellular or extracellular, reversible or network-forming, understanding the sequence determinants of such varied assembly behaviors and differential fates of the colloidal droplets will provide important insights into aberrant assembly with pathological consequences and elucidate fundamental principles for the rational design of biomimetic materials.