Why Do Disordered and Structured Proteins Behave Differently in Phase Separation?

Abstract

Intracellular membraneless organelles and their myriad cellular functions have garnered tremendous recent interest. It is becoming well accepted that they form via liquid-liquid phase separation (LLPS) of protein mixtures (often including RNA), where the organelles correspond to a protein-rich droplet phase coexisting with a protein-poor bulk phase. The major protein components contain disordered regions and often also RNA-binding domains, and the disordered fragments on their own easily undergo LLPS. By contrast, LLPS for structured proteins has been observed infrequently. The contrasting phase behaviors can be explained by modeling disordered and structured proteins, respectively, as polymers and colloids. These physical models also provide a better understanding of the regulation of droplet formation by cellular signals and its dysregulation leading to diseases.

Temperature, Hydrostatic Pressure, and Osmolyte Effects on Liquid–Liquid Phase Separation in Protein Condensates: Physical Chemistry and Biological Implications

“…Figure 13 is ag raphical summary of our proposed physical pictures. The schematics place LLPS of globular folded proteins and IDPs in the same general conceptual framework [190] with an emphasis on the critical role of void volumes (white areas), namely,t he total white area is largest when the total volume is largest( left columno fF igure 13), which corresponds to the state under low pressure. However,i ti si mportant to note that the scenarios listed herea re not exhaustive.…”

Liquid-Liquid Phase Separation of Patchy Particles Illuminates Diverse Effects of Regulatory Components on Protein Droplet Formation

“…This is not surprising, since the LLPS of disordered and globular proteins have a common physical basis, though their phase behaviors show characteristic differences. 23 Experimental studies have presented many observations on LLPS that beg for mechanistic answers. In principle, theoretical calculations and molecular simulations can provide these answers, but still face significant technical challenges.…”

“…In particular, polymer theories often provide only a very crude account of excluded-volume effects. 23 Here we used Gibbs-ensemble simulations to investigate how regulatory components affect protein droplet formation, a pressing question that has received only scant attention in previous theoretical and simulation studies. While the promotion of LLPS by crowding agents can be understood from the perspective of excluded-volume effects, 28,29 when RNA or another regulatory component is attractive toward the droplet-forming component and hence is recruited into the droplet phase, the physical rules governing the foregoing disparate effects on LLPS are yet to be established.…”

A Tug of War Between Condensate Phases in a Minimal Macromolecular System

“…6e). An H-rich droplet inside a condensate shows poor ThT 16 binding, whereas the surrounding L-rich region shows strong ThT binding. These…”

“…Droplets are a common, liquid phase of bimolecular condensates, and can assemble or disassemble by a simple compositional change, i.e., when the components go above or below their threshold concentrations 15,16 . When a macromolecular mixture can exist in multiple condensate phases, each favored by a different composition, a compositional change can likewise result in the changeover from one condensate phase to another.…”