A colloidal model for equilibrium assembly and liquid-liquid phase separation of reflectin protein*

The reflectin protein is an exceptional example of functional intrinsically disordered protein assembly for its unique ability to modulate the biophotonic camouflage of cephalopods based on its assembly-induced osmotic properties. Although its reversible assembly into discrete, size-controlled clusters and liquid-liquid phase separation droplets is known to depend sensitively on the net protein charge, the detailed molecular mechanisms of reflectin assembly have yet to be identified. Here, we show that reflectin assembly can be explained from a remarkably simple "colloidal atom" model whereby protein molecules interact via a short-range attraction and long-range repulsion (SRA-LRR) pair potential. Through an integrated combination of small angle X-ray scattering and molecular simulations, we demonstrate that coarse grained SRA-LRR interactions, parameterized by experiment, successfully capture a number of quantitative features of reflectin assembly including the pH-dependent formation of discrete-sized nanoclusters as well as liquid-liquid phase separation, resulting in a detailed phase diagram for reflectin assembly. Our results provide predictive capabilities to explain key aspects of native reflectin behavior and to design its assembly in host materials for biophotonic applications.