Charge Neutralization Tunes Dynamic Arrest of Initially Disordered Reflectin Proteins

Abstract

The intrinsically disordered reflectin proteins fill the reflective Bragg lamellae of iridescent cells in squid. In vivo, phosphorylation of the cationic reflectins leads to protein condensation and hierarchical assembly, driving osmotic dehydration of the lamellae and causing enhancement of intensity and tuning of the color of reflected light. In vitro, purified monomeric reflectin protein can be driven to cyclably and tunably assemble by pH‐neutralization or addition of salt, forming spheres of low polydispersity and reproducible size. Analysis of reflectin assembly by dynamic light scattering, x‐ray scattering, and transmission electron microscopy shows that the calibration between charge‐neutralization and assembly size is enabled by the rapid dynamic arrest of particle growth, as controlled by an electrostatic switch spatially distributed across the reflectin chain. Confocal microscopy of fluorescently labeled micron‐sized reflectin assemblies shows that they exhibit internal dynamics that rapidly slow following assembly, suggesting that assembly occurs through a transient liquid‐liquid phase separation that undergoes gelation to form stable protein‐dense condensates. Electron paramagnetic resonance (EPR) analysis shows the initially disordered reflectin monomers form ordered secondary structure that may be critical in the arrest of growth and stabilization of particles. These results provide new insights into the assembly of these unique intrinsically disordered proteins and the biophotonic systems they form, and suggest pathways for the creation of novel tunable biomaterials. Support or Funding Information This research was supported by the U.S. Department of Energy, U.S. Army Research Office, and Institute for Collaborative Biotechnologies.