Cyclable Condensation and Hierarchical Assembly of Metastable Reflectin Proteins, the Drivers of Tunable Biophotonics [Cell Biology]

December 30th, 2015 by Levenson, R., Bracken, C., Bush, N., Morse, D. E.

Reversible changes in phosphorylation of the reflectin proteins have been shown to drive the tunability of color and brightness of light reflected from specialized cells in the skin of squids and related cephalopods. We show here, using dynamic light scattering, electron microscopy, and fluorescence analyses, that reversible titration of the excess positive charges of the reflectins, comparable to that produced by phosphorylation, is sufficient to drive the reversible condensation and hierarchical assembly of these proteins. Results suggest a two-stage process in which charge neutralization first triggers condensation, resulting in the emergence of previously cryptic structures that subsequently mediate reversible, hierarchical assembly. The extent to which cyclability is seen in the in vitro formation and disassembly of complexes estimated to contain several thousand reflectin molecules suggests that intrinsic sequence- and structure-determined specificity governs the reversible condensation and assembly of the reflectins, and that these processes are thus sufficient to produce the reversible changes in refractive index, thickness and spacing of the reflectin-containing subcellular Bragg lamellae to change the brightness and color of reflected light. This molecular mechanism points to the metastability of the reflectins as the centrally important design principle governing biophotonic tunability in this system.