Abstract
Reflectin proteins are widely distributed in reflective structures in cephalopods. However, only in loliginid squids are they and the subwavelength photonic structures they control dynamically tunable, driving changes in skin color for camouflage and communication. The reflectins are block copolymers with repeated canonical domains interspersed with cationic linkers. Neurotransmitter-activated signal transduction culminates in catalytic phosphorylation of the tunable reflectins' cationic linkers; the resulting charge neutralization overcomes coulombic repulsion to progressively allow condensation, folding, and assembly into multimeric spheres of tunable well-defined size and low polydispersity. Here, we used dynamic light scattering, transmission EM, CD, atomic force microscopy, and fluorimetry to analyze the structural transitions of reflectins A1 and A2. We also analyzed the assembly behavior of phosphomimetic, deletion, and other mutants in conjunction with pH titration as an in vitro surrogate of phosphorylation. Our experiments uncovered a previously unsuspected, precisely predictive relationship between the extent of neutralization of a reflectin's net charge density and the size of resulting multimeric protein assemblies of narrow polydispersity. Comparisons of mutants revealed that this sensitivity to neutralization resides in the linkers and is spatially distributed along the protein. Imaging of large particles and analysis of sequence composition suggested that assembly may proceed through a dynamically arrested liquid-liquid phase-separated intermediate. Intriguingly, it is this dynamic arrest that enables the observed fine-tuning by charge and the resulting calibration between neuronal trigger and color in the squid. These results offer insights into the basis of reflectin-based biophotonics, opening paths for the design of new materials with tunable properties.
Highlights
Reflectin proteins are widely distributed in reflective structures in cephalopods
Bioinformatics analyses suggests that the reflectins are intrinsically disordered The tunable reflectins from the loliginid squid D. opalescens are essentially block-copolymeric, composed of unique and highly conserved peptide domains alternating with weakly polycationic linkers (Figs. 1A and S1A) [10]
Comparison of D. opalescens reflectins A1 and A2 overall net charge and mean hydropathy with those of a collection of proteins previously analyzed for structure shows that these two sequence parameters are in the range commonly observed for intrinsically disordered proteins (Fig. 1B) [17]
Summary
Reflectin proteins are widely distributed in reflective structures in cephalopods. only in loliginid squids are they and the subwavelength photonic structures they control dynamically tunable, driving changes in skin color for camouflage and communication. Imaging of large particles and analysis of sequence composition suggested that assembly may proceed through a dynamically arrested liquid–liquid phase-separated intermediate It is this dynamic arrest that enables the observed fine-tuning by charge and the resulting calibration between neuronal trigger and color in the squid. Leucophores are broadband scatterers of white light, iridocytes reflect colored, iridescent light by angleand wavelength-dependent constructive interference from intracellular Bragg reflectors. The lamellae of these reflectors are densely filled with cationic block copolymer-like proteins called reflectins and are separated from the low-refractive-index extracellular fluid by regular invaginations of the cell membrane [5,6,7,8,9]. The reflectins are essentially block copolymers, composed of highly conserved reflectin domains
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