Reflectins are a novel protein family deposited in flat structural platelets in reflective tissues of E. scolopes with no homologs outside squids, encoded by at least six genes in three subfamilies, with a highly unusual amino acid composition dominated by Tyr, Met, Arg, and Trp.

The reflectin gene likely originated from a transposon of the symbiotic bioluminescent bacterium Vibrio fischeri via horizontal gene transfer. A core repeating octapeptide (protopeptide) is shared between reflectin and the bacterial transposase. Reflectin exhibits intrinsic self-assembly and higher-order assembly modulated by aromatic compounds.

In E. scolopes, light-organ iridescence is static and based on reflectin protein platelets. In Loligo, dynamic iridescence is controlled by the muscarinic cholinergic system. Tyrosine phosphorylation of reflectin proteins, blocked by genistein, is associated with activation of dynamic iridescence by acetylcholine.

Different reflectin subtypes have distinct tissue-specific and subcellular distributions. Tunability is correlated with a specific reflectin sequence. Differential phosphorylation and dephosphorylation of reflectins in response to acetylcholine drives dynamic iridescence from intracellular Bragg reflectors in iridocytes.

Reversible charge neutralization (comparable to phosphorylation) drives cyclable condensation and hierarchical assembly of reflectins. Intrinsic sequence-determined metastability governs reversible assembly into complexes of thousands of molecules, producing changes in refractive index, thickness, and spacing of Bragg lamellae.

Reflectins are block copolymers with repeated canonical domains interspersed with cationic linkers. Phosphorylation-driven charge neutralization overcomes coulombic repulsion to progressively allow condensation, folding, and assembly into multimeric spheres. Assembly proceeds through a dynamically arrested liquid-liquid phase-separated intermediate.

Recombinant reflectin from Loligo pealeii iridophores demonstrates hierarchical assembly of nanoscale protein particles that elicit large volume changes upon condensation. These iridophores can be chemically tuned to reflect the entire visible spectrum.

Electrical stimulation of neurons in Doryteuthis pealeii skin shifts reflected spectral peak by >145 nm and increases reflectance by >245%. The reflectin protein condensation mechanism explains peak reflectance change, while a distinct mechanism causes fast color shift.

Bragg reflectors in iridocytes consist of alternating reflectin-containing high-refractive index lamellae and low-index inter-lamellar spaces. High-index lamellae have refractive index averaging 1.405 with maximum ~1.44. Tuning from red (675 nm) to blue (425 nm) requires lamellar thickness decrease from ~150 to 80 nm.

Direct measurement of the refractive index of condensed reflectin in Bragg lamellae yields n ~ 1.44, confirming reflectin as a high-refractive-index biological material suitable for biophotonic thin-film interference.

Reflectin proteins are found in both iridocytes (iridescent cells) and leucophores (broadband white-reflecting cells), with a unique complement of reflectin proteins in each cell type. Leucophores contain Mie-scattering organelles activated by acetylcholine.

Reflectins are unusual structural proteins that critically enable the optical functionality of iridocytes and leucophores in cephalopod skin. Their subcellular organization produces dynamic structural coloration for camouflage and signaling.

Reflectin is the high refractive index material composing ultrastructures in both iridophores (layered Bragg reflectors) and leucophores (Mie-scattering particles). Self-assembled reflectin-based structures have been characterized by holotomographic microscopy.

Reflectin proteins self-assemble into spherical nanoparticles tunable from 170-1000 nm. Swelling/deswelling of reflectin nanoparticles alters platelet dimensions in iridophores to control photonic patterns according to Bragg's law.

An 18-amino-acid reflectin repeat peptide (RRP) corresponding to the conserved motif region spontaneously forms beta-sheet-rich fibers and nanoparticles, similar to the behavior of full-length reflectin, demonstrating that the repeat domain is sufficient for hierarchical self-assembly.

Reflectin is present in chromatophore pigment granules of Sepia officinalis, suggesting it enhances the brightness or spectral properties of pigment by increasing refractive index contrasts inside the granule.

Reflectin concentration in iridocyte platelets is approximately 380 mg/mL, comprising roughly 18% of the dry weight of an iridocyte cell, yielding a refractive index of approximately 1.44-1.51 in the protein-rich lamellae.

Reflectin proteins are also found in eye reflective tissues (iris or retina tapetum) across cephalopods, indicating a common mechanism for making mirrors in different organs.

Reflectins contain less than 2% bulky hydrophobic residues (Leu, Ile, Val) and more than 35% charged or polar residues, leading to strong intrachain electrostatic repulsion that prevents stable folding and classifies them as intrinsically disordered proteins.

Protein charge neutralization is confirmed as the proximate driver that dynamically tunes reflectin assembly. Reflectins behave as cationic block copolymers where phosphorylation neutralizes positive charges, overcoming coulombic repulsion and allowing condensation into dense assemblies that alter optical properties.