Nano Structure, Structural Coloration and Bioinvisibility
Camouflage and warning coloration are used extensively in nature, and by humans in some form or another since the beginning of human civilization. Cephalopods, the group of animals including octopus, squid and cuttlefish, have a remarkable ability to instantly modulate body coloration and pattern in order to blend into surrounding environments or appear warningly obvious and vivid. This dynamic coloration is due to both the reversible expansion of pigments in chromatophores and the structural coloration produced by Bragg reflectors in Iridophores and Leucophores.
Reflectin, named after its high refractive index, is expressed exclusively in cephalopods and forms reflectors responsible for dynamic iridescence and structural color change. A very unusual amino acid composition marks reflectin as one of the most unique proteins ever known to protein biologists and has puzzled researchers for decades.
How tunable reflectors made from reflectin protein and how cephalopods use their tunable iridescence for camouflage remains unknown. In the past five years, we have developed this project to elucidate the molecular mechanism of reflectin-mediated dynamic structural coloration and bio-invisibility, and with a perspective for bioinspired photonic materials.
More recently, we trace the possible origin of the reflectin gene back to a transposon from the symbiotic bioluminescent bacterium Vibrio fischeri, and report the hierarchical structural architecture of reflectin protein. Intrinsic self-assembly, and higher-order assembly tightly modulated by aromatic compounds, provide insights into the formation of multilayer reflectors in iridophores and spherical microparticles in leucophores, and may form the basis of structural color change in cephalopods. Self-assembly and higher-order assembly in reflectin originated from a core repeating octapeptide (here named protopeptide) which may be from the same symbiotic bacteria. The origin of the reflectin gene and assembly features of reflectin protein are of considerable biological interest. The hierarchical structural architecture of reflectin, its domain and protopeptide not only provide insights for bioinspired photonic materials but also serve as unique ‘assembly-tags’ and feasible molecular platforms in biotechnology (Guan, Z. et al. Origin of the Reflectin Gene and Hierarchical Assembly of Its Protein. Current Biology, doi:10.1016/j.cub.2017.07.061.)