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Reconstruction of Dynamic and Reversible Color Change Using www.nature.com/scientificreports OPEN Reconstruction of Dynamic and Reversible Color Change using Refectin Protein Received: 16 November 2018 Tiantian Cai1, Kui Han2, Peilin Yang1, Zhou Zhu1, Mengcheng Jiang2, Yanyi Huang2 & Can Xie1,3 Accepted: 13 March 2019 Cephalopods have remarkable ability to change their body color across a wide range of wavelengths, Published: xx xx xxxx yet the structural basis remains largely unknown. Refectin, a protein family assumed to be responsible for structural color in cephalopods, has unique features of higher-order assembly that are tightly regulated by aromatic molecules. Here, we reconstructed the dynamic and reversible color change using purifed refectin protein and demonstrated how the conformational change and the status of assembly led to the change in optical properties. In addition, optical spectral and structural analyses indicated that the “cephalopod-blue” primarily resulted from wavelength-dependent light scattering rather than refection. Our results suggest a possible role of refectin in color dynamics. The in vitro reconstruction system we present here may serve as an initial step for designing bio-inspired optical materials based on refectin protein. Colors in nature are primarily mediated by pigmentary or structural elements or a combination of both1–3. In con- trast to pigment-derived colors, structural colors are produced by the physical interactions of the incident light with micro- or nano-structures1,4. Structural colors are bright, ofen iridescent, and widely used by living organ- isms for camoufaging or signaling5. Blue pigments are rare with limited reports in the animal kingdom, such as blue crustacyanin in lobsters6,7 and blue-green bile pigments in butterfies8. Terefore, structural colorations con- tribute to most of brilliant observed in some butterfies9, birds3,10, and mammals2 and even in the dynamic color change in cephalopods11, which represent one of the most unique and sophisticated coloration systems in nature. Cephalopods, the animal group including octopus, cuttlefsh and squid, are masters at manipulating their body coloration and patterns for camoufage or intra-species communication12,13. Tis remarkable ability of ceph- alopods depends on their unique skin structure containing multiple types of cells located in diferent layers with diferent functions14. Pigment-containing sacs in chromatophores located in the dermal layer are innervated and partially responsible for color change over the range of a few seconds15. Structural color change mediated by refectin-formed insoluble platelets in iridophores or spheres in leucophores beneath chromatophores requires a longer amount of time1. Refectin, a protein family named because of its high-refractive index, is expressed exclusively in cephalo- pods16. Te photonic properties of refectin have attracted considerable attention and intensive investigations for a long time. Te structural colors in cephalopods are suggested by multi-layer interference when incident light refected by periodically layered refectin platelets in iridophores17,18, and several experimental validations have been carried out at protein19,20 and cellular levels18, however, the structural basis of the color change pro- cess remains unknown. Refectin also exhibits multifaceted features both in vivo and in vitro1,16,17,20–27. It forms ribbon-shaped, cup-shaped and discoidal platelets in iridophores of diferent species14; spheres with various diameters in leucophores23 and chromatophores in vivo28; and either globular “refectin bricks” from self-assembly or huge platelet structures with diferent shapes from higher order assembly in vitro26. Te plasticity and the mul- tifaceted features of platelet “Bragg refector” in cephalopods can be traced to the special amino acid composition in protein sequence16, extremely low solubility in biochemistry16,26, and remarkable assembly ability of the refec- tin protein structure17,20,25,26,29. However, the connection between the complexity of structural color change in cephalopods and the structural/assembly status change of refectin protein remains unresolved. 1State Key Laboratory of Membrane Biology, Laboratory of Molecular Biophysics, School of Life Sciences, Peking University, Beijing, 100871, China. 2Biodynamic Optical Imaging Center (BIOPIC), Beijing Advanced Innovation Center for Genomics (ICG), Peking-Tsinghua Center for Life Sciences, College of Engineering, and School of Life Sciences, Peking University, Beijing, China. 3Beijing Computational Science Research Center, The Chinese Academy of Engineering Physics, Beijing, 100084, China. Tiantian Cai and Kui Han contributed equally. Correspondence and requests for materials should be addressed to Y.H. (email: yanyi@pku.edu.cn) or C.X. (email: canxie@pku.edu.cn) SCIENTIFIC REPORTS | (2019) 9:5201 | https://doi.org/10.1038/s41598-019-41638-8 1 www.nature.com/scientificreports/ www.nature.com/scientificreports a b Marker Applying the (KDa) reflectin c d e 170 protein 130 100 70 Substrate 55 40 Rotating 35 fhg 25 Drying 15 Figure 1. Film formation and coloration of Refectin. (a), SDS-PAGE of purifed Refectin (SoRef2). (b) Schematic of the spin-coating process. (c) Negative-staining EM image of Refectin (SoRef2) particles with no aromatic molecules. (d,e) Negative-staining EM image of Refectin (SoRef2) incubated with imidazole (Refectin/Imidazole) (d) and histamine (Refectin/Histamine) (e). (f) Film generated by spin coating the Refectin sample in (c). (g) Film generated by spin coating the Refectin/Imidazole sample in (d). (h) Film generated by spin coating the Refectin/Histamine sample in (e). Te scale bars in (c–e) represent 100 nm, whereas the scale bars in (f–h) represent 2 mm. One method to circumvent the question is to reconstruct the dynamic color change process with purifed refectin protein in vitro, thereby reducing the complexity of biological processes into simple molecular interac- tions, thus the structural/assembly status and optical features can be monitored at the same time. Here, refectin protein was spin coated to form thin solid flms to mimic the platelet structures in cephalopod iridophores. Using this clean-cut approach, the dynamic structural color change process from colorless to white and blue was successfully regenerated in vitro. We demonstrated that the blue coloration (here named “cephalopod-blue”) was only produced by higher-order assembly of refectin proteins in the presence of aromatic triggers. Refection and transmission spectra indicate that “cephalopod-blue” is a result of wavelength-dependent light scattering, as confrmed by structural analysis of the refectin flms. Te results outlined here may facilitate further design of bio-inspired photonic materials, and may provide biological implications to our understanding of the structural color change in cephalopods. Results Coloration of refectin. Recombinant refectin proteins have been applied in numerous studies to fabri- cate refectin flms to elucidate the origin of refectin-based structural colors17,19,20,30,31. In these studies, refectin proteins were dissolved in organic solvents, such as hexafuoroisopropanol (HFIP) or trifuoroacetic acid (TFA), followed by fow coating or spin coating. Tin-flm interference was suggested as one possible mechanism of coloration. With multiple refectin proteins purifed in mild bufer solution (represented by SoRef2 from Sepia ofcinalis, EsRef1a from Euprymna scolopes and DpRefA2 from Doryteuthis pealeii), we previously demonstrated tightly regulated hierarchical assembly of refectin, and this ability was even preserved by its single domain26. To explore how the assembly status of refectin may afect structural coloration, we revisited the structure-optics rela- tionship by spin-coated refectin at diferent assembly stages. One refectin, SoRef2 (GenBank: HE687200.1), was expressed and purifed to homogeneity (Fig. 1a), incubated with or without aromatic reagents (imidazole and his- tamine), and then spin coated as described19 (Fig. 1b). In contrast to refectin proteins in organic solvents, refec- tin proteins were added to bufer solution to maintain their conformation. Transmission electron microscopes (TEM) were used to monitor refectin conformation and assembly status with various refectin/reagents ratios (Fig. S1). We observed hierarchical assembly as reported previously26. Both small globular particles (Fig. 1c) from self-assembly and huge platelet structures (Fig. 1d,e) from higher-order assembly in diferent refectin samples in the absence or presence of aromatic stimulus were observed, respectively. To our surprise, the appearance color of the flm depended on the treatment of refectin protein. Film formed by self-assembled refectin without aromatic molecules remains colorless (Fig. 1f), whereas flms formed by higher-order assembled refectin protein incubated with imidazole (Fig. 1g) and histamine (Fig. 1h) exhibit similar blue color. Multiple refectin genes exist in cephalopods, and diferent refectin proteins exhibit specifc distributions in vivo, which may suggest diferent roles in the structural color change process32. To investigate whether diferent refectin proteins are responsible for diferent optical features, two more representative refectin proteins from Sepia ofcinalis (SoRef1, GenBank: HE687199.1 and SoRef8, GenBank: HE687206.1) were selected based on sequence similarity analysis (Fig. S2a,b), expressed and purifed (Fig. S2c,h). Similar structures and assembly fea- tures compared with SoRef2 were confrmed by TEM with or without
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