Structure, Self-Assembly, and Properties of a Truncated Reflectin Variant
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Structure, self-assembly, and properties of a truncated reflectin variant Mehran J. Umerania,1, Preeta Pratakshyab,1, Atrouli Chatterjeec, Juana A. Cerna Sanchezd, Ho Shin Kime, Gregor Ilcf, Matic Kovaciˇ cˇf, Christophe Magnang, Benedetta Marmirolih, Barbara Sartorih, Albert L. Kwansae, Helen Orinsc, Andrew W. Bartlettc, Erica M. Leungc, Zhijing Fenga, Kyle L. Naughtoni, Brenna Norton-Bakerb, Long Phana, James Longc, Alex Allevatoa, Jessica E. Leal-Cruza, Qiyin Linj, Pierre Baldig, Sigrid Bernstorffk, Janez Plavecf, Yaroslava G. Yinglinge, and Alon A. Gorodetskya,b,c,2 aDepartment of Materials Science and Engineering, University of California, Irvine, CA 92697; bDepartment of Chemistry, University of California, Irvine, CA 92697; cDepartment of Chemical and Biomolecular Engineering, University of California, Irvine, CA 92697; dDepartment of Molecular Biology and Biochemistry, University of California, Irvine, CA 92697; eDepartment of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695; fSlovenian NMR Centre, National Institute of Chemistry, 1000 Ljubljana, Slovenia; gDepartment of Computer Science, University of California, Irvine, CA 92697; hInstitute of Inorganic Chemistry, Graz University of Technology, 8010 Graz, Austria; iDepartment of Physics and Astronomy, University of California, Irvine, CA 92697; jIrvine Materials Research Institute, University of California, Irvine, CA 92697; and kElettra–Sincrotrone Trieste, 34149 Trieste, Italy Edited by Jacqueline K. Barton, California Institute of Technology, Pasadena, CA, and approved October 7, 2020 (received for review May 8, 2020) Naturally occurring and recombinant protein-based materials are Moreover, in vitro, films processed from squid reflectins not only frequently employed for the study of fundamental biological pro- exhibit proton conductivities on par with some state-of-the-art cesses and are often leveraged for applications in areas as diverse artificial materials (23–27) but also support the growth of murine as electronics, optics, bioengineering, medicine, and even fashion. and human neural stem cells (28, 29). Additionally, morphologi- Within this context, unique structural proteins known as reflectins cally variable coatings assembled from different reflectin isoforms have recently attracted substantial attention due to their key roles can enable the functionality of chemically and electrically actuated in the fascinating color-changing capabilities of cephalopods and color-changing devices, dynamic near-infrared camouflage plat- their technological potential as biophotonic and bioelectronic ma- forms, and stimuli-responsive photonic architectures (27, 30–34). terials. However, progress toward understanding reflectins has When considered together, these discoveries and demonstrations BIOPHYSICS AND COMPUTATIONAL BIOLOGY been hindered by their atypical aromatic and charged residue- constitute compelling motivation for the continued exploration of enriched sequences, extreme sensitivities to subtle changes in en- reflectins as model biomaterials. vironmental conditions, and well-known propensities for aggrega- Given reflectins’ demonstrated significance from both funda- tion. Herein, we elucidate the structure of a reflectin variant at the molecular level, demonstrate a straightforward mechanical mental biology and applications perspectives, some research ef- agitation-based methodology for controlling this variant’s hierar- fort has been devoted to resolving their three-dimensional (3D) chical assembly, and establish a direct correlation between the protein’s structural characteristics and intrinsic optical properties. Significance BIOCHEMISTRY Altogether, our findings address multiple challenges associ- ated with the development of reflectins as materials, furnish The investigation of protein-based materials has provided a molecular-level insight into the mechanistic underpinnings of better understanding of living systems and has led to the de- cephalopod skin cells’ color-changing functionalities, and may in- velopment of ubiquitous modern technologies. Within this form new research directions across biochemistry, cellular biology, context, unique cephalopod proteins called reflectins have bioengineering, and optics. exhibited promise for biophotonics and bioelectronics appli- cations, but the exploration of reflectins as materials has been reflectin | self-assembly | proteins | biomaterials | optical properties hindered by an incomplete understanding of their structures and properties. Here, we resolve the molecular-level structure aterials from naturally occurring and recombinant proteins of a model reflectin variant, establish a straightforward ap- Mare frequently employed for the study of fundamental bi- proach to controlling the assembly of this protein, and describe ological processes and leveraged for applications in fields as di- a correlation between its structural characteristics and light- verse as electronics, optics, bioengineering, medicine, and fashion manipulating properties. Taken together, our findings advance (1–13). Such broad utility is enabled by the numerous advanta- current understanding of reflectin-based materials, provide in- geous characteristics of protein-based materials, which include se- sight into the color-changing capabilities of cephalopods, and quence modularity, controllable self-assembly, stimuli-responsiveness, afford new opportunities in biochemistry, cellular biology, bio- straightforward processability, inherent biological compatibility, engineering, and optics. and customizable functionality (1–13). Within this context, unique Author contributions: A.A.G. designed research; M.J.U., P.P., A.C., J.A.C.S., H.S.K., G.I., structural proteins known as reflectins have recently attracted M.K., C.M., B.M., B.S., A.W.B., E.M.L., Z.F., K.L.N., B.N.-B., L.P., J.L., A.A., J.E.L.-C., Q.L., substantial attention because of their key roles in the fascinating P.B., S.B., J.P., and Y.G.Y. performed research; P.B., S.B., J.P., and Y.G.Y. contributed color-changing capabilities of cephalopods, such as the squid new methods/analytic tools; M.J.U., P.P., A.C., J.A.C.S., H.S.K., G.I., M.K., C.M., B.M., B.S., shown in Fig. 1A, and have furthermore demonstrated their utility A.L.K., H.O., and A.A.G. analyzed data; and M.J.U., P.P., A.C., and A.A.G. wrote the paper. for unconventional biophotonic and bioelectronic technologies The authors declare no competing interest. (11–40). For example, in vivo, Bragg stack-like ultrastructures This article is a PNAS Direct Submission. from reflectin-based high refractive index lamellae (membrane- This open access article is distributed under Creative Commons Attribution-NonCommercial- enclosed platelets) are responsible for the angle-dependent nar- NoDerivatives License 4.0 (CC BY-NC-ND). rowband reflectance (iridescence) of squid iridophores, as shown 1M.J.U. and P.P. contributed equally to this work. in Fig. 1B (15–20). Analogously, folded membranes containing 2To whom correspondence may be addressed. Email: [email protected]. distributed reflectin-based particle arrangements within sheath This article contains supporting information online at https://www.pnas.org/lookup/suppl/ cells lead to the mechanically actuated iridescence of squid doi:10.1073/pnas.2009044117/-/DCSupplemental. chromatophore organs, as shown in Fig. 1C (15, 16, 21, 22). First published December 15, 2020. www.pnas.org/cgi/doi/10.1073/pnas.2009044117 PNAS | December 29, 2020 | vol. 117 | no. 52 | 32891–32901 Downloaded by guest on September 24, 2021 AB Reflectin Incident Reflected C Platelet Light Light Membrane Reflectin Reflected Particles Light Sheath Incident Cell Light Radial muscle DFE 510152025 510152025 5101520 G 1 MNRYLNRQRLYNMYRNKYRGV MEPMSRMTMDFQGRYMDSQGRMVDPRYY DYYGRMHDHDRYYGR 64 H 65 SMFNQGHSMDSQRYGGWMDN PERYMDMSGYQMDMQGRWMDAQGRFNNP FGQMWHGRQGHY 124 125 PGYMSSHSMYGRNMYNPYHSHYASRHFDS PERWMDMSGYQMDMQGRWMDNYGRYVNP FNHHM 186 Select Sequence 187 HEMHRNYYHNGYPYCMNRGY PERYMDMSGYQMDMQGRWMDTHGRHCNP GPYYGHRNHWMQGF 248 249 HPHGRNMFQ PERWMDMSGYQMDMQGRWMDNYGRYVNP FSHNYGRHMNYPGGHYNYHHGRYM 309 310 NH PERHMDMSSYQMDMHGRWMDNQGRYIDNF DRNYYDYHMY 350 Fig. 1. (A) A camera image of a D. pealeii squid for which the skin contains light-reflecting cells called iridophores (bright spots) and pigmented organs called chromatophores (colored spots). Image credit: Roger T. Hanlon (photographer). (B) An illustration of an iridophore (Left), which shows internal Bragg stack- like ultrastructures from reflectin-based lamellae (i.e., membrane-enclosed platelets) (Inset). (C) An illustration of a chromatophore organ (Left), which shows arrangements of reflectin-based particles within the sheath cells (Inset). (D) The logo of the 28-residue-long N-terminal motif (RMN), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (E) The logo of the 28-residue-long internal motif (RMI), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (F) The logo of the 21-residue-long C-terminal motif (RMC), which depicts the constituent amino acids (Upper) and their predicted secondary structures (Lower). (G) The amino acid sequence of full-length D. pealeii reflectin A1, which contains a single RMN motif (gray oval) and five RMI motifs (orange ovals). (H) An illustration of the selection of the prototypical truncated reflectin variant (denoted as RfA1TV) from full-length D. pealeii reflectin A1. structures (30, 31, 35–39). For example,