CBMS Lecture Series

"A Novel Cu(I) Center Mediates Self-Assembly of Photonic Structures from Protein in Cephalopods" Abstract: The system of reflectin proteins in cephalopods may be the only natural example of self-assembly of biophotonic function from a conserved protein sequence; most other familiar examples of photonics in nature such as iridescent butterfly wing scales and bird feathers result from phase separation of polysaccharides in the extracellular matrix. These proteins are defined by the presence of a methionine- and aspartate-rich motif of roughly 25 amino acids. Reflectin structure-function properties remain elusive: what is the physical relationship bewteen the proteins' serially repeated, highly conserved, well-defined secondary structure and its evident ability to assemble into a broad set of precisely controlled, optically resonant, nanometer-scale arrays? Here we show that native reflectin tissue is enriched in several transition metal ions, including Cu, and native protein can bind additional Cu and Ni in the presence of EDTA and strong base. Our thermodynamic data show that two synthetic reflectin motif peptides representing different conserved region of the evolved protein bind Cu(I) ions with a dissociation constant similar to that of a strongly binding Mets motif. Further, both peptides undergo large structural shifts upon Cu(I) binding. When the synthetic peptide from the central regions of the native protein sequence binds Cu(I) it undergoes assembly with a stoichiometry dictated by the Cu(I) to peptide ratio of the system. In contrast, the peptide representing the N-terminus of the protein forms trimers in the presence of Cu(I). We conclude that a core evolved function of the reflectin domains is a geometrically and thermodynamically specific assembly via Cu(I). This peptide-metal assembly results in a volume-spanning gel whose properties may inform our understanding of the emergence of optically resonant structures from the in vivo assemblage of reflectin protein.