Dissection of the structure-forming activity from the structure-guiding activity of silicatein: a biomimetic molecular approach to print optical fibers
Silicateins, a group of proteins forming the proteinaceous axial filaments of the inorganic biosilica spicules of the siliceous sponges, are unique in their dual function to exhibit both structure-guiding (providing the structural platform for the biosilica product) and structure-forming activities (enzymatic function: biosilica synthesis from ortho-silicate). The primary translation product of the silicatein gene comprises a signal peptide, a pro-peptide and, separated by an autocatalytic cleavage site glutamine/aspartic acid [Q/D], the sequence of the mature silicatein protein. In order to dissect the biocatalytic, structure-forming activity of silicatein from its structure-guiding function, two mutated genes were constructed based on the silicatein-α gene of the demosponge Suberites domuncula. (i) A gene encoding for a non-processed silicatein that was mutated, by replacing Q/D [Gln (Q)/Asp (D)] by Q/Q, at the cleavage site within the primary translation product between the pro-peptide and the mature enzyme of wild type silicatein. (ii) A gene encoding for a mature enzymatically-active silicatein in which the S-stretch was replaced by a Q-stretch. The enzymatic activity of the mutated protein was significantly enhanced in the presence of the sponge-specific silicatein-interacting protein, silintaphin-1. The two recombinant proteins were applied for micro-contact printing. Using this technique, parallel layers (diameter 10 μm) of the enzymatically inactive, non-processed silicatein were printed onto a gold surface and used as a structure-guiding template for coating with the soluble enzymatically active silicatein. The experiments revealed that after enzymatic reaction with an ortho-silicate substrate a biosilica mantle is formed that can act as a light waveguide.