Silicon Transporters: From Membrane Proteins to Nanotechnology

Diatoms are abundant and diverse single-celled algae that are sheathed in a silica-coated outer cell wall. This structure, known as the frustule, forms hierarchical silica structures which are patterned with nanoscale precision at ambient pH and temperature. In order to form these frustules diatoms must source silicon from their environment. It is thought that this is done through membrane proteins that specifically interact with silicon and are known as silicon transporters (SITs). When the SITs are energised by an electrochemical sodium gradient they transport silicic acid (soluble silicon) into the diatom cell, where it undergoes polymerisation to form silica. This work has focused on the reconstitution of a recombinant SIT from the diatom Thalassiosira pseudonana into synthetic liposomes in order to replicate this biomineralization process. The SIT is recombinantly expressed and purified before being reconstitutedinto liposomes that contain biomineralizing peptides. The transporter is then energised by applying a transmembrane sodium gradient so that it actively pumps silicic acid from the outside to the inside of the proteoliposome. The small internal volume of the proteoliposome should cause the transported silicic acid to rapidly approach saturation; under these conditions the biomineralizing peptides should catalyze and template the formation of nanostructured silica. This novel method incorporates many of the principles of diatom biomineralization such as encapsulation and active transport, that are absent in bulk silicification experiments. Further work will attempt to synthesise novel silica structures by adjusting reaction conditions or functionalising the silica products by trapping fluorophores, enzymes and other reactive species within the silica matrix.