The Si-tag for immobilizing proteins on a silica surface.

Targeting functional proteins to specific sites on a silicon device is essential for the development of new biosensors and supramolecular assemblies. Using intracellular lysates of several bacterial strains, we found that ribosomal protein L2 binds tightly to silicon particles, which have surfaces that are oxidized to silica. A fusion of E. coli L2 and green fluorescence protein adsorbed to the silica particles with a K(d) of 0.7 nM at pH 7.5 and also adsorbed to glass slides. This fusion protein was retained on the glass slide even after washing for 24 h with a buffer containing 1 M NaCl. We mapped the silica-binding domains of E. coli L2 to amino acids 1-60 and 203-273. These two regions seemed to cooperatively mediate the strong silica-binding characteristics of L2. A fusion of L2 and firefly luciferase also adsorbed on the glass slide. This L2 silica-binding tag, which we call the "Si-tag," can be used for one-step targeting of functional proteins on silica surfaces.