Center for Ultrastructure Research, Ludwig Boltzmann Institute for Molecular Nanotechnology, University of Agricultural Sciences, Gregor-Mendel-Str. 33, A-1180 Vienna, Austria
Crystalline bacterial cell surface (S-layer) proteins represent the outermost envelope component in many bacteria and archaea. S-layers are composed of a single protein or glycoprotein species, which assemble into lattices displaying either oblique, square or hexagonal symmetry. Isolated S-layer subunits frequently recrystallize into monomolecular lattices on solid supports, such as gold chips, silicon wafers or plastics. The structure-function-relationship of distinct segments of selected S-layer proteins was investigated by producing N- and C-terminally truncated forms. Amino acid positions that were found to be located on the outer S-layer surface were exploited as fusion sites for functional peptide sequences, such as the Fc-binding ZZ-domain, core streptavidin or the major birch pollen allergen. By exploiting the specific interactions between the N-terminal part of the S-layer protein moiety and the secondary cell wall polymer as the natural anchoring molecule in the bacterial cell wall, oriented binding of the S-layer fusion protein was achieved on solid supports precoated with chemically modified secondary cell wall polymer. Thereby, the functional sequence remained located on the outer S-layer surface and accessible for binding of target molecules. Due to the crystalline structure of S-layer lattices, fused sequences show a well-defined distance down to the sub-nanometer length scale.