Self-assembly of the S-layer protein of Sporosarcina ureae ATCC 13881

Increasing the integration density of electron device components will necessitate the use of new nanofabrication paradigms that complement and extend existing technologies. One potential approach to overcome the current limitations of electron-beam lithography may involve the use of hybrid systems, in which existing lithographic techniques are coupled with “bottom up” approaches such as supramolecular self-assembly. In this respect, biological systems offer some unique possibilities as they combine both self-organization and spatial patterning at the nanometer length scale. In particular, Surface Layer Proteins (S-layers) can facilitate high order organization and specific orientation of inorganic structures as they are two-dimensional porous crystalline membranes with regular structure at the nanometer scale. In this framework, the aim of the present work was the characterization of the S-layer of Sporosarcina ureae ATCC 13881 (SslA) with respect to its self-assembling properties and modification that would allow it to be employed as a patterning element and a new building block for nanobiotechnology. In vitro recrystallization experiments have shown that wild type SslA self-assembles into monolayers, multilayers or tubes. Factors such as initial monomer concentration, Ca2+ ions, pH of the recrystallization buffer and the presence of a silicon substrate have a strong influence on the recrystallization process. SslA monolayers proved to be an excellent biotemplate for ordered assembly of gold nanoparticle arrays. The recombinant SslA after expression and purification formed micrometer sized, crystalline monolayers exhibiting the same lattice structure as the wild type protein (p4 symmetry). This remarkable property of self-assembling has been preserved even when SslA was truncated. The deletion of both, N- and C-terminal SslA domains does not hinder self-assembly; the resulting protein is able to form extended monolayers that exhibit the p4 lattice symmetry. The central SslA-domain is self sufficient for the self-assembly. The possibility to change the natural properties of S-layers by genetic engineering techniques opens a new horizon for the tuning of their structural and functional features. The SslA-streptavidin fusion protein combines the remarkable property of self-assembling with the ligand i.e. biotin binding function. On silicon wafers, this chimeric protein recrystallized into coherent protein layers and exposes streptavidin, fact demonstrated by binding studies using biotinylated quantum dots. In this way, it can serve as a functional surface for controlled immobilization of biologically active molecules but also as a platform for the synthesis of planar arrays of quantum dots. Furthermore, the results open up exciting possibilities for construction of hybrid S-layers, structures that may ultimately promote the fabrication of miniaturized, nanosized electronic devices