Self-assembly offers striking advantages in achieving perfect positional control at the molecular level since the replication of a single constituent subunit defines the properties of a whole array. Crystalline bacterial cell Surface layer (S-layer) proteins  have been optimized during billions of years of biological evolution as one of the simplest biological self-assembly systems with the appealing properties to be perfectly suited as basic building blocks in a biomolecular construction kit. S-layer proteins have the intrinsic property to reassemble into two-dimensional arrays at a broad spectrum of surfaces (e.g. silicon wafers) and interfaces (e.g. planar lipid films), (ii) functional groups are repeated with the periodicity of the S-layer lattice at a distance of approximately 10nm, leading to (iii) regular arrays of bound functional molecules and particles.
S-layer proteins (40,000 to 200,000 Mw) form the outermost cell envelope component of a broad spectrum of eubacteria and archaea. In general, S-layer proteins are isolated from cells by hydrogen-bond breaking agents. The information of building up two-dimensional arrays resides in the amino acid sequence of the S-layer protein itself. S-layers exhibit either oblique, square or hexagonal lattice symmetry with unit cell dimensions in the range of 3 to 30nm. S-layers are generally 5 to 10 nm thick and show pores of identical size (diameter, 2 to 8nm) and morphology.
Microlithographic procedures using deep ultraviolet laser radiation have already been used for patterning S-protein monolayers on silicon surfaces. Upon irradiation with laser pulses the protein layer could be completely removed in the exposed areas but retained its structural and functional integrity in the unexposed regions. Although the wavelength of the laser radiation imposes a resolution limit in the 100nm range, this technique is still of great technological importance since it allows to establish an interface between the nano- and micrometer world.
Electron microscopical studies have already shown that S-layers are ideal matrices for immobilizing functional molecules (e.g. enzymes, antibodies) in a well defined way since functional groups are located at identical positions at each individual protein subunit and replicated with the periodicity of the crystalline array. Molecules can be bound in dense monomolecular crystalline packing either by non-covalent forces or by covalent bonds. Genetically modified S-layer proteins incorporating functional domains are currently selected for reassembly on silicon wafers in order to provide higher specificity in the binding of functionalized molecules and particles.
S-layers can also be used to induce the formation of inorganic nanocrystal superlattices (e.g. CdS, Au, Ni, Pt, or Pd) with a broad range of particle sizes (5 to 15nm in diameter), interparticle spacings (up to 30nm) and lattice symmetries (oblique, square or hexagonal) as required for molecular electronics and non-linear optics.
This contribution will summarize the characteristic properties of S-layer proteins as building blocks in a biomolecular construction kit with particular attention to applications in molecular nanotechnology.
Sleytr, U. B., Messner, P., Pum, D., Sára, M. 1999. Angew.Chemie Int. Ed. 38:1034-1054.
Univ.Prof. Dr. Dietmar Pum
Ctr Ultrastructure Research, Universität für Bodenkultur Wien
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