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S-layer

Molecular nanotechnology, especially nanobiotechnology starts to use and modify functionalize surfaces. Especially the immobilization of self-assembling biomolecules draws an increasing attention. The advantages of using immobilized enzymes in well-defined positions on nano-structured surfaces may even be greater. Self-assembly is an organization of molecules into defined structures, lowering the free energy of the system. Interaction between the molecules is non-covalent (e.g. hydrophobic-hydrophobic, van der Waals forces, molecular stacking ([http://onlinelibrary.wiley.com/doi/10.1002/smll.200700200/abstract Schäffer et al., 2007]).

Many biomolecules such as protein, polysaccharides and lipid have the ability to self-assemble into different shapes (e.g. spherical, rod- or sheet-like shapes), allowing several specific functions as virus capsids, cytoskeleton components or extracellular surface layer protein. The so-called paracrystalline cell surface-layer (S-layer) are build up on S-layer proteins and are one of the most common surface structures in bacteria and archaea. They are regarded as the outmost cell envelope of prokaryotic organisms ([http://onlinelibrary.wiley.com/doi/10.1111/j.1574-6968.2006.00573.x/full Sleytr et al., 2007]).

S-layer in general

S-layer proteins fulfill various functions as molecular sieves, ion traps and protective coats ([http://onlinelibrary.wiley.com/doi/10.1002/3527600035.bpol7011/abstract Sleytr et al., 2005]). They build up periodic structures, posses pores of identical size and morphology and show equal physicochemical properties on each molecular unit. Isolated they have the special ability to reassemble into two-dimensional crystals equal to structures found on intact bacterial cells. S-layer are mainly composed of a single (glyco)protein species, assembled into a layer completely covering the cell. In organisms the may represent up to 20% of the total protein content of a bacterial cell. Most S-layer proteins are weak acidic (pI 4-6) and contain a high proportion of hydrophobic amino acids as well as few or no sulphur-containing amino acids, their molecular mass varies between 40 – 200 kDa and is often strain-specific. The assembled S-layer lattices are generally 5 – 20 nm thick, in archaea lattices are up to 70 nm thick. The protein subunits of S-layer are arranged in lattices with different symmetry; olique (p1, p2), square (p4) or hexagonal (p3, p6) with a center-to-center spacing of the subunits of 3 – 35 nm. S-layer are highly porous with a porosity of 30 – 70% ([http://onlinelibrary.wiley.com/doi/10.1111/j.1742-4658.2006.05606.x/abstract Sleytr et al., 2007]).

Various S-layer proteins from archaea and eubacteria are glycosylated, with strain-specific modifications. S-layer proteins were the first prokaryotic proteins that were shown to exhibit this remarkable characteristic. Up until now glycosylation has been proven for several archaeal S-layer proteins. Among the bacterial species glycosylation was demonstrated only for S-layer proteins of Bacillaceae ([http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2958.1997.d01-1868.x/abstract Chami et al., 1997], [http://onlinelibrary.wiley.com/doi/10.1002/smll.200700200/abstract Schäffer et al., 2007]).

In gram-negative archaea, S-layer are the exclusive cell wall component. In gram-positive bacteria and archaea, S-layer assemble on the outmost part of a firm wall matrix, which is composed mostly of peptidoglycan, pseudomurein. In gram-negative bacteria, S-layer are linked to specific lipopolysaccharides (LPS) ([http://onlinelibrary.wiley.com/doi/10.1002/3527600035.bpol7011/abstract Sleytr et al., 2005]). For gram-positive bacteria a cell-wall-targeting domain could be identified at the N-terminal end of many S-layer proteins. The domain facilitates binding to a specific secondary cell wall polymer (SCWP) by a lectin-type binding ([http://onlinelibrary.wiley.com/doi/10.1111/j.1742-4658.2006.05606.x/abstract Sleytr et al., 2007]). It was found that some S-layer proteins consist of two distinct domains with different functions. One domain ist involved in the assembly with other S-layer protein monomers and the other domain mediates the interaction with the cell wall. Several SLH domains have been identified at the amino-terminal region of different S-layer proteins and at the carboxy-terminal region of cell-assiociated exoroteins. The domain may be repeated within the sequence and is involved in anchoring the S-layer proteins to the cell surface ([http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2958.1997.d01-1868.x/abstract Chami et al., 1997]).

The supramolecular structure as well as the mechanism of binding the outmost cell wall vary between S-layers of different species, leading to the development different isolation procedures. S-layers normally are attached to the cell wall through non-covalent binding, and can therefore be isolated in dissociating agents (e.g. lithium chloride), metal-chelating agents (e.g. ethylendiaminetetraacetic acid EDTA) or chaotropic denaturants such as urea or guanidine hydrochloride ([http://onlinelibrary.wiley.com/doi/10.1002/3527600035.bpol7011/abstract Sleytr et al., 2005]).