S-layers are regular crystalline surface layers in pro-karyotic organisms composed of protein or glycoprotein subunits (Beveridge, 1994; Sleytr 1993, 1988a, 1986b; Sleytr & Messner, 1992, 1988a, b, 1983; Beveridge & Graham, 1991; Hovmöller 1988; Koval, 1988; Baumeister & Engelhardt, 1987). Most of the presently known S-layers are composed of identical proteins or glycoproteins of molecular mass 30-220 kDa (Sleytr 1994; Messner & Sleytr, 1992). Such layers have been recognized as common features of both prokaryote domains (archaeobacteria and eubacteria). S-layers can be associated with quite different cell envelope structures, such as peptidoglycan, pseudo-murein or components of the outer plasma membrane (Fig. 1). In Gram-negative eubacteria (e.g. ), S-layers are associated with the outer membrane and in some archaeobacteria (e.g. ), S-layers are the sole cell wall structure and therefore associated only with the plasma membrane. The subunits are linked together and also to the underlying cell envelope layers by non-covalent forces (Messner & Sleytr, 1992; Beveridge & Graham, 1991; König, 1988). S-layers can be exclusive wall components in cell envelope structures of Gram-negative archaeobacteria (Sleytr & Messner, 1992). Most of the presently known S-layers are composed of a single (glyco)protein species endowed with the ability to assemble into two-dimensional arrays on a supporting envelope layer, and they are characterized by defined symmetry and pores of uniform size (Hovmöller 1988; Sleytr & Messner, 1983). When S-layers are compared with heteroporous materials, they can be considered as an ideal matrix (e.g. and species), because functional groups are not only aligned in high density on the S-layer surface but also show identical position and orientation on each protomer (Sára 1993a, b). On the outer surface of S-layers from most members of the family an equimolar amount of amino and carboxyl groups is present, and in the case of S-layer glycoproteins there is also a high number of surface-located hydroxyl groups available (Sleytr 1994). Comparative analysis of amino acid and genetic studies on S-layers showed that crystalline arrays are usually composed of weakly acidic proteins at neutral pH. The content of hydrophobic amino acids is generally high and the cysteine or methionine content low (Sleytr 1993). S-layers are porous crystalline membranes 5-15 nm thick (Sleytr 1994), which completely cover the cell surface and can provide micro-organisms with a selective advantage by functioning as protective coats, molecular sieves, molecule and ion traps, and structures involved in cell adhesion and surface recognition (Sleytr 1993; Messner & Sleytr, 1992; Hovmöller 1988; Sleytr & Messner, 1983). S-layers were identified as contributing to virulence when present as a structural component of pathogens and possess great potential for various biotechnological, biomedical and non-bio-medical applications (Sleytr 1993).


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