VI. Applications of S-layers

Abstract

The wealth of information existing on the general principle of S-layers has revealed a broad application potential. The most relevant features exploited in applied S-layer research are: (i) pores passing through S-layers show identical size and morphology and are in the range of ultrafiltration membranes; (ii) functional groups on the surface and in the pores are aligned in well-defined positions and orientations and accessible for binding functional molecules in very precise fashion; (iii) isolated S-layer subunits from many organisms are capable of recrystallizing as closed monolayers onto solid supports at the air-water interface, on lipid monolayers or onto the surface of liposomes. Particularly their repetitive physicochemical properties down to the subnanometer scale make S-layers unique structures for functionalization of surfaces and interfaces down to the ultimate resolution limit. The following review focuses on selected applications in biotechnology, diagnostics, vaccine development, biomimetic membranes, supramolecular engineering and nanotechnology. Despite progress in the characterization of S-layers and the exploitation of S-layers for the applications described in this chapter, it is clear that the field lags behind others (e.g. enzyme engineering) in applying recent advances in protein engineering. Genetic modification and targeted chemical modification would allow several possibilities including the manipulation of pore permeation properties, the introduction of switches to open and close the pores, and the covalent attachment to surfaces or other macromolecules through defined sites on the S-layer protein. The application of protein engineering to S-layers will require the development of straightforward expression systems, the development of simple assays for assembly and function that are suitable for the rapid screening of numerous mutants and the acquisition of structural information at atomic resolution. Attention should be given to these areas in the coming years.

Biophysical Characterization of the Entire Bacterial Surface Layer Protein SbsB and Its Two Distinct Functional Domains

“…The intrinsic ability of isolated S-layer subunits to recrystallize into their original regular arrays either in suspension or on surfaces (e.g. noble metals, lipid films (4)) makes S-layer proteins unique building blocks in molecular nanobiotechnology and biomimetics (5)(6)(7)(8)(9). The mature S-layer protein SbsB (10) of Geobacillus stearothermophilus PV72/p2 (11) is processed from a 920-amino acid-long preprotein by cleavage of a 31-residue-long signal peptide and assembles into an oblique (p1) lattice type.…”

A temperature‐sensitive expression system based on the Geobacillus stearothermophilus NRS 2004/3a sgsE surface‐layer gene promoter

“…The biosynthesis of S-layers is a complex process in which the amount of protein synthesis, protein translocation through the cell wall and its incorporation into the existing S-layer lattice have to be co-ordinated with the growth rate of the bacterium and the synthesis of other cell-wall components [3]. Strong expression of S-layer genes generally seems to result from a combination of efficient transcription initiation and usually high mRNA stability [4][5][6][7]. All promoter regions described for S-layer genes have so far revealed the conserved hexanucleotide −10 and −35 regions typical of prokaryotic promoters.…”

Novel Bacterial Membrane Surface Display System Using Cell Wall-Less L-Forms of Proteus mirabilis and Escherichia coli

“…In gramnegative bacteria, outer membrane proteins (21), pili and flagella (26), modified lipoproteins (6,7,11), ice nucleation proteins (17,23), and autotransporters (19,27,35) have been used as anchors. In gram-positive bacteria, surface anchors have been derived from lipoproteins, cell wall proteins, or S-layer proteins (24,32,33). However, depending on the displayed protein and the desired application, each system has its own advantages and disadvantages, such as the size limitation of the displayed protein, mislocalization or formation of inclusion bodies, association with lipopolysaccharides (LPS), or destabilization of the outer membrane (10,17,22).…”