Composite S-layer lipid structures

Abstract

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state of the art survey how S-layer proteins, lipids, and polysaccharides may be used as basic building blocks for the assembly of S-layer supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas for application of composite S-layer membrane systems concern sensor systems involving specific membrane functions.

Fig. 1

Atomic force microscopical images the S-layer protein from Geobacillus stearothermophilus PV72/p2 exhibiting an oblique S-layer lattice (a) and the S-layer protein from Lysinibacillus sphaericus CCM 2177 exhibiting a square S-layer lattice (b). In the latter image crystalline patches forming the closed S-layer lattice are visible. The bars correspond to 50 nm. (c) Electron micrograph of a freeze-etched and Pt/C-shadowed preparation of a Gram-positive organism exhibiting a square S-layer lattice. The bar corresponds to 100 nm. Schematic drawing illustrating the various S-layer lattice types. In the oblique lattice (d), one morphological unit (dark grey) consists of one (p1) or two (p2) identical subunits. Four subunits constitute one morphological unit in the square (p4) lattice type (e), whereas the hexagonal lattice type is either composed of three (p3) or six (p6) subunits (f). Modified from Sleytr et al. (1999) with permission from Wiley–VCH.

Fig. 2

Supramolecular structure of an archaeal (a) and Gram-positive bacterial cell envelope (b). Schematic illustrations of various S-layer-supported lipid membranes. In (c), a folded or painted membrane spanning a Teflon aperture is shown. A closed S-layer lattice can be self-assembled on either one or both (not shown) sides of the lipid membranes. (d) A bilayer lipid membrane is generated across an orifice of a patch clamp pipette by the tip–dip method. Subsequently a closely attached S-layer lattice is formed on one side of the lipid membrane. (e) Schematic drawing of a lipid membrane generated on an S-layer ultrafiltration membrane (SUM). Optionally, an S-layer lattice can be attached on the external side of the SUM-supported lipid membrane (right part). (f) Schematic drawing of a solid support covered by a layer of modified secondary cell wall polymer (SCWP). Subsequently a closed S-layer lattice is assembled and bound via the specific interaction between S-layer protein and SCWP. On this biomimetic structure a lipid membranes is generated. As shown in (e), a closed S-layer lattice can be recrystallized on the external side of the solid supported lipid membrane (right part). (g) Schematic drawing of (1) an S-layer-coated emulsome (left part) and S-liposome (right part) with entrapped water-soluble (blue) or lipid-soluble (brown) functional molecules and (2) functionalized by reconstituted integral membrane proteins. S-layer-coated emulsomes and S-liposomes can be used as immobilization matrix for functional molecules (e.g. IgG) either by direct binding (3), by immobilization via the Fc-specific ligand protein A (4), or biotinylated ligands can be bound to S-layer-coated emulsome and S-liposome via the biotin–streptavidin system (5). Alternatively, emulsomes and liposomes can be coated with S layer fusion proteins incorporating functional domains (6). Modified after Sleytr et al., 2002, Copyright (2002) and Sleytr et al., 2004, Copyright (2004), with permission from Wiley–VCH.

Fig. 3

Chemical structures of (a) the phospholipid 1,2-diphytanoyl-sn-glycero-3-phosphatidylcholine, (b) the membrane-spanning tetraetherlipids glycerol dialkyl nonitol tetraetherlipid (GDNT) extracted and purified from Sulfolobus and Metallosphaera archaea, and (c) the Main Phospholipid (MPL) isolated from Thermoplasma acidophilum.

Fig. 4

Schematic illustration of the sandwich structure of an S-layer supported lipid membrane with an additional S-layer cover (not drawn to scale). (a) The S-layer lattices may show a congruent coverage resulting in opposing immobilized phospholipids (left side). In the case of the tetraetherlipids, the molecules are immobilized on both polar lipid head groups (right side). The immobilized lipid head groups are drawn in black, whereas the free-lipid head groups are drawn in white. In (b), the two S-layer lattices sandwiching the membrane are displaced. In such an arrangement, the immobilized phospholipids do not oppose each other (left side) and the tetraetherlipids are now anchored to the S-layer just by one lipid head group (right side). Please consider this is only a two-dimensional schematic drawing showing a cut where the lipid head groups interact with binding domains on the S-layer proteins. Modified after Schuster and Sleytr, 2006, Copyright (2006), with permission from Bentham Science Publishers Ltd.

Fig. 5

Schematic drawing of a lipid membrane on an S-layer (yellow) covered porous or solid support (grey). The lipid membrane (black) is bound by incorporated tethered lipid molecules (magenta) reaching through the S-layer pores to the substrate. In this lipid membrane, transmembrane proteins (blue) can be reconstituted. Finally, a further proteinaceous lattice composed of S-layer glycoproteins (with carbohydrate moieties in green) can be recrystallized on the top via SCWP-linked phospholipids (orange) which are anchored in the outer leaflet of the lipid membrane.