S-layer stabilized lipid membranes (Review)

Abstract

The present review focuses on a unique bio-molecular construction kit based on surface-layer (S-layer) proteins as building blocks and patterning elements, but also major classes of biological molecules such as lipids, membrane-active peptides and membrane proteins, and glycans for the design of functional supported lipid membranes. The biomimetic approach copying the supramolecular building principle of most archaeal cell envelopes merely composed of a plasma membrane and a closely associated S-layer lattice has resulted in robust and fluid lipid membranes. Most importantly, S-layer supported lipid membranes spanning an aperture or generated on solid and porous substrates constitute highly interesting model membranes for the reconstitution of responsive transmembrane proteins and membrane-active peptides. This is of particular challenge as one-third of all proteins are membrane proteins such as pore-forming proteins, ion channels, and receptors. S-layer supported lipid membranes are seen as one of the most innovative strategies in membrane protein-based nanobiotechnology with potential applications that range from pharmaceutical (high-throughput) drug screening over lipid chips to the detection of biological warfare agents.

FIG.1

In (a), freeze-etching preparation of a whole cell of Bacillus sphaericus with a square S-layer lattice is shown. Bar corresponds to 200 nm. Schematic illustration of the supramolecular architecture of the three major classes of prokaryotic cell envelopes containing crystalline bacterial cell surface layers (S-layers). (b) Cell envelope structure of gram-negative archaea with S-layers as the only component external to the cytoplasmic membrane. (c) Cell envelope as observed in gram-positive archaea and bacteria. In bacteria the rigid wall component is primarily composed of peptidoglycan. In archaea other wall polymers (e.g., pseudomurein) are found. (d) Cell envelope profile of gram-negative bacteria composed of a thin peptidoglycan layer and an outer membrane. If present, the S-layer is closely associated with the lipopolysaccharide of the outer membrane. Modified after Ref. . Copyright 1999 Reprinted with permission from Wiley-VCH.

FIG. 2

Schematic drawings of possible S-layer lattice types. Owning to the chirality of proteins, space group symmetries with mirror-reflection lines or glide-reflection lines are not possible in S-layer lattices. Modified after Ref. (Bacterial surface layer proteins: a simple versatile biological self-assembly system in nature; Fig. 2). Copyright 2006 with kind permission of Springer Science and Business Media.

FIG. 3

Schematic illustration of (A) an archaeal cell envelope structure composed of the cytoplasmic membrane with integral membrane proteins and a S-layer lattice, integrated into the cytoplasmic membrane. Using this supramolecular construction principle, biomimetic membranes can be generated (B,C). The cytoplasmic membrane is replaced by a phospholipid bilayer (B) or a tetraetherlipid monolayer (C) and S-layer proteins derived from Bacillaceae are recrystallized to form a closed lattice on the lipid film. Subsequently integral model membrane proteins can be reconstituted into the S-layer-supported lipid membrane. As indicated in (B) and (C), a second S-layer lattice may be recrystallized on the top to stabilize the layered architecture and provide a nanoporous filter function. Modified after Ref. . Copyright (2003) with permission from Wiley-VCH.

FIG. 4

Schematic illustration of the composite S-layer/GDNT-monolayer structure. The chemical structure of the glycerol dialkyl nonitol tetraetherlipid (GDNT) molecules is shown in the top of the figure. The black colored GDNT molecules represent the most favored lipids in the GDNT monolayer whose associated head groups may interact with defined domains in the S-layer lattice (not drawn to scale). Reprinted from Ref. . Copyright 1998 Reprinted with permission from Elsevier.

FIG. 5

SFM images (deflection mode) of (a) the S-layer protein SbpA from Bacillus sphaericus CCM 2177 recrystallized on a QCM-D gold-coated sensor surface (image size 800×800 nm²) and (b) on a SCWP-covered gold-coated sensor surface (image size 700×700 nm²).

FIG. 6

Increase in mass vs time upon the adsorption/binding and recrystallization of the S-layer protein SbpA on a gold-covered QCM-D sensor (blue line) and on a SCWP-coated gold-covered QCM-D sensor (green line).

FIG. 7

Schematic illustrations of various S-layer-supported lipid membranes. (a) S-layer protein has been recrystallized from the aqueous phase on a phospholipid monolayer. In (b), a folded or painted membrane has been generated to span a Teflon aperture. Subsequently S-layer protein can be injected into one or both compartments whereby the protein self-assembles to form closely attached S-layer lattices on the BLMs. (c) On a SUM a BLM can be generated by a modified LB technique. As a further option, a closed S-layer lattice can be attached on the external side of the SUM-supported BLM (left part). (d) Solid supports can be covered by a closed S-layer lattice and subsequently BLMs can be generated using combinations of the LB and Langmuir–Schaefer techniques and vesicle fusion. As shown in (c), a closed S-layer lattice can be recrystallized on the external side of the solid supported BLM (left part). Modified after Ref. . Copyright 2004 Reprinted with permission from Wiley-VCH.