S-Layer Protein-Based Biosensors

Abstract

The present paper highlights the application of bacterial surface (S-) layer proteins as versatile components for the fabrication of biosensors. One technologically relevant feature of S-layer proteins is their ability to self-assemble on many surfaces and interfaces to form a crystalline two-dimensional (2D) protein lattice. The S-layer lattice on the surface of a biosensor becomes part of the interface architecture linking the bioreceptor to the transducer interface, which may cause signal amplification. The S-layer lattice as ultrathin, highly porous structure with functional groups in a well-defined special distribution and orientation and an overall anti-fouling characteristics can significantly raise the limit in terms of variety and the ease of bioreceptor immobilization, compactness of bioreceptor molecule arrangement, sensitivity, specificity, and detection limit for many types of biosensors. The present paper discusses and summarizes examples for the successful implementation of S-layer lattices on biosensor surfaces in order to give a comprehensive overview on the application potential of these bioinspired S-layer protein-based biosensors.

Keywords: S-layer protein; biomimetics; bioreceptor; biosensor; crystalline 2D protein lattice; linking matrix; lipid membrane platform.

Figure 1

Elements and selected components of an S-layer protein-based quartz crystal microbalance with dissipation monitoring (QCM-D) biosensor. (A) A biosensing element or bioreceptor comprising of accessible functions like, e.g., an antibody to which the analyte binds with highly specific affinity. (B) An interface architecture comprising of a QCM-D sensor surface covered by a recrystallized S-layer lattice, which provides an environment for the proper functioning of the biosensing element. Here, the specific biological event takes place, which gives rise to a certain physical phenomenon. (C) A transducer converting the physical phenomenon (piezoelectricity) resulting from the analyte’s interaction with the biological element into an electrical signals. (D) Associated electronics comprising of signal amplifier, signal processor and a display allowing for a user-friendly visualization and evaluation of the data.

Figure 2

Transmission electron microscopy (TEM) image of a freeze-etched and metal shadowed preparation of (a) an archaeal cell (from Methanocorpusuculum sinense), and (b) a bacterial cell (from Desulfotomaculum nigrificans). Bars, 200 nm. Adopted from [15], copyright (2014) with permission from John Wiley & Sons Ltd.

Figure 3

Scheme of natural and surface layer (S-layer) supported lipid membranes. Supramolecular structure of an archaeal cell envelope comprising of a cytoplasma membrane, archaeal S-layer proteins incorporated in the lipidic matrix and integral membrane proteins (A). Schematic illustrations of various S-layer-supported lipid membranes. (B) Lipid monolayer films at the air/water interphase with a recrystallized S-layer lattice underneath. (1) Tetraether lipid monolayer in the upright conformation. (2) Tetraether lipid monolayer in the U-shaped (bent) conformation. (3) Phospholipid monolayer. (C) A tetraether lipid monolayer 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 by bacterial S-layer proteins on one side of the lipid membrane. In (D), a folded or painted bilayer phospholipid membrane spanning a Teflon aperture is shown. A closed bacterial S-layer lattice can be self-assembled on either one or both (not shown) sides of the lipid membrane. (E) Schematic drawing of a solid support where a closed bacterial S-layer lattice has been assembled. On this biomimetic structure, a tetraether lipid membrane was generated by the modified Langmuir-Blodgett method. Optionally as shown on the left side, a bacterial S-layer lattice can be attached on the external side of the solid supported lipid membrane. (F) Scheme of a bilayer lipid membrane generated on an S-layer ultrafiltration membrane. Optionally as shown on the left side, a bacterial S-layer lattice can be attached on the external side of the S-layer ultrafiltration membrane (SUM)-supported lipid membrane. In B to F, the head groups of the lipid molecules interacting with the S-layer protein are marked in dark. As indicated in C to F, all S-layer-supported model lipid membranes can be functionalized by biomolecules like membrane-active peptides and integral membrane proteins. Modified after [63], copyright (2004) with permission from Wiley-VCH.

Figure 4

S-layer-coated liposomes and emulsomes. TEM images of emulsomes coated with the S-layer protein SbsB from Geobacillus stearothermophilus PV72/p2 (A) wildtype SbsB and (B) recombinant SbsB. The bars correspond to 100 nm. Adopted from [191], copyright (2013) with permission from Wiley-VCH. (C) Schematic drawing of (1) an S-layer coated emulsomes (left) and Iiposome (right) with entrapped functional molecules and (2) functionalized by reconstituted integral proteins. Note, S-layer coated emulsomes can only transport hydrophobic molecules but with a much higher transport capacity. S-layer coated emulsomes and liposomes can be used as immobilization matrix for functional molecules (e.g., human IgG) either by direct binding (3) or by immobilization via the Fc-specific ligand protein A (4), or biotinylated ligands can be bound to the S-layer coated liposome or emulsomes via the biotin—avidin system (5). Alternatively, emulsomes or liposomes can be coated with genetically modified S-layer subunits incorporating functional domains (6). Modified after [61], copyright (2002) with permission from Wiley-VCH.