2D-Protein Crystals (S-Layers) as Support for Lipid Membranes

Abstract

Abstract Two-dimensional, crystalline bacterial cell surface layers, termed S-layers, are one of the most commonly observed outermost cell envelope structures of prokaryotic organisms. The highly porous, water-filled protein lattices are composed of a single (glyco)protein species and cover the cell surface completely. The uniqueness of S-layers resides in their periodicity and in the capability of isolated S-layer protomers to recrystallize by an intrinsic, entropy-driven process into monomolecular arrays on intact bacterial cells, in suspension, and on various surfaces or interfaces particularly on lipid films. The biomimetic approach copying the supramolecular principle of plasma membrane with associated S-layer proteins observed in many archaea has the potential to lead to novel technologies for stabilizing functional lipid membranes. Among others, S-layer stabilized membranes can be used for structure–function studies on reconstituted integral proteins, for preparing biocompatible surfaces and drug targeting and delivery systems, and for the application as biosensing devices (e.g., lipid chip or lab-on-a-chip), but also in the ion channel-based high-throughput screening. Numerous benefits caused by the attachment of coherent S-layer lattices on mono- and bilayer lipid membranes (BLMs) but also restrictions will be discussed. Spatial well-defined domains of the S-layer protein interact predominantly electrostatically with few lipid headgroups within the BLM. But most important, no impact on the hydrophobic core region (e.g., thickness, induction of defects), but also on the function of reconstituted integral proteins have been determined. A striking important feature is the enhanced stability of these composite S-layer/lipid structures as a slower widening of induced defects and a higher resistance against an applied hydrostatic pressure was observed. Beside the higher mechanical stability of lipid membranes generated on S-layer-covered solid supports, the ionic reservoir between the BLM and the electrode can be tuned using further components of the S-layer-associated plasma membrane. Basic and applied S-layer research has demonstrated that nature provides most elegant paradigms for versatile nanometer size self-assembly systems. The fascinating intrinsic features of native S-layer proteins and the possibility for producing recombinant S-layer fusion proteins and combining S-layer lattices with other biological molecules, particularly lipids bring these unique separating and stabilizing architectures in an excellent position for application in the broad field of membrane-based molecular nanotechnology and biomimetics.