Applications Of S-Layer Protein

GURGAON, India - Nov. 6, 2015 - PRLog -- Applications of S-layer protein

Surface layers are cell components on prokaryotic cells that consist of two dimensional crystalline arrays of protein subunits. The self-assembled paracystalline protein lattices cover almost all archaea and many bacteria. Biologically, the surface layer proteins have various functions that are important to the organism on which they are. In bacteria, the surface layer constitute the cell envelop, where it occurs as a regularly ordered protein layer, while in archaea, the S-layer forms the only cell wall as it is the only component. S-layer protein is of importance to man as it is found in pathogenic and biotechnologically useful Gram positive and negative bacteria. The layer is attached to the lipopolysaccharide components of the Gram negative bacterial outer membrane, while in some archaea and Gram positive bacteria it is non-covalently bound to the peptidoglycan, pseudomurine or secondary cell wall polymers (Howorka 2011).

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         The composition of the surface layer can either be one protein or glycoprotein species. In some organisms, different protein species form the complex s-layer lattices and single strains may also express different S-layer proteins since protein synthesis is dependent on the growth conditions. These proteins have low homology and their molecular weight ranges from 25-200kDa. They have high content of acidic and hydrophobic amino acids and low or lack sulfur-containing amino acids hence have a net negative charge and a pI values in the acidic range except lactobacilli’s S-layer that has pI values in the basic range. Structurally, S-layer proteins form lattices that are regular that have oblique, hexagonal or square symmetry with a thickness of 5-20nm. Electron microscopy has enables a low resolution three-dimensional structure of the S-layer to be obtained using negatively stained samples.

Figure 1: different structures of S-layer lattices (Ilk et al 2011).

S-layer proteins have two separate morphological regions, one for self-assembly and the other for cell wall adhesion. Binding of the protein in most Gram positive and negative bacteria is through N-terminal to the peptidoglycan, where it recognizes secondary cell wall polymers. It attaches to the components of the outer membrane of a Gram-negative with N-or C-terminus. Surface layer homology motifs mediate the interaction of the protein with polysaccharides and are made up of approximately 55 amino acids. Various characteristics of the S-layer convey essential importance to the organism on which they are. Their high porous structures provide sieving properties for molecules such as nutrients and lytic enzymes. It also acts as a barrier against bacterial phage and viral adhesion as well as resistance to extreme low pH. S-layer helps to stabilize the membrane, but it has been found to also act as an attachment region for some phages (Lederberg 2004). Non-covalent association of the S-layer with the cell surface molecules enables it solubilization using high concentrations of agent that act on the hydrogen bonds. Isolated S-layers subunits have the ability to re-crystallize into the regular arrays on surfaces or in suspension after the isolation agent is removed (Sara & Sleytr). After isolation, the protein can be used in various fields to produce other macromolecules, as blue print for synthesis of nanoparticles, as a vehicle for molecule and substances delivery to cells and as a genetic engineering tool.

Figure 2: Solubilization and self-assembly of S-layer protein (Pum et al 2000).

Applications of S-layer protein

There are various principles that characterize the S-layer making it have great potential for broad applications. Some of the important features exploited in the S-layer protein application include; identical pore in morphology and size comparable to ultrafiltration membranes; binding sites for functional molecules on the functional groups found on the surface and pores of the layer; recrystallization capability as closed monolayers on liposome surface and lipid monolayers or solid support. This makes the protein essential for application in biotechnology, nanotechnology, diagnostics, vaccine development, supramolecular engineering and biomimetic membranes.

In molecular nanotechnology, S-layer protein is used for controlled binding of functional biomolecules in biosensors and dipstick immunoassays; in semiconductor technology for making of nanometer-thick resistance; in molecular electronics as templates for formation of nanoparticles that are regularly arranged and in lipid membrane as supporting structures.  In biosensors, the characteristic of S-layer to have identical subunits of protein, identical orientation and position of the functional group to the sub-nanometer scale makes it suitable to for molecule mobilization in predictable way that is geometrically defined. An example is development of optical and amperometric biosensors, where enzymes are linked covalently to S-layer fragments or self assembly products and latter deposited on polymeric ultrafiltration membranes that are rigid and porous. The sensing layer is then attached to tip of optical fiber for biochemical detection, while electrical contact in amperometric biosensors is achieved through sputtering thin gold layer on the sensing layer (Pum & Sleytr 1999). According to Mark et al (2006), nanoscale metallic and semiconductor nanoparticles arrays can be made from exploitation of physiochemical diversity of the S-layers of the prokaryotes. This was after they investigated and compared two dimensional surface layers of Deinococcus radiodurans and sufolobus acidocaldarius ability to be biotemplates for self-assembled, inorganic nanoparticles.

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         Development of regularly arranged nanoparticles using s-layer involves recrystallizing the s-layer proteins as monolayers on solid support or as products of self assembly on substrates to induce formation of nanoparticles of gold and cadmium. In development of biomimetic membranes, plain lipid membranes are usually not considered for practical devices due to their susceptibility to damage during manual handling and storage for a long period.