Abstract
Bacterial surface layers (S-layers) have been observed as the outermost cell envelope component in a wide range of bacteria and most archaea. They are one of the most common prokaryotic cell surface structures and cover the cells completely. It is assumed that S-layers provide selection advantages to prokaryotic cells in their natural habitats since they act as protective envelopes, as structures involved in cell adhesion and surface recognition, as molecular or ion traps, and as molecular sieves in the ultrafiltration range. In order to contribute to the question of the function of S-layers for the cell, we merged high-resolution cryo-EM and small-angle X-ray scattering datasets to build a coarse-grained functional model of the S-layer protein SbpA from Lysinibacillus sphaericus ATCC 4525. We applied the Navier–Stokes and the Poisson equations for a 2D section through the pore region in the self-assembled SbpA lattice. We calculated the flow field of water, the vorticity, the electrostatic potential, and the electric field of the coarse-grained model. From calculated local changes in the flow profile, evidence is provided that both the characteristic rigidity of the S-layer and the charge distribution determine its rheological properties. The strength of turbulence and pressure near the S-layer surface in the range of 10 to 50 nm thus support our hypothesis that the S-layer, due to its highly ordered repetitive crystalline structure, not only increases the exchange rate of metabolites but is also responsible for the remarkable antifouling properties of the cell surface. In this context, studies on the structure, assembly and function of S-layer proteins are promising for various applications in nanobiotechnology, biomimetics, biomedicine, and molecular nanotechnology.
Highlights
Crystalline bacterial surface layers are known to be one of the most common cell surface structures in archaea and bacteria [1,2,3,4,5]
In the presence of calcium ions, SbpA is able to reassemble in suspension, on solid supports, at the air–water interface, on planar lipid S-layers, on liposomes, nano-capsules, and carbon nanotubes, following a non-classical crystallization pathway [30,31,32,33,34,35]
The results were in excellent agreement with the experimental findings, showing, for example, that liquid-like cluster formation precedes crystallization
Summary
Crystalline bacterial surface layers (called S-layers) are known to be one of the most common cell surface structures in archaea and bacteria [1,2,3,4,5]. S-layers are monomolecular arrays of a single protein or glycoprotein species (Mw 40 to 200 kDa) and completely cover the archaeal or bacterial cell (Figure 1). The unit cell dimensions of S-layers range from 3 to 30 nm, while the thickness is between 5 and 10 nm (up to 70 nm in archaea). Due to their crystalline nature, S-layers are porous protein networks (30–70% porosity) with pores of uniform size (2–8 nm) and morphology [7,8]
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