islandicus still assemble partial S-layers, consisting of only SlaA ( 11, 12). Interestingly, it was recently shown by negative stain electron microscopy (EM) that SlaB knockout strains of S. islandicus are shorter by 1 β-sandwich domain ( 10). sedula, SlaB is comprised of an N-terminal Sec-dependent signal sequence, followed by 3 consecutive β-sandwich domains, an α-helical coiled-coil domain, and 1 C-terminal transmembrane helix. Comparative sequence analysis and molecular modeling of SlaB revealed that it exists in 2 species-dependent variants. Biochemical analysis of SlaA and SlaB from different Sulfolobus species revealed molecular masses ranging from 120 kDa to 180 kDa or from 40 kDa to 45 kDa, respectively ( 9, 10). The S-layer of all Sulfolobus species consists of 2 proteins, SlaA and SlaB. They are well-established model organisms, as they can be relatively easily cultured in the laboratory, and the genomes of a number of strains have been sequenced ( 6– 8). Sulfolobus is a genus of hyperthermophilic acidophilic archaea, which grow in low-pH terrestrial hot springs at 75–80 ☌ all around the globe. S-layers are therefore highly interesting for various applications in nanotechnology ( 5). S-layers are intrinsically capable of self-assembly in vitro, resulting in tube-like, spherical 2D crystals ( 4). Furthermore, it has been suggested that S-layers provide protection against immunological defense systems and viruses, act as pathogenic virulence factors, serve as phage receptors, promote surface adhesion, establish a quasi-periplasmic space, provide anchoring scaffolds for membrane proteins, sequester ions, and facilitate biomineralization ( 2). In addition, in vitro experiments have shown that S-layers change the physical and biochemical properties of lipid layers, rendering them less flexible, less fluid, more stable and heat-resistant, and possibly more resistant to hydrostatic pressure.
They therefore define cellular shape and provide osmotic, thermal and mechanical stability ( 2). For archaea, S-layers are of particular importance as they often comprise the only cell wall component.
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