Characterization of three different unusual S-layer proteins from Viridibacillus arvi JG-B58 that exhibits two super-imposed S-layer proteins

Viridibacillus arvi JG-B58 that was previously isolated from heavy metal contaminated environment expresses three different surface layer (S-layer) proteins namely Slp1, Slp2, and Slp3. Two of the V. arvi JG-B58 S-layer proteins were visualized on the surface of living cells via atomic force microscopy (AFM). These S-layer proteins form a double layer with p4 symmetry. All examined S-layer proteins lack some typical S-layer protein features. They possess no SLH domains that are usually responsible for the anchoring of the proteins to the cell wall. Further, the pI values are relatively high ranging from 7.84 to 9.25 for the matured proteins. These features are typical for S-layer proteins of Lactobacillus although sequence comparisons indicate a close relationship to S-layer proteins of Lysinibacillus and Bacillus strains. The three S-layer proteins were isolated from the cells using two different strategies. Purified S-layer proteins were recrystallized on SiO2 substrates in order to study the structure of the arrays and self-assembling properties. There are only a few studies reporting the concomitant existence of two different S-layer proteins on cell surfaces. Together with the genomic data, this is the first description of a novel type of S-layer proteins showing features of Lactobacillus as well as of Bacillus-S-layer proteins and the first study of the cell envelope of Viridibacillus arvi.