Molecular analysis of the S-layer proteins of Bacillus sphaericus strains JG-A12 and NTCC 9602

Uranium mining waste pile isolate Bacillus sphaericus JG-A12 and it's closest relative B. sphaericus NTCC 9602 possess S-layer proteins with a similar size of about 135 kDa and with a square lattice symmetry. The two proteins differ significantly in their ability to interact with metals and in their stability against proteases and pH changes. By using ICP-MS, the "Stain-all" dye, and colourimetric method it was demonstrated that both proteins are phosphorylated but the S-layer of the strain JG-A12 contains six times more phosphorous than those of the reference strain NTCC9602. Surprisingly, the two proteins share an extremely high homology of 98 % in the region of the N-terminal S-layer homologous domains and of 91 % in the region of the central domains. It is noticeable that the minor amino acid differences between the S-layers of the studied strains consist of threonine and serine enrichments in the case of JG-A12. Bearing in mind that the majority of the phosphoproteins are usually modified at threonine and serine residues, our observation explains the higher level of phosphorylation of the S-layer of the uranium mining isolate JG-A12. The higher amount of phosphate groups of this protein explains also it's ability to complex more effectively uranium and other metals.
In addition, there are evidences that the two Bacillus sphaericus strains studied possess an additional S-layer or an S-layer-like copy which seems to be located on large indigenous plasmids.