Heterologous expression of a surface layer-like protein in E. coli causes a drastic morphological change of the cell

Bacterial envelope proteins, so called surface layers (S-layer) are widely spread pararcrystalline surface structures which coat the cells of lots of bacterial strains and all archaea. They are mostly composed of protein monomers which form via self-assembling high regular two dimensional arrays. The S-layer proteins we investigate are from bacterial strains recovered from uranium mining waste pile Haberland in Saxony, Germany.
Their S-layer proteins selectively bind uranium and protect the cells from its toxicity. These special S-layer characteristics make them interesting for many technological applications such as filter materials, biosensors, as functional surfaces, or for example as drug containers.
In order to produce S-layer proteins in a high efficient way a heterologous expression in Escherichia coli is essential. In our study, the S-layer-like protein SllB of Lysinibacillus sphaericus JG-A12 was expressed in E. coli Bl21. Noteworthy, recombinant protein production resulted in a high stability of the cells against mechanical and chemical treatment. These unusual cells were analyzed by light microscopy, AFM and TEM. All methods demonstrated a total changed cell morphology with long filaments in the beginning of the exponential growth stage and 5-200 µm long tube like transparent structures at the end of the exponential growth stage containing E. coli single cells. Analyses by SDS-PAGE, N-terminal sequencing and IR-spectroscopy showed that the tube-like structures consist of outer membrane associated with recombinant surface layer proteins. These findings point to a disordered cell division. However, the underlying mechanism of these morphological changes are not known and will be analyzed in future. The long filaments, in combination with high expression level, good growth and high stability make these unusual E. coli cells interesting for biotechnological applications. In addition, these results cast a new light on one of the best studied microorganisms.