S-layer of Lactobacillus helveticus ATCC 12046: isolation, chemical characterization and re-formation after extraction with lithium chloride

Summary: In a previous study, electron microscopic examinations of thin sections of Lactobacillus helveticus ATCC 12046 revealed a three-layered structure of the cell wall. The outermost component was identified as a layer of a non-glycosylated 52 kDa protein. Freeze-etched preparations of intact cells have now demonstrated that this protein layer is an oblique surface layer (S-layer) lattice (a = 4·5 nm, b = 9·6 nm, γ = 77 °) which completely covers the cell surface. Treatment with 5 m-LiCl extracted the S-layer protein from intact cells efficiently and selectively. Viability did not decrease significantly. Moreover, the S-layer reappeared when treated cells were allowed to grow again. In vitro self-assembly products obtained upon aggregation of isolated S-layer subunits exhibited the same oblique S-layer symmetry as observed on intact cells in vivo. The purified S-layer protein had a high content (44%) of hydrophobic amino acids. The N-terminal sequence was mainly composed of alanine, threonine, asparagine and aspartic acid.