Heterologous expression and self-assembly of the S-layer protein SbsA of Bacillus stearothermophilus in Escherichia coli

The cell surface of Bacillus stearothermophilus PV72 is covered by a regular surface layer (S-layer) composed of a single species of protein, SbsA, with a molecular weight of 130 000. Recently, the sequence of the corresponding gene (sbsA) has been determined. The SbsA coding region including the signal sequence was cloned as a polymerase chain reaction (PCR) product into a low-copy-number vector under the transcriptional control of the lambda pL promoter. Expression of sbsA was shown to be thermally inducible from the resulting vector pBK4 in a strain of Escherichia coli expressing the lambda cl857 from the chromosome. As shown by ultrathin sectioning of whole cells and immunogold labelling using SbsA-specific antibodies, expression of sbsA in E. coli led to accumulation of sheet-like self-assembling products of the protein in the cytoplasm. No SbsA protein was detected either in the periplasm or in the supernatant fractions. Long-term expression of sbsA from pBK4, including in the late stationary phase, did not lead to degradation of SbsA.