In vitro folding of Escherichia coli outer-membrane phospholipase A

Recombinant outer-membrane phospholipase A (OMPLA) of Escherichia coli was expressed without its signal sequence from the T7 phi 10 promoter. As a result of the cloning strategy the protein had an N-terminal extension of six amino acid residues. The protein accumulated in the cytosol in inclusion bodies. Conditions were established for the efficient folding of OMPLA in vitro in the presence of Triton X-100. After in vitro folding, the protein was present as a mixture of folded and unfolded forms. Ion-exchange chromatography was used for the purification of OMPLA and the separation of correctly folded, enzymically active enzyme from unfolded inactive protein. The final protein preparation was pure and fully heat-modifiable based on SDS/PAGE. The recombinant enzyme had a specific activity of 71 U/mg, which is similar to the value of the wild-type enzyme, purified from the membrane. The final yield of active enzyme was 35 mg protein/l culture of an A600 of 6. Circular dichroism spectroscopy revealed a high content of beta strand, in good agreement with a predicted beta-barrel structure of this outer-membrane protein.