Penicillin-binding protein 2a of Streptococcus pneumoniae: expression in Escherichia coli and purification and refolding of inclusion bodies into a soluble and enzymatically active enzyme.

Penicillin-binding proteins (PBPs), targets of beta-lactam antibiotics, are membrane-bound enzymes essential for the biosynthesis of the bacterial cell wall. PBPs possess transpeptidase and transglycosylase activities responsible for the final steps of the bacterial cell wall cross-linking and polymerization, respectively. To facilitate our structural studies of PBPs, we constructed a 5'-truncated version (lacking bp from 1 to 231 encoding the N-terminal part of the protein including the transmembrane domain) of the pbp2a gene of Streptococcus pneumoniae and expressed the truncated gene product as a GST fusion protein in Escherichia coli. This GST fusion form of PBP2a, designated GST-PBP2a*, was expressed almost exclusively as inclusion bodies. Using a combination of high- and low-speed centrifugation, large amounts of purified inclusion bodies were obtained. These purified inclusion bodies were refolded into a soluble and enzymatically active enzyme using a single-step refolding method consisting of solubilization of the inclusion bodies with urea and direct dialysis of the solubilized preparations. Using these purification and refolding methods, approximately 37 mg of soluble GST-PBP2a* protein was obtained from 1 liter of culture. The identity of this refolded PBP2a* protein was confirmed by N-terminal sequencing. The refolded PBP2a*, with or without the GST-tag, was found to bind to BOCILLIN FL, a beta-lactam, and to hydrolyze S2d, an analog of the bacterial cell wall stem peptides. The S2d hydrolysis activity of PBP2a* was inhibited by penicillin G. In conclusion, using this expression system, and the purification and refolding methods, large amounts of the soluble GST-PBP2a* protein were obtained and shown to be enzymatically active.