Expression and purification of a trivalent pertussis toxin-diphtheria toxin-tetanus toxin fusion protein in Escherichia coli

Pertussis toxoid, diphtheria toxoid, and tetanus toxoid are key components of diphtheria-tetanus-acellular pertussis vaccines. The efficacy of the vaccines is well documented, however, the vaccines are expensive partly because the antigens are derived from three different bacteria. In this study, a fusion protein (PDT) composed of the immunoprotective S1 fragment of pertussis toxin, the full-length non-toxic diphtheria toxin, and fragment C of tetanus toxin was constructed via genetic means. The correct fusion was verified by restriction endonuclease analysis and Western immunoblotting. Escherichia coli carrying the recombinant plasmid (pCoPDT) produced a 161 kDa protein that was recognized by antibodies specific to the three toxins. The expression of the PDT protein was inducible by isopropyl-β-d-thio-galactoside but the total amount of protein produced was relatively low. Attempts to improve the protein yield by expression in an E. coli strain (Rosetta-gami 2) that could alleviate rare-codon usage bias and by supplementation of the growth media with amino acids deemed to be a limiting factor in translation were not successful. The PDT protein remained in the insoluble fraction when the recombinant E. coli was grown at 37 °C but the protein became soluble when the bacteria were grown at 22 °C. The PDT protein was isolated via affinity chromatography on a NiCAM column. The protein was associated with five other proteins via disulfide bonds and non-covalent interactions. Following treatment with β-mercaptoethanol, the PDT fusion was purified to homogeneity by preparative polyacrylamide gel electrophoresis with a yield of 45 μg/L of culture. Antisera generated against the purified PDT protein recognized the native toxins indicating that some, if not all, of the native epitopes were conserved.