Recombinant fusion protein was expressed by isopropyl-β-d-thiogalactopyranoside induction at a final concentration of 0.5 mM at 30 °C for 5 h. Bacterial cells were harvested by centrifugation at 70,000× g for 10 min at 4 °C. The cells were lysed by sonication in buffer (50 mM NaH2PO4, 300 mM NaCl and 10 mM imidazole), and the supernatant was collected after centrifugation at 13,000× g for 30 min. The fusion protein was purified by Ni-NTA affinity chromatography following the manufacturer’s instructions (Qiagen, Gaithersburg, MD, USA). The purified recombinant protein was examined by 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and stained by Coomassie brilliant blue G-250.