Escerichia coli DH5a and E. coli BL21 were used as hosts for plasmid amplification and expression, respectively. The plasmids pMD19-T (TaKaRa, Japan) and pET28a (+) (Invitrogen, U.S.) were used as vectors. The recombinant cells were inoculated in a lysogeny broth (LB) medium with kanamycin (50 mM) and operated at 37uC for 3 h. The recombinant protein was induced by adding 0.4 mM isopropyl-b-D-thiogalactopyranoside (IPTG).