A series of complementary oligonucleotides (Table 1) coding for wild-type and truncated motif SVQYHPL were synthesized, annealed, and cloned into the BamHI/XhoI sites of the prokaryotic expression vector pGEX-6p-1 (Pharmacia, Piscataway, NY, USA), producing a group of recombinant plasmids. All the resulting recombinant plasmids were validated by restriction analysis and DNA sequencing. Expression plasmids were transformed into BL21 (DE3) competent cells, followed by the addition of 1 mM isopropyl-D-thioga-lactopyranoside (IPTG; GE Healthcare, USA) for induction.