(A) Cleavage efficiency of the thioredoxin-BirA fusion protein with different amounts of TEV protease as followed by SDS–PAGE (12%). The reactions were allowed to proceed for 16 h at room temperature. Lane 1–6, 0, 2, 4, 6, 8, 10 μg of TEV protease was added to 1 mg of fusion protein, respectively; Lane 7, protein standards. (B) Separation of the target protein from the carrier thioredoxin by using a HisTrap HP column (5 ml). Lane 1–3, fractions that flowed through the column and which contain the target protein; Lane 4–5, factions that bound to the column and eluted with increasing concentration of imidazole, Lane 6, protein standards.