sentence: The protein was purified by nickel chelate chromatography ( HisTrap , Ä KTAprimeTM Plus ; GE Healthcare ) and stored in FT freezing buffer ( 50 mM Tris , pH 7.5 ,, 50 μ M ZnCl , 5 mM MgCl , 10 mM β - mercaptoethanol , and 10 % glycerol ) .