Purified recombinant sweet potato MT-I and MT-II proteins. Crude extracts (5 μg protein) from E. coli (M15) transformed with pQE30 (lane 1) or with pQE31- MT-I (lane 2) or pQE31- MT-II (lane 2) were analyzed by 15% (w/v) SDS-PAGE, and then the gels were stained with Coomassie brilliant blue G-250. Molecular masses of standard proteins were indicated at the left of the figure. His-tagged MT-I protein or MT-II (lane 3) was purified by Ni2+-chelated affinity chromatography. “M” indicated the see Blue™ pre-stained markers for SDS-PAGE. Each data show the mean ± SD of one experiment performed in triplicate.