Functional analysis of metallothionein MTT5 from Tetrahymena thermophila

Metallothioneins (MTs) constitute a superfamily of cysteine-rich proteins that bind heavy-metal ions by metal-thiolate clusters. Five MT genes from Tetrahymena thermophila was subdivided into 7a (MTT1, MTT3, and MTT5) and 7b (MTT2 and MTT4) subfamilies. In the study, MTT5 was knocked out in Tetrahymena. The mutant cells were sensitive to Cd²⁺ and Pb²⁺ but poorly sensitive to Cu⁺ . In the MTT5 knockout cells, the expression levels of MTT1 and MTT3 were significantly up-regulated under Cd²⁺ and Pb²⁺ stresses, whereas the expression levels of MTT2 and MTT4 were significantly up-regulated under Cu⁺ stress relative to those in the wild-type cells. Furthermore, recombinant GST-MTT5 was expressed in Escherichia coli/pGEX-MTT5 and purified by affinity chromatography. Fluorescence quenching analysis showed that apoMTT5 can bind 8 Cd²⁺ , 8 Pb²⁺ , and 12 Cu⁺ . The metal-binding ability of the MTT5 complex followed the order of Pb²⁺  > Cd²⁺  > Cu⁺ . Meanwhile, the half-maximal inhibitory concentrations of the heavy-metal ions for E. coli/pGEX-MTT5 were as follows: Cu⁺ (483.9 µM) > Pb²⁺ (410.7 µM) > Cd²⁺ (130.8 µM). The accumulation of Cd²⁺ , Pb²⁺ , and Cu⁺ in the E. coli/pGEX-MTT5 was enhanced relative to that of E. coli/pGEX-4T. Results suggested that different MTs functionally compensated in Tetrahymena, and MTT5 was a potential candidate for cadmium and lead bioremediation.