Functional comparison of metallothioneins MTT1 and MTT2 from Tetrahymena thermophila

Metallothioneins MTT1 and MTT2 from Tetrahymena thermophila have been characterized. The MTT1 contains mainly characteristic Cys-Cys-Cys and Cys-Cys clusters, but MTT2 contains mainly Cys-X-Cys cluster. Cd(16)-MTT1 mainly consists of α-helix and β-turns, in contrast, Cd(11)-MTT2 mainly consists of random coils. Reaction of Cd(16)-MTT1 and Cd(11)-MTT2 with nitric oxide leads to intramolecular disulfide bond formation, respectively. Binding stabilities of Cd(2+), Hg(2+) and Zn(2+) to MTT1 are stronger than those to MTT2. Cu(2+) can not replace Cd(2+) from Cd(16)-MTT1 complex, but can replace Cd(2+) from Cd(11)-MTT2 complex. The analysis of qRT-PCR revealed MTT2 mRNA levels were 31-fold higher than those of MTT1 under basal conditions. These results further suggest MTT1 possibly play a role in the detoxification of heavy metal ions, and MTT2 may be involved in the homeostasis of copper ions.