Heterologous Expression and Metal-Binding Characterization of a Type 1 Metallothionein Isoform (OsMTI-1b) from Rice (Oryza sativa)

Metallothioneins (MTs) are ubiquitous, low molecular mass and cysteine-rich proteins that play important roles in maintaining intracellular metal homeostasis, eliminating metal toxification and protecting the cells against oxidative damages. MTs are able to bind metal ions through the thiol groups of their cysteine residues. Plants have several MT isoforms which are classified into four types based on the arrangement of cysteine residues. In the present study, a rice (Oryza sativa) gene encoding type 1 MT isoform, OsMTI-1b, was inserted in vector pET41a and overexpressed in Escherichia coli as carboxy-terminal extensions of glutathione-S-transferase (GST). The recombinant protein GST-OsMTI-1b was purified using affinity chromatography and its ability to bind with Ni²⁺, Cd²⁺, Zn²⁺ and Cu²⁺ ions was analyzed. The results demonstrated that this isoform has ability to bind Ni²⁺, Cd²⁺ and Zn²⁺ ions in vitro, whereas it has no substantial ability to bind Cu²⁺ ions. From competitive reaction with 5,5′-dithiobis(2-nitrobenzoic acid), DTNB, the affinity of metal ions for recombinant form of GST-OsMTI-1b was as follows: Ni²⁺/Cd²⁺ > Zn²⁺ > Cu²⁺