Structural modification of cyanobacterial metallothionein enhanced its affinity to a heavy metal other than Zn²⁺

Abstract

Cyanobacterial metallothionein, for example, SmtA from Synechococcus PCC 7942 traps Zn²⁺ in cells to detoxify metal ions as well as to retain Zn²⁺ homeostasis. SmtA consists of 56 amino acids and is enriched in Cys and His residues that ligate the metal ions. Simple structure of SmtA would be helpful to create a genetically modified metal-binding protein applicable to the phytoremediation. In this study, we modified the amino acid sequence in the C-terminal region of the SmtA homologue from Synechococcus PCC 7002 by inserting one to three amino acid residues. Insertion of two residues between H49 and G50 increased binding affinity to Cd²⁺ and decreased the affinity to Zn²⁺ indicating that insertion might create larger space for binding the heavy metal ions. In contrast, any insertions between G46 and C47 were ineffective against affinities to both Cd²⁺ and Zn²⁺. Further we will discuss the effect of structural modification of SmtA homologue on the affinity to the heavy metal ions.