Metal Binding of Polypeptides Including Tandemly-connected N-terminal Regions of Cyanobacterial SmtA

Abstract

SmtA, a metallothionein homologue of Synechococcus PCC 7942, binds heavy metal ions and is involved in response to the heavy metal stress. SmtA has two metal-binding regions, namely, the N-terminal (Met1-Cys47) region and C-terminal region (Gly43-Gly56), which binds two and one molecules of Zn²⁺, respectively. We have constructed the polypeptides which include tandemly-connedted N-terminal regions of SmtA and analyzed their metal binding ability.
The number of Zn²⁺ bound to isolated recombinant proteins increased depending on the number of included N-terminal regions. For example, a modified SmtA with additional 10 N-terminal regions bound 26 Zn²⁺ molecules. The binding mode of Zn²⁺ was estimated to be similar to that in the native SmtA. In addition, E. coli expressing either the native or modified SmtA contained increased amount of Zn²⁺, indicating that these proteins are functioning in accumulating heavy metals in the cells.