Abstract
Metallothioneins (MTs) are small cysteine-rich proteins coordinating various transition metal ions, including ZnII, CdII, and CuI. MTs are ubiquitously present in all phyla, indicating a successful molecular concept for metal ion binding in all organisms. The plant MT Ec-1 from Triticum aestivum, common bread wheat, is a ZnII- binding protein that comprises two domains and binds up to six metal ions. The structure of the C-terminal four metal ion binding beta-E domain was recently described. Here we present the structure of the N-terminal second domain, gama-Ec-1, determined by NMR spectroscopy. The gamma-Ec-1 domain enfolds an MIICys cluster and was characterized as part of the full-length Zn6Ec-1 protein as well as in the form of the separately expressed domain, both in the ZnII-containing isoform and the CdII-containing isoform. Extended X-ray absorption fine structure analysis of Zn2g- Ec-1 clearly shows the presence of a ZnS4 coordination sphere with average Zn–S distances of 2.33 A ̊. 113Cd NMR experiments were used to identify the MII-Cys connectivity pattern, and revealed two putative metal cluster conformations. In addition, the general metal ion coordination abilities of g-Ec-1 were probed with CdII binding experiments as well as by pH titrations of the ZnII and CdII forms, the latter suggesting an interaction of the c domain and the bE domain within the full-length protein.
Loebus, J