Protein and metal cluster structure of the wheat metallothionein domain γ-Ec-1. The second part of the puzzle.
Jens Loebus, Estevão A. Peroza, Nancy Blüthgen, Thomas Fox, Wolfram Meyer Klaucke, Oliver Zerbe and Eva Freisinger[1]
Molecular Tour
Ec-1 (Early Cys-labelled protein) from Triticum aestivum (common bread wheat) belongs to the family of plant metallothioneins. Metallothioneins in general are small (up to approx. 8 kDa) Cys-rich proteins with the ability to coordinate d10 transition metal ions with high thermodynamic affinity. The function of these protein is presumably to be found in metal ion homeostasis (Zn(II), Cu(I)), detoxification (Cd(II), Hg(II), etc.), and possibly also in reactive oxygen species scavenging. Wheat Ec-1 consists of 81 amino acids and forms two distinctive domains. The NMR structure of the larger C-terminal β(E)-domain has been already described (PDB ID 2kak, [2]) and accommodates four Zn(II) ions. The NMR solution structure of the smaller N-terminal γ-domain shown here consists of the first 24 amino acids of the protein (plus two additional N-terminal vector-derived amino acid residues). The six Cys residues of the γ-domain form a cluster with two Zn(II) or Cd(II) ions similar to the one found in the yeast transcription factor GAL4 (PDB ID 1aw6,[3]). The resulting NMR γ-Ec-1 structures containing the metal cluster (Zn(II) or Cd(II)) arrangements or were obtained. , revealed that . In the case with Cys-9/21 connectivity forms bridge between two Zn(II) atoms (in addition to Cys9), whereas in the case with Cys-9/3 arrangement participate in the bridge formation. Cd(II) isoforms with or connectivity were similar to the corresponding Zn(II) isoforms. Alignments of and isoforms highlighted this similarity. As well as in the case of Zn(II) isoforms, revealed that only small structural adaptations were necessary to transform one form into the other. This two-metal ion cluster, as well as the metal ion arrangement found in the larger β(E)-domain, represent new coordination modes that were unprecedented for the superfamily of metallothioneins so far. The functional significance of these findings has to be awaited.
