Journal:JBIC:9

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. <scene name='Journal:JBIC:9/Cv/9'>Thereby metal-thiolate cluster structures are formed.</scene> 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]], <ref name="Peroza">PMID:19361445</ref>) 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]],<ref name="Baleja">PMID:9460244</ref>). The resulting NMR γ-Ec-1 structures containing the metal cluster (Zn(II) or Cd(II)) arrangements <scene name='Journal:JBIC:9/Cv/16'>Cys-9/21</scene> or <scene name='Journal:JBIC:9/Cv/17'>Cys-9/3</scene> were obtained. <scene name='Journal:JBIC:9/Cv/13'>A superposition of the backbone atoms of conformers, in which either Cys-3 or Cys-21 were constrained to be the bridging ligands</scene>, revealed that <scene name='Journal:JBIC:9/Cv2/2'>only small structural adaptations were necessary to transform one form into the other</scene>. In the case with Cys-9/21 connectivity <scene name='Journal:JBIC:9/Cv/14'>Cys21</scene> forms bridge between two Zn(II) atoms (in addition to Cys9), whereas in the case with Cys-9/3 arrangement <scene name='Journal:JBIC:9/Cv/15'>Cys3</scene> participate in the bridge formation. Cd(II) isoforms with <scene name='Journal:JBIC:9/Cv1/2'>Cys-9/21</scene> or <scene name='Journal:JBIC:9/Cv1/3'>Cys-9/3</scene> connectivity were similar to the corresponding Zn(II) isoforms. Alignments of <scene name='Journal:JBIC:9/Cv1/6'>Zn(II)/Cd(II) Cys-9/21</scene> and <scene name='Journal:JBIC:9/Cv1/5'>Zn(II)/Cd(II) Cys-9/3</scene> isoforms highlighted this similarity. As well as in the case of Zn(II) isoforms, <scene name='Journal:JBIC:9/Cv1/4'>a superposition of the backbone atoms of Cd(II) Cys-9/21 and Cys-9/3 isoforms</scene> revealed that <scene name='Journal:JBIC:9/Cv2/3'>only small structural adaptations were necessary to transform one form into the other</scene>. 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.