An unusual chordate metallothionein gene in Ciona intestinalis genome: structure and expression studies

Metallothioneins (MTs) are able to bind essential and non-essential heavy metal ions, thus controlling cellular homeostasis and detossification. In addition, they act as scavengers for reactive oxygen species (ROS), thanks to their abundant thiols groups. They have a role also in the regulation of inflammatory responses through the modulation of immunomodulatory humoral components. Chordata represents the major phylum of Deuterostomes, including about 45,000 species distributed in three subphyla: Tunicata (Urochordata), Cephalochordata and Vertebrata. Invertebrate Chordata, about 3 % of the total chordate species, are collectively named Protochordata. Unfortunately, no MT genes have been annotated so far in Protochordates. In order to allow a comparison with the vertebrate MTs we undertook a search for MT genes in the genome of the solitary tunicate Ciona intestinalis. We were able to find a MT gene (CiMT1), which represents the first MT gene identified in Tunicates. Its expression is limited to hemocytes and modulated by Cd, Zn and Cu. The deduced protein is only 39 amino acids in length with no typical α and β domains. However the sequence shows that this protein shares the usual percentage (≥ 30) of Cys residues arranged in typical conserved motifs reported for vertebrates.