Individuation of a new metallothionein from the urochordate Ciona intestinalis

Metallothioneins (MTs) are metal-chelating proteins occurring in animals, plants and prokaryotes, involved in detosification and immunity. They do not constitute a monophyletic protein family, but rather a superfamily of heterogeneous, low molecular weight, cysteine-rich peptides. Current knowledge on MTs comes mainly from vertebrate molecules which are composed of approximately 60 amino acids, including 20 cysteine residues, and folded into two independent domains when coordinating divalent metal ions. Up to now, there are no descriptions of MTs in invertebrate Chordates althought it seems that the vertebrate structure is maintained also in other deuterostomes such as the echinoderms. In this study, we present some data on MTs of the solitary urochordate Ciona intestinalis. We have cloned the transcript and characterized the gene of a new MT, codifying for 39 amino acids, including 12 cys residues (30 % of total amino acids, in accordance with other MTs); moreover, the typical organization of cysteine residues in C-X-C motifs is conserved. The gene is composed of two introns (one inside the coding region and the other inside the 3’ UTR region) and three exons. The 5’ untrascribed region contains several cis elements similar to those found in vertebrate MT genes such as: metal responding elements (MRE), antioxidant responding elements and STAT3, which are involved in constitutive and metal-related, ROS and cytochines induction, respectively. The amino acid sequence of C. intestinalis MT shows only limited similarity with other MTs, e.g., Mytilus edulis MT (28.8 % identity), Strongylocentrotus purpuratus MT (23.4 % identity) and Sparus aurata MT (36.7 % identity). Phylogenetic analyses with various methods are in progress.