Individuation of a new metallothionein from the urochordate Ciona intestinalis.

Metallothioneins (MTs) are metal-chelating proteins occurring in animals, plants and prokaryotes, involved in detossification and immunity. In 2001, Canpolat and Lynes showed that exogenous MT can affect cell proliferation macrophage and cytotoxic T lymphocyte function, and humoral immunity to T-dependent antigens in mice. These evidences strengthen the hypothesis that MTs have an active role in extracellular environment as immunomodulatory proteins. 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. As we are interested in studing the involvement and role of MTs ascidian immune responses, we undertook a preliminary investigation amied to identify these molecules in Ciona intestinalis, the genome of which has been fully sequenced but no MTs have been annotated. 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) involved in constitutive and metal-related induction, antioxidant responding elements, important for ROS-dependent MT expression and STAT3, having a role in cytokine-related induction. The amino acid sequence of C. intestinalis MT shows only limited similarity with other known MTs: Mytilus edulis MT (28,8% identity), Strongylocentrotus purpuratus MT (23,4% identity) and Sparus aurata MT (36,7% identity).