The sequences making up Metallothio_Cad are found repeated in metallothionein proteins expressed by several different Tetrahymena species. Metallothioneins are low molecular mass, cysteine-rich metal-binding proteins that are thought to be involved in the regulation of levels of trace metals, and detoxification of these metals when present in excess [1]. Some of the metallothioneins found in this family (for example, Swiss:Q8T6B3) are known to be induced by cadmium and are thought to be involved in the cellular sequestration of toxic metal ions. The high proportion of cysteine residues allows the metal ions to be bound by the formation of clusters of metal-thiolate complexes [1]. Tetrahymena spp. metallothioneins differ from other eukaryotic metallothioneins mainly in the length of their sequences and in the cysteine-containing motifs they exhibit.

1: Purification and primary structure of metallothioneins induced by cadmium in the protists Tetrahymena pigmentosa and Tetrahymena pyriformis. Piccinni E, Staudenmann W, Albergoni V, De Gabrieli R, James P; Eur J Biochem 1994;226:853-859. PMID:7813475