Metallothioneins are a group of small proteins, which are extremely rich in cysteine. They are used for the transport and storage of soft, "thiophilic" metal ions like Zn2+ and Cu2+.
They are also used to remove toxic ions like Hg2+ and Cd2+ from cells.
The metal ions can be bound in different ways. Both tetrahedrally bound, isolated metal ions and clusters of metal ions can be found, as is shown here.
Coordination of the Zn2+ ions in cluster I
Coordination of the Zn2+ ions in cluster II