Metallothionein (MT) is a low–molecular weight, cysteine-rich stress response protein with a high affinity for divalent heavy metals. MT was originally identified as a cadmium-rich protein in the equine renal cortex. A large amount of subsequent work has shown MT to serve in many cell types in the management of essential divalent metal cations, to interfere with the toxic effects of xenobiotics, heavy metals, and free radicals, and to serve as a regulator of specific transcription factors. As a consequence of these various activities, MT synthesis has an impact on developmental processes and on the cellular responses to stressful conditions (reviewed in (Klaassen et al., 1999; Vasak, 2005)).
MT is primarily synthesized on free polysomes (Shapiro et al., 1980). Based on these observations, MT has often been considered to function exclusively as an intracellular protein. While MT lacks the signal peptide sequences or other protein trafficking