Nucleocytoplasmic Functionality of Metallothionein¹ Free

Appropriate nucleocytoplasmic partitioning of proteins can direct diverse cellular processes. Metallothioneins (MTs) are thiol-rich, stress-inducible proteins that can afford protection against oxidants, mutagens, and anticancer drugs. MTs display discrete nucleocytoplasmic sequestration patterns despite their small size (M_r 6,000). We demonstrate subcellular location-specific functionality of MT using a regulated expression system that restricts MT expression to the nucleus or the cytoplasm in MT-null fibroblasts. Specifically, we found that cytoplasmic but not nuclear expression of MT decreases the level of intracellular reactive oxygen species and is more cytoprotective against the prototypic oxidizing agent tert-butyl hydroperoxide. Cytoplasmic MT expression also protects against the cytotoxicity of the heavy metal CdCl₂, whereas nuclear expression protects against the cytotoxicity of the mutagenic agent N-methyl-N′-nitro-N-nitrosoguanidine. These data support the hypothesis that essential cytotoxic targets of both oxidants and heavy metals reside in the cytoplasm and establish the importance of nucleocytoplasmic partitioning for the function of small protective proteins such as MTs.