Nuclear import of metallothionein requires its mRNA to be associated with the perinuclear cytoskeleton

The influence of mRNA localization on metallothionein-l protein distributio n was studied by immunocytochemistry. We used Chinese hamster ovary cells t hat had been transfected with either a native metallothionein-l gene constr uct or metallothionein-l 5'-untranslated region and coding sequences linked to the 3'-untranslated region from glutathione peroxidase. The change in t he 3'-untranslated region caused the delocalization of the mRNA with a loss of the perinuclear localization and association with the cytoskeleton. Clo nes were selected which expressed similar levels of metallothionein-l prote in, as assessed by radioimmunoassay. The results showed that loss of metall othionein-l mRNA localization was associated with a loss of metallothionein -l protein localization, most notably with a lack of metallothionein-l prot ein in the nucleus of synchronized cells which were beginning to synthesize DNA This indicates that the association of metallothionein-l mRNA with the cytoskeleton around the nucleus is essential for efficient shuttling of th e protein into the nucleus during the G(1) to S phase transition. This is t he first demonstration of a physiological role for perinuclear mRNA localiz ation and we propose that such localization may be important for a wide ran ge of nuclear proteins, including those that shuttle between nucleus and cy toplasm in a cell cycle dependent manner.