The transport mechanism of metallothionein is different from that of classical NLS‐bearing protein

Abstract

A nuclear localization signal (NLS) has been detected in several nuclear proteins. Classical NLS-mediated nuclear pore targeting is performed by using the cytosolic factors, importin alpha and importin beta, whereas nuclear translocation requires the small GTPase, Ran. In the present study, we demonstrated that nuclear localization of metallothionein (MT) differs from that of classical NLS-mediated substrates. In digitonin-permeabilized BALB/c3T3 cells, biotinylated MT was localized in the nucleus in the presence of ATP and erythrocyte cytosol in the same manner as for SV40 large T NLS-conjugated allophycocyanin (APC-NLS). Under ATP-free conditions, nuclear rim-binding was observed in both transport substrates. Rim-binding of labeled MT was competitively inhibited by the addition of an excess amount of unlabeled MT. Different elution profiles were observed for the localization-promoting activities of MT in the cytosol compared to those of APC-NLS. Furthermore, nuclear localization of MT was determined to be a wheat germ agglutinin-insensitive, GTPgammaS-sensitive, and anti-Ran antibody-sensitive process. Green fluorescent protein-metallothionein (GFP-MT) fusion protein was also localized in the nucleus in the stable transformant of CHL-IU cells. These results strongly suggest that the targeting by MT of the nuclear pore is mediated by cytosolic factor(s) other than importins and that MT requires Ran for its nuclear localization.

A zinc‐finger like metal binding site in the nucleosome

“…The literature contains further, albeit indirect indications in favor of the physiological binding of Zn(II) ions to nucleosomes. In particular, metallothionein (MT) was shown to translocate to the nucleus just in S-phase, and thought to deliver zinc to ZF and enzymes [54][55][56][57][58]. In a recent paper, micro-PIXE maps demonstrated additionally a very strong colocalization of phosphorus and zinc in cells undergoing such MT-dependent Zn(II) transport [59].…”