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A Galvanizing Story—Protein Stability and Zinc Homeostasis

Zinc is an essential component of protein function in all living cells. The intracellular availability of zinc ions must be balanced against their potential toxicity. Maintaining optimal zinc ion levels requires the integration of zinc enzyme expression with zinc transport. Much of the effort to understand bacterial zinc homeostasis, as well as that of other metals, has focused on transcriptional regulation and the biochemical properties of the relevant transcriptional regulators. Less is known about the translational and posttranslational mechanisms of metal homeostasis in bacteria, although these are clearly important (9–11). The significance of protein stability and turnover is emphasized in the results of a study of the zinc-dependent ZntR transcriptional activator, which are presented in this issue (15). These new data provide a way to reconcile the results of previous biochemical and biological studies of ZntR function (6, 13, 18) and provide a better model for understanding zinc homeostasis and one which can be applied to other metals. This study underscores the importance of designing experiments that account for the dynamic nature of cell physiology and its effect on intracellular transition metal utilization. There are many known zinc proteins in Escherichia coli (50), several of which are essential (e.g., RNA polymerase and tRNA synthetases). Growth conditions that change the level of zinc protein expression will require adjustments of intracellular zinc levels to ensure that sufficient metal is available for the generation of active proteins. Access to zinc will depend on the concentration of both extracellular zinc and that recovered from the turnover of other Zncontaining proteins. Zinc ions are transported into the cytoplasm via ZnuABC (14), although other import pathways exist (5). ZnuABC expression is regulated by Zur (14), a Zn-dependent transcriptional repressor homologous to the well-known Fur family of metal-dependent regulators (8). When Zur binds available intracellular zinc, it represses ZnuABC transcription, ultimately leading to cessation of zinc import via this pathway. Excess zinc ions are exported by the zinc-inducible exporter ZntA (1, 16), whose expression is regulated by ZntR (2), a member of the MerR family of transcriptional regulators (7).