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ZIPping Zinc across a Cell♦

Zinc is a component of many proteins ( 10% of characterized proteomes) and is thus an essential element for cell growth. Zrt-, Irt-like proteins (ZIPs), a ubiquitous family of transporters found at all phylogenetic levels, represent the primary known routes of zinc import into cells, although how ZIPs promote zinc uptake and how it relates to overall zinc equilibrium remains unclear. In this Paper of the Week, Wei Lin and colleagues managed to purify and functionally reconstitute a ZIP homolog from the bacterium Bordetella bronchiseptica (ZIPB), allowing for a detailed characterization of transport activity. Their kinetic analysis showed that ZIPB-mediated zinc flux was nonsaturable, electrogenic, voltage-dependent, and selective for zinc and cadmium. The observed kinetics indicate that ZIPB is a selective electrodiffusional channel facilitating passive uptake driven by zinc concentration gradients. These intriguing results challenge the common assumption that cellular zinc uptake is an active process and should be useful for future physiological studies on cytosolic zinc transport and homeostasis.