Iron Sequestration by the Yeast Vacuole. A Study with Vacuolar Mutants of Saccharomyces cerevisiae

Abstract

To investigate iron uptake by the yeast vacuole, we examined iron accumulation and subcellular distribution in two Saccharomyces cerevisiae mutant strains with distinct alterations in vacuolar morpholgy and functions, the vacuolar protein targeting mutants vpt13 and vpt16. The mutant vpt13 is deficient in vacuolar acidification but possesses a morphologically normal vacuole, whereas vpt16 is devoid of any vacuole-like structure. The vacuole-lacking mutant vpt16 displayed reduced iron accumulation at high and low extracellular iron concentrations. Iron accumulation by vpt13 was not reduced at low extracellular iron concentrations, and only slightly reduced at high external iron. Subcellular fractionation of vpt13 after iron loading confirmed the vacuolar localization of the accumulated iron. Vacuolar iron sequestration thus is not dependent on vacuolar acidification, in contrast to vacuolar uptake of other metal cations such as calcium, strontium, manganese, and zinc. Conclusively, the vacuolar iron-uptake mechanism is different from the cation-proton exchangers mediating uptake of these other metal cations. Regarding the lacking dependence on vacuolar acidification as well as the high amounts of iron accumulated by the vacuole, vacuolar iron uptake could be mediated by an iron pump similar to the known ATP-dependent mechanisms of active transport.