Evidence for iron channeling in the Fet3p-Ftr1p high-affinity iron uptake complex in the yeast plasma membrane

In high-affinity iron uptake in the yeast Saccharomyces cerevisiae, Fe-II is oxidized to Fe-III by the multicopper oxidase, Fet3p, and the Fe-III produced is transported into the cell via the iron permease, Ftr1p. These two proteins are likely part of a heterodimeric or higher order complex in the yeast plasma membrane. We provide kinetic evidence that the Fet3p-produced Fe-III is trafficked to Ftr1p for permeation by a classic metabolite channeling mechanism. We examine the Fe-59 uptake kinetics for a number of complexes containing mutant forms of both Fet3p and Ftr1p and demonstrate that a residue in one protein interacts with one in the other protein along the iron trafficking pathway as would be expected in a channeling process. We show that, as a result of some of these mutations, iron trafficking becomes sensitive to an added FeIII chelator that inhibits uptake in a strictly competitive manner. This inhibition is not strongly dependent on the chelator strength, however, suggesting that FeIII dissociation from the iron uptake complex, if it occurs, is kinetically slow relative to iron permeation. Metabolite channeling is a common feature of multifunctional enzymes. We constructed the analogous ferroxidase, permease chimera and demonstrate that it supports iron uptake with a kinetic pattern consistent with a channeling mechanism. By analogy to the FeIII trafficking that leads to the mineralization of the ferritin core, we propose that ferric iron channeling is a conserved feature of iron homeostasis in aerobic organisms.