The FET3 gene product required for high affinity iron transport in yeast is a cell surface ferroxidase

The yeast FET3 gene is required for high affinity iron transport (Askwith, C., Eide, D., Ho, A. V., Bernard, P. S., Li, L., Davis-Kaplan, S., Sipe, D. M., and Kaplan, J. (1994) Cell 76, 403-410). The gene has extensive sequence homology to the family of multi-copper oxidases. In this communication, we demonstrate that the gene product is a cell surface ferroxidase involved in iron transport. Cells that contain a functional FET3 gene product exhibited an iron-dependent non-mitochondrial increase in oxygen consumption. Comparison of the rate of iron oxidation to O₂ consumption yielded an approximate value of 4:1, as predicted for a ferroxidase. Spheroplasts obtained from cells grown under low iron conditions also displayed an iron- dependent increase in O2 consumption. Treatment of spheroplasts with trypsin or affinity-purified antibodies directed against the putative external ferroxidase domain of Fet3 had no effect on basal O₂ consumption but inhibited the iron-dependent increase in O₂ consumption. Anti-peptide antibodies directed against the cytosolic domain of Fet3 had no effect on O₂ consumption. These studies indicate that Fet3 is a plasma membrane ferroxidase required for high affinity iron uptake, in which the ferroxidase- containing domain is localized on the external cell surface.