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 (Askw ith, 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 e xtensive sequence homology to the family of multi-copper oxidases. In this communication, we demonstrate that the gene product is a cell sur face ferroxidase involved in iron transport. Cells that contain a func tional FET3 gene product exhibited an iron-dependent non-mitochondrial increase in oxygen consumption, Comparison of the rate of iron oxidat ion to O-2, consumption yielded an approximate value of 4:1, as predic ted for a ferroxidase. Spheroplasts obtained from cells grown under lo w iron conditions also displayed an iron-dependent increase in O-2 con sumption. Treatment of spheroplasts with trypsin or affinity-purified antibodies directed against the putative external ferroxidase domain o f Fet3 had no effect on basal O-2 consumption but inhibited the iron-d ependent increase in O-2 consumption. Anti-peptide antibodies directed against the cytosolic domain of Fet3 had no effect on O-2 consumption . These studies indicate that Fets is a plasma membrane ferroxidase re quired for high affinity iron uptake, in which the ferroxidase-contain ing domain is localized on the external cell surface.