Yeast SMF1 mediates H⁺-coupled iron uptake with concomitant uncoupled cation currents

Yeast membrane proteins SMF1, SMF2, and SMF3 are homologues of the DCT1 metal ion transporter family. Their functional characteristics and the implications of these characteristics in vivo have not yet been reported. Here we show that SMF1 expressed in Xenopus oocytes mediates H⁺-dependent Fe²⁺ transport and uncoupled Na⁺ flux. SMF1-mediated Fe²⁺ transport exhibited saturation kinetics (K(m) = 2.2 μM), whereas the Na⁺ flux did not, although both processes were electrogenic. SMF1 is also permeable to Li⁺, Rb⁺, K⁺, and Ca²⁺, which likely share the same uncoupled pathway. SMF2 (but not SMF3) mediated significant increases in both Fe²⁺ and Na⁺ transport compared with control oocytes. These data are consistent with the concept that uptake of divalent metal ions by SMF1 and SMF2 is essential to yeast cell growth. Na⁺ inhibited metal ion uptake mediated by SMF1 and SMF2 expressed in oocytes. Consistent with this, we found that increased sensitivity of yeast to EGTA in the high Na⁺ medium is due to inhibition of SMF1- and SMF2-mediated metal ion transport by uncoupled Na⁺ pathway. Interestingly, DCT1 also mediates Fe²⁺-activated uncoupled currents. We propose that uncoupled ion permeabilities in metal ion transporters protect cells from metal ion overload.