We have recently shown that a member of the Nramp family of metal transporters, Saccharomyces cerevisiae Smf1p, is tightly regulated at the level of protein stability and protein sorting. Under metal replete conditions, Smf1p is targeted to the vacuole for degradation in a manner dependent on the S. cerevisiae BSD2 gene product, but under metal starvation conditions, Smf1p accumulates at the cell surface. Here, we have addressed whether Smf1p activity may be necessary for its regulation by metal ions and Bsd2p. Well conserved residues within transmembrane domain 4 and the transport signature sequence of Smf1p were mutagenized. We identified two mutants G190A and G424A, which destroyed Smf1p activity as monitored by complementation of a smf1 mutation. Notably, these mutations also abolished control by metal ions and Bsd2p, suggesting that Smf1p metal transport function may be necessary for its regulation. Two additional mutants isolated (Q419A and E423A) exhibited wild-type complementation activity and were properly targeted for vacuolar degradation in a Bsd2-dependent manner. However, these mutants failed to re-distribute to the plasma membrane under conditions of metal starvation. A model is proposed herein describing the probable role of Smf1 protein conformation in directing its movement to the vacuole versus cell surface in response to changes in metal ion availability.
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