Acr3p is a plasma membrane antiporter that catalyzes As(III)/H(+) and Sb(III)/H(+) exchange in Saccharomyces cerevisiae

Resistance to arsenical compounds in Saccharomyces cerevisiae as well as in a growing number of prokaryotes and eukaryotes is mediated by members of the Acr3 family of transporters. In yeast cells, it has been clearly shown that Acr3p is localized to the plasma membrane and facilitates efflux of trivalent arsenic and antimony. However, until now, the energy dependence and kinetic properties of Acr3 proteins remained uncharacterized. In this work, we show that arsenite and antimonite uptake into everted membrane vesicles via the yeast Acr3 transporter is coupled to the electrochemical potential gradient of protons generated by the plasma membrane H(+)-translocating P-type ATPase. These results strongly indicate that Acr3p acts as a metalloid/H(+) antiporter. Two differential kinetic assays revealed that Acr3p-mediated arsenite/H(+) and antimonite/H(+) exchange demonstrates Michaelis-Menten-type saturation kinetics characterized by a maximum flux for permeating metalloids. The approximate K(m) values for arsenite and antimonite transport were the same, suggesting that Acr3p exhibits similar low affinity for both metalloids. Nevertheless, the maximal velocity of the transport at saturation concentrations of metalloids was approximately 3 times higher for arsenite than for antimonite. These findings may explain a predominant role of Acr3p in conferring arsenite tolerance in S. cerevisiae.