Role of the yeast ABC transporter Yor1p in cadmium detoxification

Growth of yeast strains, either deleted for the vacuolar ABC transporter Ycf1 or deleted for the plasma membrane ABC transporter Yor1p or overexpressing Yor1p, were compared for their sensitivity to cadmium. On solid medium cell death (or growth inhibition) was observed at cadmium concentrations higher than 100 μM when yeasts were grown at 30 °C for 24 h. However, for all tested strains cell death (or growth inhibition) was already observed at 40 μM cadmium when incubated at 23 °C for 60 h. Thus cadmium is more toxic to yeast at 23 °C than at 30 °C. At 23 °C, the Δyor1 strain grew more slowly than the wild-type strain and the double Δyor1, Δycf1 deleted strain was much more sensitive to cadmium than each single Δyor1 or Δycf1 deletant. Overexpression of Yor1p in a Δycf1 strain restores full growth. Cadmium uptake measurements show that Δycf1 yeast strains expressing or overexpressing Yor1p store less cadmium than the corresponding Δycf1, Δyor1 strain. The strains expressing Yor1p display an energy-dependent efflux of cadmium estimated for the yeast overexpressing Yor1p to be about 0.02 nmol 109Cd/mg protein/min. Yeast cells loaded with radiolabeled glutathione and then with radioactive cadmium displayed a twice-higher efflux of glutathione than that of cadmium suggesting that Yor1p transports both compounds as a bis-glutathionato-cadmium complex. All together, these results suggest that in addition to being accumulated in the yeast vacuole by Ycf1p, cadmium is also effluxed out of the cell by Yor1p.