THE PLANT CDNA LCT1 MEDIATES THE UPTAKE OF CALCIUM AND CADMIUM IN YEAST

Nonessential metal ions such as cadmium are most likely transported ac ross plant membranes via transporters for essential cations, To identi fy possible pathways for Cd2+ transport we tested putative plant catio n transporters for Cd2+ uptake activity by expressing cDNAs in Sacchar omyces cerevisiae and found that expression of one clone, LCT1, render s the growth of yeast more sensitive to cadmium. Ion flux assays showe d that Cd2+ sensitivity is correlated with an increase in Cd2+ uptake. LCT1-dependent Cd2+ uptake is saturable, lies in the high-affinity ra nge (apparent K-M for Cd2+ = 33 mu M) and is sensitive to block by La3 + and Ca2+. Growth assays demonstrated a sensitivity of LCT1-expressin g yeast cells to extracellular millimolar Ca2+ concentrations. LCT1-de pendent increase in Ca2+ uptake correlated with the observed phenotype , Furthermore, LCT1 complements a yeast disruption mutant in the MIDI gene, a non-LCT1-homologous yeast gene encoding a membrane C-a2+ influ x system required for recovery from the mating response, We conclude t hat LCT1 mediates the uptake of Ca2+ and Cd2+ in yeast and may therefo re represent a first plant cDNA encoding a plant Ca2+ uptake or an org anellar Ca2+ transport pathway in plants and may contribute to transpo rt of the toxic metal Cd2+ across plant membranes.