OmZnT1 and OmFET, two metal transporters from the metal-tolerant strain Zn of the ericoid mycorrhizal fungus Oidiodendron maius, confer zinc tolerance in yeast

Two full-length cDNAs (OmZnT1 and OmFET) encoding membrane transporters were identified by yeast functional screening in the heavy metal tolerant ericoid mycorrhizal isolate Oidiodendron maius Zn. OmZnT1 belongs to the Zn-Type subfamily of the cation diffusion facilitators, whereas OmFET belongs to the family of iron permeases. Their properties were investigated in yeast by functional complementation of mutants affected in metal uptake and metal tolerance. Heterologous expression of OmZnT1 and OmFET in a Zn-sensitive yeast mutant restored the wild-type phenotype. Additionally, OmZnT1 expression also restored cobalt tolerance in a Co-sensitive mutant. A GFP fusion protein revealed that OmZnT1 was targeted to the endoplasmic reticulum membrane, a result consistent with a function for OmZnT1 in metal sequestration. Similarly to other iron permeases, OmFET-GFP was localized on the plasma membrane. OmFET restored the growth of uptake-defective strains for iron and zinc. Zinc-sensitive yeast mutants expressing OmFET specifically accumulated magnesium, as compared to cells transformed with the empty vector. We suggest that OmFET may counteract zinc toxicity by increasing entry of magnesium into the cell.

► Mycorrhizal fungi are known to protect host plants against metal toxicity.
► We isolated and characterized two fungal genes involved in Zn tolerance.
► The genes encode metal transport proteins with a new function in Zn tolerance.
► Expression of an Fe permease in yeast increase Zn tolerance through the entry of Mg.
► Our data suggest new mechanisms of metal tolerance in mycorrhizal fungi.