Characterization of three distinct metallothionein genes of the Ag-hyperaccumulating ectomycorrhizal fungus Amanita strobiliformis

• Amanita strobiliformis has multiple functional metallothionein genes (AsMTs).
• AsMT1s (1a, 1b and 1c isoforms) are adjusted for the handling of Ag and Cu.
• AsMT3 is specifically adapted for the handling of Zn.
• AsMT3 is the first characterized member of the peptides conserved in agaricomycetes.
• Unlike AsMT1s and AsMT3, AsMT2 did not exhibit metal-specific function.

Mechanisms evolved in eukaryotes to handle heavy metals involve cytosolic, metal-binding metallothioneins (MTs). We have previously documented that the sequestration of silver (Ag) in the Ag-hyperaccumulating Amanita strobiliformis is dominated by 34-amino-acid (AA) AsMT1a, 1b, and 1c isoforms. Here we show that in addition to AsMT1a, 1b, and 1c isogenes, the fungus has two other MT genes: AsMT2 encoding a 34-AA AsMT2 similar to MTs known from other species, but unrelated to AsMT1s; AsMT3 coding for a 62-AA AsMT3 that shares substantial identity with as-yet-uncharacterized conserved peptides predicted in agaricomycetes. Transcription of AsMT1s and AsMT3 in the A. strobiliformis mycelium was specifically inducible by treatments with Ag or copper (Cu) and zinc (Zn) or cadmium (Cd), respectively; AsMT2 showed a moderate upregulation in the presence of Cd. Expression of AsMTs in the metal-sensitive Saccharomyces cerevisiae revealed that all AsMTs confer increased Cd tolerance (AsMT3 proved the most effective) and that, unlike AsMT1 and AsMT2, AsMT3 can protect the yeasts against Zn toxicity. The highest level of Cu tolerance was observed with yeasts expressing AsMT1a. Our data indicate that A. strobiliformis can specifically employ different MT genes for functions in the cellular handling of Ag and Cu (AsMT1s) and Zn (AsMT3).