The fungus Tremella mesenterica encodes the longest metallothionein currently known: gene, protein and metal binding characterization

Abstract

Fungal Cu-thioneins, and among them, the paradigmatic Neurospora crassa metallothio- nein (MT) (26 residues), were once considered as the shortest MTs -the ubiquitous, versa- tile metal-binding proteins- among all organisms, and thus representatives of their primeval forms. Nowadays, fungal MTs of diverse lengths and sequence features are known, follow- ing the huge heterogeneity of the Kingdom of Fungi. At the opposite end of N . crassa MT, the recently reported Cryptococcus neoformans CnMT1 and CnMT2 (122 and 186 aa) con- stitute the longest reported fungal MTs, having been identified as virulence factors of this pathogen. CnMTs are high-capacity Cu-thioneins that appear to be built by tandem amplifi- cation of a basic unit, a 7-Cys segment homologous to N . crassa MT. Here, we report the in silico , in vivo and in vitro study of a still longer fungal MT, belonging to Tremella mesenter- ica (TmMT), a saprophytic ascomycete. The TmMT gene has 10 exons, and it yields a 779-bp mature transcript that encodes a 257 residue-long protein. This MT is also built by repeated fragments, but of variable number of Cys: six units of the 7-Cys building blocks- CXCX 3 CSCPPGXCXCAXCP-, two fragments of six Cys, plus three Cys at the N-terminus. TmMT metal binding abilities have been analyzed through the spectrophotometric and spectrometric characterization of its recombinant Zn-, Cd- and Cu-complexes. Results allow it to be unambiguous classified as a Cu-thionein, also of extraordinary coordinating capacity. According to this feature, when the TmMT cDNA is expressed in MT-devoid yeast cells, it is capable of restoring a high Cu tolerance level. Since it is not obvious that T . mesenterica shares the same physiological needs for a high capacity Cu-binding protein with C . neoformans , the existence of this peculiar MT might be better explained on the basis of a possible role in Cu-handling for the Cu-enzymes responsible in lignin degradation pathways.