Full characterization of the Cu-, Zn-, and Cd-binding properties of CnMT1 and CnMT2, two metallothioneins of the pathogenic fungus Cryptococcus neoformans acting as virulence factors

Abstract

We report here the full characterization of the metal binding abilities of CnMT1 and CnMT2, two Cryptococcus neoformans proteins recently identified as metallothioneins (MTs), which have been shown to play a crucial role in the virulence and pathogenicity of this human-infecting fungus. In this work, we first performed a thorough in silico study of the CnMT1 and CnMT2 genes, cDNAs and corresponding encoded products. Subsequently, the Zn(II)-, Cd(II)- and Cu(I) binding abilities of both proteins were fully determined through the analysis of the metal-to-protein stoichiometries and the structural features (determined by ESI-MS, CD, ICP-AES and UV-vis spectroscopies) of the corresponding recombinant Zn-, Cd- and Cu-MT preparations synthesized in metal-enriched media. Finally, the analysis of the Zn/Cd and Zn/Cu replacement processes of the respective Zn–MT complexes when allowed to react with Cd(II) or Cu(I) aqueous solutions was performed. Comprehensive consideration of all gathered results allows us to consider both isoforms as genuine copper-thioneins, and led to the identification of unprecedented Cu₅-core clusters in MTs. CnMT1 and CnMT2 polypeptides appear to be evolutionarily related to the small fungal MTs, probably by ancient tandem-duplication events responding to a highly selective pressure to chelate copper, and far from the properties of Zn- and Cd-thioneins. Finally, we propose a modular structure of the Cu–CnMT1 and Cu–CnMT2 complexes on the basis of Cu₅ clusters, concordantly with the modular structure of the sequence of CnMT1 and CnMT2, constituted by three and five Cys-rich units, respectively.