Understanding the 7-Cys module amplification of C. neoformans metallothioneins: how high capacity Cu-binding polypeptides are built to neutralize host nutritional immunity

Experimental recombining of repetitive motifs leads to large functional metallothioneins and demonstrates their modular evolvability potential

Protein modularity is acknowledged for promoting the emergence of new protein variants via domain rearrangements. Metallothioneins (MTs) offer an excellent model system for experimentally examining the consequences of domain rearrangements due to the possibility to assess the functional properties of native and artificially created variants using spectroscopic methods and metal tolerance assays. In this study, we have investigated the functional properties of AbiMT4 from the snail Alinda biplicata (Gastropoda, Mollusca), a large MT comprising 10 putative β domains (β39β1), alongside four artificially designed variants differing in domain number, type, or order. Our findings reveal that AbiMT4 is a cadmium-selective protein with a high metal-binding capacity, characterized by structurally and functionally independent domains repeated in tandem along the protein. Our results indicate that due to its modular organization, AbiMT4 remains functional even when the number, type, and order of the domains are significantly altered. Furthermore, we demonstrate that the metal-binding properties of AbiMT4 are not dictated by the overall architecture of the protein but primarily arise from the properties of each individual domain. Using MTs as example, this work provides empirical evidence that domain rearrangements are an effective strategy for exploring new viable sequences and creating novel protein variants subject to adaptive selection. Thus, our study highlights the importance of the modular structure of proteins, as increasing their functional flexibility enhances their evolvability. Additionally, our work demonstrates a simple way to design and model new proteins for predefined functions.

Fluconazole induces ROS in Cryptococcus neoformans and contributes to DNA damage in vitro

Pathogenic basidiomycetous yeast, Cryptococcus neoformans, causes fatal meningitis in immunocompromised individuals. Fluconazole (FLC) is a fungistatic drug commonly administered to treat cryptococcosis. Unfortunately, FLC-resistant strains characterized by various degree of chromosomal instability were isolated from clinical patients. Importantly, the underlying mechanisms that lead to chromosomal instability in FLC-treated C. neoformans remain elusive. Previous studies in fungal and mammalian cells link chromosomal instability to the reactive oxygen species (ROS). This study provides the evidence that exposure of C. neoformans to FLC induces accumulation of intracellular ROS, which correlates with plasma membrane damage. FLC caused transcription changes of oxidative stress related genes encoding superoxide dismutase (SOD1), catalase (CAT3), and thioredoxin reductase (TRR1). Strikingly, FLC contributed to an increase of the DNA damage in vitro, when complexed with iron or copper in the presence of hydrogen peroxide. Strains with isogenic deletion of copper response protein metallothionein were more susceptible to FLC. Addition of ascorbic acid (AA), an anti-oxidant at 10 mM, reduced the inhibitory effects of FLC. Consistent with potential effects of FLC on DNA integrity and chromosomal segregation, FLC treatment led to elevated transcription of RAD54 and repression of cohesin-encoding gene SCC1. We propose that FLC forms complexes with metals and contributes to elevated ROS, which may lead to chromosomal instability in C. neoformans.

The Fungus Tremella mesenterica Encodes the Longest Metallothionein Currently Known: Gene, Protein and Metal Binding Characterization

Fungal Cu-thioneins, and among them, the paradigmatic Neurospora crassa metallothionein (MT) (26 residues), were once considered as the shortest MTs -the ubiquitous, versatile metal-binding proteins- among all organisms, and thus representatives of their primeval forms. Nowadays, fungal MTs of diverse lengths and sequence features are known, following 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) constitute 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 amplification 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 mesenterica (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-CXCX₃CSCPPGXCXCAXCP-, 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.

Does Variation of the Inter-Domain Linker Sequence Modulate the Metal Binding Behaviour of Helix pomatia Cd-Metallothionein?

Snail metallothioneins (MTs) constitute an ideal model to study structure/function relationships in these metal-binding polypeptides. Helix pomatia harbours three MT isoforms: the highly specific CdMT and CuMT, and an unspecific Cd/CuMT, which represent paralogous proteins with extremely different metal binding preferences while sharing high sequence similarity. Preceding work allowed assessing that, although, the Cys residues are responsible for metal ion coordination, metal specificity or preference is achieved by diversification of the amino acids interspersed between them. The metal-specific MT polypeptides fold into unique, energetically-optimized complexes of defined metal content, when binding their cognate metal ions, while they produce a mixture of complexes, none of them representing a clear energy minimum, with non-cognate metal ions. Another critical, and so far mostly unexplored, region is the stretch linking the individual MT domains, each of which represents an independent metal cluster. In this work, we have designed and analyzed two HpCdMT constructs with substituted linker segments, and determined their coordination behavior when exposed to both cognate and non-cognate metal ions. Results unequivocally show that neither length nor composition of the inter-domain linker alter the features of the Zn(II)- and Cd(II)-complexes, but surprisingly that they influence their ability to bind Cu(I), the non-cognate metal ion.