The major function of a metallothionein from the aquatic fungus Heliscus lugdunensis is cadmium detoxification

Abstract

A spring from a former copper shale mine in the area of Mansfelder Land, Germany, shows extremely high transition metal ion concentrations, i.e. 40mM ZnII, 208μM CuII, 61μM AsV, and 25μM CdII. This makes it a challenging habitat for living organisms as they have to cope with metal ion concentrations that by far exceed the values usually observed in spring water. One of the surviving species found is the aquatic fungus Heliscus lugdunensis (teleomorph: Nectria lugdunensis). Investigation of its redox related heavy metal tolerance revealed the presence of small thiol containing compounds as well as a small metallothionein, Neclu_MT1 (MT1_NECLU: P84865). While CdII-induction of metallothioneins is observed in many species, the fact that exclusively CdII, but not ZnII, CuI, AsIII or oxidative stress can induce Neclu_MT1 protein synthesis is unparalleled. To complement the physiological studies performed in the fungus H. lugdunensis, the CdII and ZnII binding characteristics of the recombinantly expressed protein were spectroscopically analysed in vitro aiming to demonstrate the observed CdII specificity also on the protein level. Stoichiometric analyses of the recombinant protein in combination with photospectrometric metal ion titrations and 113Cd-NMR experiments reveal that metal ion binding capacities and consequently the structures formed at physiological Neclu_MT1 concentrations differ from each other. Concluding, we describe the first solely CdII-inducible metallothionein, Neclu_MT1, from H. lugdunensis, featuring a difference in the structure of the CdIIversus the ZnII metalated protein in a physiologically relevant concentration range.