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

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 Zn(II), 208μM Cu(II), 61μM As(V), and 25μM Cd(II). 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 Cd(II)-induction of metallothioneins is observed in many species, the fact that exclusively Cd(II), but not Zn(II), Cu(I), As(III) or oxidative stress can induce Neclu_MT1 protein synthesis is unparalleled. To complement the physiological studies performed in the fungus H. lugdunensis, the Cd(II) and Zn(II) binding characteristics of the recombinantly expressed protein were spectroscopically analysed in vitro aiming to demonstrate the observed Cd(II) specificity also on the protein level. Stoichiometric analyses of the recombinant protein in combination with photospectrometric metal ion titrations and (113)Cd-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 Cd(II)-inducible metallothionein, Neclu_MT1, from H. lugdunensis, featuring a difference in the structure of the Cd(II)versus the Zn(II) metalated protein in a physiologically relevant concentration range.