Zinc transfer from the embryo-specific metallothionein E from wheat: A case study

The direct observation of binding and release of spectroscopically silent metal ions such as Zn and Ca by proteins has been challenging before the advent of native electrospray ionisation mass spectrometry. This report highlights the powerful capability of ESI-MS to provide insight into metalloprotein speciation that is independent of any spectroscopic property. Using the zinc-binding plant metallothionein E from wheat as a study case, we show that ESI-MS is unique amongst other techniques in capturing intermediary metallospecies that evolve during the course of metal transfer to the chelator EDTA, as a model reaction to mimic the biological function of the protein as a zinc donor. Zinc release from the two-domain protein E appears to be extremely rapid and non-cooperative, and progresses with loss of one zinc ion from the fully loaded Zn species, and a transient build-up of Zn and Zn species, which further react to give species with 0-3 zinc ions bound. H NMR data has provided further insights into the different behaviour of the two domains upon metal depletion. © 2010 the Owner Societies.