Brain-specific metallothionein-3 has higher metal-binding capacity than ubiquitous metallothioneins and binds metals noncooperatively

Zinc metabolism in the cells is largely regulated by ubiquitous small proteins, metallothioneins (MT). Metallothionein-3 is specifically expressed in the brain and is down regulated in Alzheimer's disease. We demonstrate by mass spectrometry that MT-3, in contrast to common MTs, binds Zn2+ and Cd2+ in a noncooperative manner and can also bind higher stoichiometries of metals than seven. MT-3 reconstituted with seven metals exists in a dynamic equilibrium of different metalloforms, where the prevalent metalloform is Me7MT-3, but metalloforms with 6, 8, and even 9 metals are also present. The results from pH and stability studies demonstrate that the heterogeneity of metalloforms originates from the N-terminal beta-cluster, whereas the C-terminal alpha-cluster of MT-3 binds four metal ions such as that of common MTs. Experiments with EDTA demonstrate that the beta-cluster of ZnMT-3 has a higher metal transfer potential than the (beta-cluster of Zn7MT-2. Moreover, ZnMT-3 loses metals during ultrafiltration. MT-3, reconstituted with an excess of Zn2+ or Cd2+, exists as a dynamic mixture of metalloforms with higher than 7 metal stoichiometries (8-11). Such forms of ZnMT-3 are unstable and decompose partly already during a rapid gel filtration, whereas CdMT-3 forms are more-stable. Extra metal ions may bind to the beta-cluster region as well as to the carboxylates of MT-3. The specific metal-binding properties of MT-3 could be functionally implemented for buffering of fluctuating concentrations of zinc in zincergic neurons and for transfer of zinc to synaptic vesicles.