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Palumaa, P. et al.

Palumaa, Eriste, Njunkova, Pokras, Jörnvall, Sillard,

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 Zn(2+) and Cd(2+) 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 Me(7)MT-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 Zn(7)MT-2. Moreover, ZnMT-3 loses metals during ultrafiltration. MT-3, reconstituted with an excess of Zn(2+) or Cd(2+), 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.[1]

References

Brain-specific metallothionein-3 has higher metal-binding capacity than ubiquitous metallothioneins and binds metals noncooperatively. Palumaa, P., Eriste, E., Njunkova, O., Pokras, L., Jörnvall, H., Sillard, R. Biochemistry (2002) [Pubmed]

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