Spectroscopic characterization of a fruit-specific metallothionein: M. acuminata MT3

Abstract

Metallothioneins (MTs) are ubiquitous low molecular mass, cysteine-rich proteins with the ability to bind d 10 metal ions in the form of metal-thiolate clusters. In contrast to the vertebrate forms, knowledge about the properties of members of the plant metallothionein family is still scarce. The amino acid sequences of plant MTs are distinctively different to the sequences of other MT species. The protein under investigation, Musa acuminata (banana) MT3, belongs to the plant MT fruit-specific p3 subfamily. With a total of 10 cysteine residues, MT3 features a cysteine content and percentage that is more comparable to fungal and prokaryotic MTs than to the well characterized mammalian iso-forms. The gene sequence encoding MT3 was cloned into a suitable vector and the protein was recombinantly overexpressed in Escherichia coli. MT3 is able to coordinate a maximum of four divalent d 10 metal ions under the formation of metal-thiolate clusters. The hitherto unknown spectroscopic behavior of MT3 in combination with the metal ions Zn 2+, Cd 2+, Pb 2+, and Hg 2+ will be presented and gives rise to the existence of a weaker metal ion coordination site. The pH stability of the investigated zinc and cadmium clusters is comparable to the values found for other plant metallothioneins though significantly lower than for the mammalian iso-forms. Possible metal-thiolate cluster structures will additionally be discussed.