Is MtnE, the fifth Drosophila metallothionein, functionally distinct from the other members of this polymorphic protein family?

Metallothioneins (MTs) are a super-family of small, Cys-rich, non-homologous proteins that bind metal ions through the formation of metal-thiolate bonds. Although universally ubiquitous, they exhibit distinct metal-binding preferences, either for divalent (Zn-thioneins) or monovalent (Cu-thioneins) metal ions. Drosophila constitutes a bizarre exception, since it is currently the only case of metazoans synthesizing only Cu-thioneins, which are similar to the paradigmatic yeast Cup1 protein. Until recently, the Drosophila MT system was assumed to be composed of 4 isoforms (MtnA, MtnB, MtnC and MtnD), all of them responsive to heavy metal load through the dMTF1 transcription factor. The significance of this polymorphism has been analyzed in depth both at genomic and proteomic levels. Singularly, a fifth MT isoform was recently annotated and named MtnE. The analysis of the MtnE expression pattern revealed some differential traits with regard to the other MTs. We analyze here the peculiarities of the metal binding abilities of the MtnE polypeptide and compare them with those of the other Drosophila MTs determined through the same rationale. Characterization by ESI-MS spectrometry and CD and UV-visible spectrophotometry of the Zn(II)-, Cd(II)- and Cu(I)-MtnE complexes obtained by recombinant synthesis demonstrates that MtnE is the least metal-specific isoform of the Drosophila MTs, and therefore it could play a role when/where a broad spectrum of metal coordination abilities are advantageous in terms of physiological needs.