Abstract
BackgroundThe degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu+ or Cd2+. They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. However, it has not as yet been discerned if their specific metal occupation is the result of a rigid control of metal availability, or isoform expression programming in the hosting tissues or of structural differences of the respective peptides determining the coordinative options for the different metal ions. In this study, the Roman snail (Helix pomatia) Cu-loaded and Cd-loaded isoforms (HpCuMT and HpCdMT) were used as model molecules in order to elucidate the biochemical and evolutionary mechanisms permitting pulmonate MTs to achieve specificity for their cognate metal ion.ResultsHpCuMT and HpCdMT were recombinantly synthesized in the presence of Cd2+, Zn2+ or Cu2+ and corresponding metal complexes analysed by electrospray mass spectrometry and circular dichroism (CD) and ultra violet-visible (UV-Vis) spectrophotometry. Both MT isoforms were only able to form unique, homometallic and stable complexes (Cd6-HpCdMT and Cu12-HpCuMT) with their cognate metal ions. Yeast complementation assays demonstrated that the two isoforms assumed metal-specific functions, in agreement with their binding preferences, in heterologous eukaryotic environments. In the snail organism, the functional metal specificity of HpCdMT and HpCuMT was contributed by metal-specific transcription programming and cell-specific expression. Sequence elucidation and phylogenetic analysis of MT isoforms from a number of snail species revealed that they possess an unspecific and two metal-specific MT isoforms, whose metal specificity was achieved exclusively by evolutionary modulation of non-cysteine amino acid positions.ConclusionThe Roman snail HpCdMT and HpCuMT isoforms can thus be regarded as prototypes of isoform families that evolved genuine metal-specificity within pulmonate molluscs. Diversification into these isoforms may have been initiated by gene duplication, followed by speciation and selection towards opposite needs for protecting copper-dominated metabolic pathways from nonessential cadmium. The mechanisms enabling these proteins to be metal-specific could also be relevant for other metalloproteins.
Highlights
The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy
Binding of Zn2+ and Cd2+ by HpCdMT Mass spectrometric analysis documents that recombinant synthesis of HpCdMT in E. coli cultured in Cd2+ or Zn2+-enriched media led to the production of only a single species with a fixed content of either six equivalents of cadmium or zinc (Figure 1a and 1b)
The two metal complexes - Cd6-HpCdMT and Zn6-HpCdMT display optical spectra with steep rises of absorbance below 270 nm and below 240 nm which is typical of tetrahedral bonding of both metal ions to multiple thiolate ligands (Figure 2a and 2c)
Summary
The degree of metal binding specificity in metalloproteins such as metallothioneins (MTs) can be crucial for their functional accuracy. Unlike most other animal species, pulmonate molluscs possess homometallic MT isoforms loaded with Cu+ or Cd2+ They have, so far, been obtained as native metal-MT complexes from snail tissues, where they are involved in the metabolism of the metal ion species bound to the respective isoform. Apparent metal specificity results from a disproportional oversupply of a certain metal ion, due to particular physiological conditions such as metabolic trace element disorders [14] or cellular overload due to metal exposure [15] In such cases, this metal ion occupies all the binding sites of an MT molecule which would otherwise form heterometallic complexes. True metal specificity requires an exclusive binding preference for a certain metal to an MT peptide due to its innate structural configuration
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