title: Earthworm Lumbricus rubellus MT-2: Metal Binding and Protein Folding of a True Cadmium-MT.
abstract: Earthworms express, as most animals, metallothioneins (MTs)-small, cysteine-rich proteins that bind d(10) metal ions (Zn(II), Cd(II), or Cu(I)) in clusters. Three MT homologues are known for Lumbricus rubellus, the common red earthworm, one of which, wMT-2, is strongly induced by exposure of worms to cadmium. This study concerns composition, metal binding affinity and metal-dependent protein folding of wMT-2 expressed recombinantly and purified in the presence of Cd(II) and Zn(II). Crucially, whilst a single Cd₇wMT-2 species was isolated from wMT-2-expressing E. coli cultures supplemented with Cd(II), expressions in the presence of Zn(II) yielded mixtures. The average affinities of wMT-2 determined for either Cd(II) or Zn(II) are both within normal ranges for MTs; hence, differential behaviour cannot be explained on the basis of overall affinity. Therefore, the protein folding properties of Cd- and Zn-wMT-2 were compared by ¹H NMR spectroscopy. This comparison revealed that the protein fold is better defined in the presence of cadmium than in the presence of zinc. These differences in folding and dynamics may be at the root of the differential behaviour of the cadmium- and zinc-bound protein in vitro, and may ultimately also help in distinguishing zinc and cadmium in the earthworm in vivo.
authors.affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. GKowald@acciuk.co.uk.
authors.forename: Gregory R
authors.initials: GR
authors.lastname: Kowald
authors.affiliation: MRC-PHE Centre for Environment &amp; Health, King´s College London, London SE1 9NH, UK. stephen.sturzenbaum@kcl.ac.uk.|Analytical and Environmental Sciences Division, Faculty of Life Science &amp; Medicine, King's College London, London SE1 9NH, UK. stephen.sturzenbaum@kcl.ac.uk.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Stürzenbaum
authors.affiliation: Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK. c.blindauer@warwick.ac.uk.
authors.forename: Claudia A
authors.initials: CA
authors.lastname: Blindauer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002104:Cadmium / Q000378:metabolism*; D008668:Metallothionein / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D009835:Oligochaeta / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D017510:Protein Folding; D016415:Sequence Alignment; D015032:Zinc / Q000378:metabolism*
chemical_list: D002104:Cadmium; D008668:Metallothionein; D015032:Zinc
keywords: NMR spectroscopy; cadmium; mass spectrometry; metallothionein; protein folding; selectivity; zinc