title: Zinc ion coordination as a modulating factor of the ZnuA histidine-rich loop flexibility: a molecular modeling and fluorescence spectroscopy study.
abstract: ZnuA is the soluble component of the high-affinity ZnuABC zinc transporter belonging to the ATP-binding cassette-type periplasmic Zn-binding proteins. The zinc transporter ZnuABC is composed by three proteins: ZnuB, the membrane permease, ZnuC, the ATPase component and ZnuA, the soluble periplasmic metal-binding protein which captures Zn and delivers it to ZnuB. The ZnuA protein contains a charged flexible loop, rich in histidines and acidic residues, showing significant species-specific differences. Various studies have established that this loop contributes to the formation of a secondary zinc binding site, which has been proposed to be important in the acquisition of periplasmic Zn for its delivery to ZnuB or for regulation of zinc uptake. Due to its high mobility the structure of the histidine-rich loop has never been solved by X-ray diffraction studies. In this paper, through a combined use of molecular modeling, mutagenesis and fluorescence spectroscopy, we confirm the presence of two zinc binding sites characterized by different affinities for the metal ion and show that the flexibility of the loop is modulated by the binding of the zinc ions to the protein. The data obtained by fluorescence spectroscopy have then be used to validate a 3D model including the unsolved histidine-rich loop.
authors.affiliation: Department of Biology, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Castelli
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Stella
authors.forename: Patrizia
authors.initials: P
authors.lastname: Petrarca
authors.forename: Andrea
authors.initials: A
authors.lastname: Battistoni
authors.forename: Alessandro
authors.initials: A
authors.lastname: Desideri
authors.forename: Mattia
authors.initials: M
authors.lastname: Falconi
mesh_terms: D018528:ATP-Binding Cassette Transporters; D001665:Binding Sites; D027682:Cation Transport Proteins / Q000737:chemistry* / Q000235:genetics; D002413:Cations, Divalent / Q000737:chemistry; D006639:Histidine / Q000737:chemistry* / Q000235:genetics; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D013050:Spectrometry, Fluorescence; D015032:Zinc / Q000737:chemistry*
chemical_list: D018528:ATP-Binding Cassette Transporters; D027682:Cation Transport Proteins; D002413:Cations, Divalent; D006639:Histidine; D015032:Zinc