title: Role of four conserved aspartic acid residues of EF-loops in the metal ion binding and in the self-assembly of ciliate Euplotes octocarinatus centrin.
abstract: Ciliate Euplotes octocarinatus centrin (EoCen) is an EF-hand calcium-binding protein closely related to the prototypical calcium sensor protein calmodulin. Four mutants (D37K, D73K, D110K and D146K) were created firstly to elucidate the importance of the first aspartic acid residues (Asp37, Asp73, Asp110 and Asp146) in the beginning of the four EF-loops of EoCen. Aromatic-sensitized Tb3+fluorescence indicates that the aspartic acid residues are very important for the metal-binding of EoCen, except for Asp73 (in EF-loop II). Resonance light scattering (RLS) measurements for different metal ions (Ca2+and Tb3+) binding proteins suggest that the order of four conserved aspartic acid residues for contributing to the self-assembly of EoCen is Asp37 > Asp146 > Asp110 > Asp73. Cross-linking experiment also exhibits that Asp37 and Asp146 play critical role in the self-assembly of EoCen. Asp37, in site I, which is located in the N-terminal domain, plays the most important role in the metal ion-dependent self-assembly of EoCen, and there is cooperativity between N-terminal and C-terminal domain (especially the site IV). In addition, the dependence of Tb3+induced self-assembly of EoCen and the mutants on various factors, including ionic strength and pH, were characterized using RLS. Finally, 2-p-toluidinylnaphthalene-6-sulfonate (TNS) binding, ionic strength and pH control experiments indicate that in the process of EoCen self-assembly, molecular interactions are mediated by both electrostatic and hydrophobic forces, and the hydrophobic interaction has the important status.
authors.affiliation: School of Basic Medical Sciences, Shanxi Medical University, Taiyuan, 030001, China. liuwen@sxmu.edu.cn.
authors.forename: Wen
authors.initials: W
authors.lastname: Liu
authors.affiliation: Department of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China.
authors.forename: Lian
authors.initials: L
authors.lastname: Duan
authors.affiliation: School of Basic Medical Sciences, Shanxi Medical University, Taiyuan, 030001, China.
authors.forename: Tijian
authors.initials: T
authors.lastname: Sun
authors.affiliation: Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Shanxi University, Taiyuan, 030006, China. yangbs@sxu.edu.cn.
authors.forename: Binsheng
authors.initials: B
authors.lastname: Yang
mesh_terms: D000595:Amino Acid Sequence; D001224:Aspartic Acid / Q000737:chemistry; D001665:Binding Sites; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D002942:Circular Dichroism; D017124:Conserved Sequence; D016812:Euplotes / Q000737:chemistry*; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions / Q000378:metabolism; D009154:Mutation; D009282:Naphthalenesulfonates / Q000378:metabolism; D009994:Osmolar Concentration; D013725:Terbium / Q000378:metabolism
chemical_list: D002135:Calcium-Binding Proteins; D007477:Ions; D009282:Naphthalenesulfonates; D013725:Terbium; D001224:Aspartic Acid; C009497:2-(4-toluidino)-6-naphthalenesulfonic acid
keywords: Aspartic acid; Centrin; Self-assembly; Terbium(III)