title: The crystal structure of the sorcin calcium binding domain provides a model of Ca2+-dependent processes in the full-length protein.
abstract: Sorcin is a 21.6 kDa calcium binding protein, expressed in a number of mammalian tissues that belongs to the small, recently identified penta-EF-hand (PEF) family. Like all members of this family, sorcin undergoes a Ca2+-dependent translocation from cytosol to membranes where it binds to target proteins. For sorcin, the targets differ in different tissues, indicating that it takes part in a number of Ca2+-regulated processes. The sorcin monomer is organized in two domains like in all PEF proteins: a flexible, hydrophobic, glycine-rich N-terminal region and a calcium binding C-terminal domain. In vitro, the PEF proteins are dimeric in their Ca2+-free form, but have a marked tendency to precipitate when bound to calcium. Stabilization of the dimeric structure is achieved by pairing of the uneven EF-hand, EF5. Sorcin can also form tetramers at acid pH. The sorcin calcium binding domain (SCBD, residues 33-198) expressed in Escherichia coli was crystallized in the Ca2+-free form. The structure was solved by molecular replacement and was refined to 2.2 A with a crystallographic R-factor of 22.4 %. Interestingly, the asymmetric unit contains two dimers. The structure of the SCBD leads to a model that explains the solution properties and describes the Ca2+-induced conformational changes. Phosphorylation studies show that the N-terminal domain hinders phosphorylation of SCBD, i.e. the rate of phosphorylation increased twofold in the absence of the N-terminal region. In addition, previous fluorescence studies indicated that hydrophobic residues are exposed to solvent upon Ca2+ binding to full-length sorcin. The model accounts for these data by proposing that Ca2+ binding weakens the interactions between the two domains and leads to their reorientation, which exposes hydrophobic regions facilitating the Ca2+-dependent binding to target proteins at or near membranes.
authors.affiliation: CNR, Centro Studio sulla Biologia Molecolare and Dipartimento di Scienze Biochimiche, Università La Sapienza, P.le Aldo Moro 5, Roma, 00185, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Ilari
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Johnson
authors.forename: Vassilios
authors.initials: V
authors.lastname: Nastopoulos
authors.forename: Daniela
authors.initials: D
authors.lastname: Verzili
authors.forename: Carlotta
authors.initials: C
authors.lastname: Zamparelli
authors.forename: Gianni
authors.initials: G
authors.lastname: Colotti
authors.forename: Demetrius
authors.initials: D
authors.lastname: Tsernoglou
authors.forename: Emilia
authors.initials: E
authors.lastname: Chiancone
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000737:chemistry* / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D018360:Crystallography, X-Ray; D019281:Dimerization; D020832:EF Hand Motifs; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011485:Protein Binding; D015202:Protein Engineering; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D013329:Structure-Activity Relationship; D013431:Sulfates / Q000378:metabolism
chemical_list: D002135:Calcium-Binding Proteins; D013431:Sulfates; D002118:Calcium