title: Calcium ion exchange in crystalline gelsolin.
abstract: Gelsolin is a calcium and pH-sensitive modulator of actin filament length. Here, we use X-ray crystallography to examine the extraction and exchange of calcium ions from their binding sites in different crystalline forms of the activated N and C-terminal halves of gelsolin, G1-G3 and G4-G6, respectively. We demonstrate that the combination of calcium and low pH activating conditions do not induce conformational changes in G4-G6 beyond those elicited by calcium alone. EGTA is able to remove calcium ions bound to the type I and type II metal ion-binding sites in G4-G6. Constrained by crystal contacts and stabilized by interdomain interaction surfaces, the gross structure of calcium-depleted G4-G6 remains that of the activated form. However, high-resolution details of changes in the ion-binding sites may represent the initial steps toward restoration of the arrangement of domains found in the calcium-free inactive form of gelsolin in solution. Furthermore, bathing crystals with the trivalent calcium ion mimic, Tb3+, results in anomalous scattering data that permit unequivocal localization of terbium ions in each of the proposed type I and type II ion-binding sites of both halves of gelsolin. In contrast to predictions based on solution studies, we find that no calcium ion is immune to exchange.
authors.affiliation: Department of Medical Biochemistry and Microbiology, Uppsala Biomedical Center, Uppsala University, Uppsala 751 23, Sweden.
authors.forename: Sakesit
authors.initials: S
authors.lastname: Chumnarnsilpa
authors.forename: Anantasak
authors.initials: A
authors.lastname: Loonchanta
authors.forename: Bo
authors.initials: B
authors.lastname: Xue
authors.forename: Han
authors.initials: H
authors.lastname: Choe
authors.forename: Dunja
authors.initials: D
authors.lastname: Urosev
authors.forename: Hui
authors.initials: H
authors.lastname: Wang
authors.forename: Uno
authors.initials: U
authors.lastname: Lindberg
authors.forename: Leslie D
authors.initials: LD
authors.lastname: Burtnick
authors.forename: Robert C
authors.initials: RC
authors.lastname: Robinson
mesh_terms: D000199:Actins / Q000378:metabolism; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism; D018360:Crystallography, X-Ray; D018260:Gelsolin / Q000737:chemistry* / Q000378:metabolism*; D011237:Predictive Value of Tests; D017434:Protein Structure, Tertiary
chemical_list: D000199:Actins; D002135:Calcium-Binding Proteins; D018260:Gelsolin; D002118:Calcium