Calcium ion exchange in crystalline gelsolin.
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.
Chumnarnsilpa
Sakesit
S
Department of Medical Biochemistry and Microbiology, Uppsala Biomedical Center, Uppsala University, Uppsala 751 23, Sweden.
Loonchanta
Anantasak
A
Xue
Bo
B
Choe
Han
H
Urosev
Dunja
D
Wang
Hui
H
Lindberg
Uno
U
Burtnick
Leslie D
LD
Robinson
Robert C
RC
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
D000199:Actins; D002135:Calcium-Binding Proteins; D018260:Gelsolin; D002118:Calcium