title: Global structure changes associated with Ca2+ activation of full-length human plasma gelsolin.
abstract: Gelsolin regulates the dynamic assembly and disassembly of the actin-based cytoskeleton in non-muscle cells and clears the circulation of filaments released following cell death. Gelsolin is a six-domain (G1-G6) protein activated by calcium via a multi-step process that involves unfolding from a compact form to a more open form in which the three actin-binding sites (on the G1, G2, and G4 subdomains) become exposed. To follow the global structural changes that accompany calcium activation of gelsolin, small-angle x-ray scattering (SAXS) data were collected for full-length human plasma gelsolin at nanomolar to millimolar concentrations of free Ca2+. Analysis of these data showed that, upon increasing free Ca2+ levels, the radius of gyration (Rg) increased nearly 12 A, from 31.1+/-0.3 to 43+/-2 A, and the maximum linear dimension (Dmax) of the gelsolin molecule increased 55 A, from 100 to 155A. Structural reconstruction of gelsolin from these data provided a striking visual tracking of the gradual Ca2+-induced opening of the gelsolin molecule and highlighted the critical role played by the flexible linkers between homologous domains. The tightly packed architecture of calcium-free gelsolin, seen from both SAXS and x-ray crystallographic models, is already partially opened up in as low as 0.5 nM Ca2+. Our data confirm that, although the molecule springs open from 0 to 1 microM free Ca2+, even higher calcium concentrations help to stabilize a more open structure, with increases in Rg and Dmax of approximately 2 and approximately 15 A, respectively. At these higher calcium levels, the SAXS-based models provide a molecular shape that is compatible with that of the crystal structures solved for Ca2+/gelsolin C-terminal and N-terminal halves+/-monomeric G-actin. Placement of these crystal structures within the boundaries of the SAXS-based model suggests a movement of the G1/G2 subunits that would be required upon binding to actin.
authors.affiliation: Department of Chemistry, University of North Carolina, Charlotte, North Carolina 28223-0001, USA.
authors.lastname: Ashish
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Paine
authors.forename: Paul B
authors.initials: PB
authors.lastname: Perryman
authors.forename: Lin
authors.initials: L
authors.lastname: Yang
authors.forename: Helen L
authors.initials: HL
authors.lastname: Yin
authors.forename: Joanna K
authors.initials: JK
authors.lastname: Krueger
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D003599:Cytoskeleton / Q000737:chemistry / Q000378:metabolism; D018260:Gelsolin / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008958:Models, Molecular; D011485:Protein Binding / Q000502:physiology; D020836:Protein Structure, Quaternary / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology
chemical_list: D000199:Actins; D018260:Gelsolin; D002118:Calcium