Global Shapes of F-actin Depolymerization-competent Minimal Gelsolins

Abstract

Background: Shape-function studies are necessary to design better therapeutic alternatives of the plasma gelsolin. Results: N-terminal fragment 30 -161 is the smallest segment with F-actin depolymerization potential, and G1-G3 can function independent of Ca 2ϩ ions or low pH. Conclusion: The g2-g3 linker plays a role in imparting pH/Ca 2ϩ insensitivity to G1-G3. Significance: We provide the first evidence that g2-g3 linker regulates mobility of the G1 domain.

Structural Basis for pH-mediated Regulation of F-actin Severing by Gelsolin Domain 1

“…Recent truncation studies have shown that the minimal gelsolin fragment (residues 28–161) consisting of domain 1 (G1) and the linker between domains G1 and G2 depolymerizes F-actin more efficiently than the full-length protein and other truncation mutants in vitro and in animal models10. This minimal fragment, denoted as G1+ hereafter, is nearly identical to the construct (residues 25–160) used previously for structural determination in complex with actin11.…”

“…For the full-length gelsolin, the gyration radius starts to increase at about pH 614, correlating with its activity dependence on pH1213. Although it is unclear at which pH the gyration radius of G1+ starts to change, G1+ is inactive at pH 7–8 and becomes partially active at pH 6 in the absence of Ca 2+  10. SAXS-based structure reconstructions suggested a pH-activated “open” state of gelsolin in which G1 is detached from domain G3 and domains G2-G6 still retain the inactive-like structure.…”

“…Thus gelsolin replacement may be considered as a potential therapy89. Besides full-length gelsolin, some gelsolin fragments have F-actin severing activity and also have a therapeutic effect in disease conditions accompanying cell injury810.…”

“…Previous reports have demonstrated that gelsolin requires no Ca 2+ for in vitro binding to G-actin, nucleating F-actin formation, and severing F-actin filaments at a pH less than 6.01213. According to recent small angle X-ray scattering (SAXS) data, lowering pH from 8 to 5 induces the increase of the gyration radii of gelsolin and G1+ in the absence of Ca 2+ by 9.7% and 11.6%, respectively1014. For the full-length gelsolin, the gyration radius starts to increase at about pH 614, correlating with its activity dependence on pH1213.…”

“…Due to the unavailability of atomic resolution structure of gelsolin at pHs lower than 6 in the absence of ligands, the exact activation mechanism by pH is still unclear. In addition, G1+ was proposed to assume a “closed” state in which the linker between domains G1 and G2 seems to be in contact with G1 in the absence of Ca 2+ at pH 8, but to adopt an “open” state in which the linker may extend away from G1 at pH 510. According to the activity dependence on pH, the Ca 2+ -free G1+ should exist in a “closed” state at a pH larger than 7.…”