VISUALIZING THE NUCLEATING AND CAPPED STATES OF F-ACTIN BY Ca2+-GELSOLIN: SAXS DATA BASED STRUCTURES OF BINARY AND TERNARY COMPLEXES
Structural insight eludes on how full-length gelsolin depolymerizes and caps F-actin, while the same entity can nucleate polymerization of G-actins. Employing small angle X-ray scattering (SAXS) data analysis, we deciphered these two contrasting assemblies. Mixing Ca2+-gelsolin with F-actin in high salt F-buffer resulted in depolymerization of ordered F-actin rods to smaller sized species which became monodisperse upon dialysis with low salt G-buffer. These entities were the ternary (GA2) and binary (GA) complexes of gelsolin and actin with radius of gyration and maximum linear dimension of 4.55 and 4.68 nm, and 15 and 16 nm, respectively. In contrast, upon mixing G-actin with Ca2+-gelsolin in G-buffer, rapid association of higher order species started. Using size exclusion chromatography in-line with SAXS, we confirmed that initially GA and GA2 species are formed as seen upon depolymerization of F-actin, followed by dialysis. Interestingly, while GA2 could seed formation of native-like F-actin in both G- and F-buffer, GA failed in G-buffer. Thus, GA2 and GA are the central species formed via depolymerization or towards nucleation. SAXS profile referenced modeling revealed that: 1) in GA, actin is bound to the C-terminal half of gelsolin, and 2) in GA2, second actin binds to the open N-terminal half accompanied by dramatic rearrangements across gelsolins g1-g2 and g3-g4 linkers. Importantly, first structural insight is provided into the two probable models for GA2 with two actins in parallel, but differentially stacked: one in polymerization competent, and other in incompetent manner, suggesting latter to represent capped state along with the inert GA. TOC Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=129 SRC="FIGDIR/small/584204v1_ufig1.gif" ALT="Figure 1"> View larger version (45K): org.highwire.dtl.DTLVardef@9b7ab4org.highwire.dtl.DTLVardef@194d85aorg.highwire.dtl.DTLVardef@108151dorg.highwire.dtl.DTLVardef@1a24e88_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIOrderly decrement in the length of F-actin by Ca2+-gelsolin was tracked by SAXS. C_LIO_LIResidual re-association in 1:2 ratio in F-buffer was quenched by dialysis in G-buffer. C_LIO_LIIdentical GA and GA2 entities formed upon mixing F- or G-actin with Ca2+-gelsolin. C_LIO_LIModels of nucleation competent, GA2 showed differential stacking of two G-actins. C_LIO_LIN-terminal half of gelsolin reposition as GA2 changes to or from GA, the capped state. C_LI