Visualizing the nucleating and capped states of f-actin by Ca²⁺-gelsolin: Saxs data based structures of binary and...

Structural insight eludes on how full-length gelsolin depolymerizes and caps filamentous (F-)actin, while the same entity can nucleate polymerization of G-actins. Analyzing small angle X-ray scattering (SAXS) data, we deciphered assemblies which enable these contrasting processes. 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 monodispersed 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. 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 g1-g2 and g3-g4 linkers.