Visualizing Temperature Mediated Activation of Gelsolin and Its Deactivation By Pip₂: A Saxs Based Study

This is the first report describing temperature based initiation of gelsolin's F-actin depolymerization activity, even in absence of free Ca²⁺ or low pH. Small angle X-ray scattering (SAXS) and circular dichroism (CD) studies revealed that temperature in the range of 30-40 °C is capable of opening the G1 domain alone, as remaining domains are held together by the Ca²⁺-sensitive C-tail latch without any loss in the secondary structural content. Full opening of all domains of tail-less gelsolin, and retention of closed shape for G2-G6 gelsolin merely by heating, further substantiated our findings. The Ca²⁺/pH independent activity of gelsolin near physiological temperature brought out a query: whether gelsolin is always active, and if not, what might deactivate it? Earlier, PIP₂ has been reported to render gelsolin inactive with no structural insight. Reduction in shape parameters and modeling revealed that PIP₂ reverses the temperature induced extension of g1-g2 linker leading to a compact shape seen for Ca²⁺-free gelsolin. Similar results for partially activated gelsolin (by low pH or Ca²⁺ ions below 0.1 μM) imply that inside cells, depolymerization, capping, and nucleation of F-actin by gelsolin is regulated by the culmination of local Ca²⁺ ion concentration, pH, temperature and PIP₂ levels.