Visual Insight into How Low pH Alone Can Induce Actin-severing Ability in Gelsolin under Calcium-free Conditions

Abstract Gelsolin is a key actin cytoskeleton modulating protein primarily regulated by calcium and phosphoinositides. In addition, low pH has also been suggested to activate gelsolin in absence of Ca2+ ions, although no structural insight on this pathway is available except for a reported decrement in its diffusion coefficient at low pH. We also observed ~1.6 fold decrease in the molecular mobility of recombinant gelsolin when buffer pH was lowered from 9 to 5. Analysis of the small angle X-ray scattering data collected over same pH range indicated that the radius of gyration and maximum linear dimension of gelsolin molecules increased from 30.3 to 34.1 A and from 100 to 125 A, respectively. Models generated for each dataset indicated that similar to Ca2+ induced process, low pH also promotes unwinding of this six-domain protein but only partially. It appeared that pH is able to induce extension of G1 domain from rest of the five domains while the Ca2+-sensitive latch between G2 and G6 domains remains closed. Interestingly, increasing free Ca2+ level to merely ~40 nM, the partially open pH 5 shape ′sprung-open′ to a shape seen earlier for this protein at pH 8 and 1 mM free Ca2+. Also, pH alone could induce a shape where g3-g4 linker of gelsolin was open when we truncated the C-tail latch from this protein. Our results provide insight into how under physiological conditions, a drop in pH can fully activate F-actin severing shape of gelsolin with micromolar levels of Ca2+ available.