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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.

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 the absence of Ca 2؉ 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 ϳ1.6-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 the same pH range indicated that the radius of gyration and maximum linear dimension of gelsolin molecules increased from 30.3 to 34.1 Å and from 100 to 125 Å , respectively. Models generated for each dataset indicated that similar to the Ca 2؉ -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 the G1 domain from the rest of the five domains, whereas the Ca 2؉ -sensitive latch between G2 and G6 domains remains closed. Interestingly, increasing the free Ca 2؉ level to merely ϳ40 nM, the partially open pH 5 shape "sprung open" to a shape seen earlier for this protein at pH 8 and 1 mM free Ca 2؉ . Also, pH alone could induce a shape where the 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 the F-actin-severing shape of gelsolin with micromolar levels of Ca 2؉ available.