The amyloidogenicity of gelsolin is controlled by proteolysis and pH

Background: Normally, gelsolin functions in plasma as part of the actin-scavenging system to assemble and disassemble actin filaments. The Asp187 → Asn (D187N) and Asp187 → Tyr (D187Y) gelsolin mutations facilitate two proteolytic cuts in the parent protein generating a 71-residue fragment that forms amyloid fibrils in humans, putatively causing Finnish type familial amyloidosis (FAF). We investigated the role of the D187N mutation in amyloidogenicity using biophysical studies in vitro. Results: Both the recombinant wild-type and D187N FAF-associated gelsolin fragments adopt an ensemble of largely unfolded structures that do not self-associate into amyloid at pH 7.5. Incubation of either fragment at low pHs (6.0-4.0) leads to the formation of well-defined fibrils within 72 hours, however. Conclusions: The D187N mutation has been suggested to destabilize the structure of the gelsolin parent protein (specifically domain 2), facilitating two proteolytic cleavage events. Our studies demonstrate that generating the largely unstructured peptide is not sufficient alone for amyloid formation in vitro (on a time scale of months). A drop in pH or an analogous environmental change appears necessary to convert the unstructured fragment into amyloid fibrils, probably through an associative mechanism. The wild-type gelsolin fragment will make amyloid fibrils from pH 6 to 4 in vitro, but neither the wild-type fragment nor fibrils have been observed in vivo. It is possible that domain 2 of wild-type gelsolin is stable in the context of the whole protein and not susceptible to the proteolytic degradation that affords the 71-residue FAF-associated peptide.