The amyloidogenicity of gelsolin is controlled by proteolysis and pH

Background: Normally, gelsolin functions in plasma as part of the actin-sca venging system to assemble and disassemble actin filaments, The Asp187 --> Asn (D187N) and Asp187 --> Tyr (D187Y) gelsolin mutations facilitate two pr oteolytic cuts in the parent protein generating a 71-residue fragment that forms amyloid fibrils in humans, putatively causing Finnish type familial a myloidosis (FAF). We investigated the role of the D187N mutation in amyloid ogenicity using biophysical studies in vitro. Results: Both the recombinant wild-type and D187N FAF-associated gelsolin f ragments 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, h owever. Conclusions: The D187N mutation has been suggested to destabilize the struc ture of the gelsolin parent protein (specifically domain 2), facilitating t wo 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 environ mental change appears necessary to convert the unstructured fragment into a myloid fibrils, probably through an associative mechanism, The wild-type ge lsolin fragment will make amyloid fibrils from pH 6 to 4 in vitro, but neit her the wildtype fragment nor fibrils have been observed in vivo. It is pos sible that domain 2 of wild-type gelsolin is stable in the context of the w hole protein and not susceptible to the proteolytic degradation that afford s the 71-residue FAF-associated peptide.