The pathogenesis of protein misfolding diseases is attributed to the cytotoxicity caused by amyloidogenic prefibrillar aggregates, rather than mature fibrils. The presence of one or more amyloidogenic stretches in different proteins has been proven critical for initiating fibril formation. In the present study, we show that two natural compounds, curcumin and emetine, bind tightly (Kd < 1.6 μM) to the core amyloidogenic stretch (182-192) of gelsolin (AGel). Binding happens in different structural orientations, distinctly modulating the amyloidogenic pathway of AGel. While AGel alone undergoes sigmoidal transition to thioflavin T (ThT)-responsive fibrillar aggregates with clear lag phase, the presence of curcumin or emetine abolishes the lag phase and produces starkly different, noncytotoxic end products. Atomic force microscopy revealed that while curcumin augments fibril formation, emetine arrests it at an intermediate aggregated stage with no fibrillar morphology. FTIR spectroscopy, dynamic light scattering, and ANS fluorescence experiments also suggest that these two species are distinct. Curcumin and emetine also differentially affect the preformed amyloids with the former thickening the fibrils and the latter releasing reclusive oligomers. MD simulations further provided mechanistic insights of differential interaction by the two compounds modulating amyloid formation. The results were also confirmed on the disease-associated amyloidogenic fragment of gelsolin (fAGel). Thus, our findings suggest that targeting amyloidogenic stretches in proteins could be useful in designing novel molecules against protein misfolding diseases.
Keywords: Amyloid; curcumin; cytotoxicity; emetine; gelsolin; oligomer.