Environmental conditions affect the kinetics of nucleation of amyloid fibrils and determine their morphology.

To understand and tackle amyloid-related diseases, it is crucial to investigate the factors that modulate amyloid formation of proteins. Our previous studies proved that the N47A mutant of the α-spectrin SH3 (Spc-SH3) domain forms amyloid fibrils quickly under mildly acidic conditions. Here, we analyze how experimental conditions influence the kinetics of assembly and the final morphology of the fibrils. Early formation of curly fibrils occurs after a considerable conformational change of the protein and the concomitant formation of small oligomers. These processes are strongly accelerated by an increase in salt concentration and temperature, and to a lesser extent by a reduction in pH. The rate-limiting step in these events has a high activation enthalpy, which is significantly reduced by an increase in NaCl concentration. At low-to-moderate NaCl concentrations, the curly fibrils convert to straight and twisted amyloid fibrils after long incubation times, but only in the presence of soluble species in the mixture, which suggests that the curly fibrils and the twisted amyloid fibrils are diverging assembly pathways. The results suggest that the influence of environmental variables on protein solvation is crucial in determining the nucleation kinetics, the pathway of assembly, and the final fibril morphology.