Abstract
The ability to self-propagate is one of the most intriguing characteristics of amyloid fibrils, and is a feature of great interest both to stopping unwanted pathological amyloid, and for engineering functional amyloid as a useful nanomaterial. The sequence and structural tolerances for amyloid seeding are not well understood, particularly concerning the propagation of distinct fibril morphologies (polymorphs) across species. This study examined the seeding and cross-seeding reactions between two unique fibril polymorphs, one long and flexible (formed at pH 2) and the other short and rigid (formed at pH 6.3), of human lysozyme and hen egg-white lysozyme. Both polymorphs could cross-seed aggregation across species, but this reaction was markedly reduced under physiological conditions. For both species, the pH 6.3 fibril polymorph was dominant, seeding fibril growth with a faster growth rate constant at pH 2 than the pH 2 polymorph. Based on fibrillation kinetics and fibril morphology, we found that the pH 2 polymorph was not able to faithfully replicate itself at pH 6.3. These results show that two distinct amyloid polymorphs are both capable of heterologous seeding across two species (human and hen) of lysozyme, but that the pH 6.3 polymorph is favored, regardless of the species, likely due to a lower energy barrier, or faster configurational diffusion, to accessing this particular misfolded form. These findings contribute to our better understanding of amyloid strain propagation across species barriers.
Highlights
Understanding the structure and assembly mechanisms of amyloid fibrils is of great interest owing to their broad importance
Hen egg-white lysozyme and human lysozyme (HLZ) fibril formation was carried out under two different conditions that resulted in two distinct fibril polymorphs (Figure 1)
Following the kinetics of fibril formation using ThT (Figure 2) revealed that fibrillation was complete by 6 days at pH 2 and by 3 days at pH 6.3, for both hen egg-white lysozyme (HEWL) and HLZ
Summary
Understanding the structure and assembly mechanisms of amyloid fibrils is of great interest owing to their broad importance. Much remains to be understood about how proteins change their folded structure, self-assemble into protofilaments, and how these (typically 2–6) further combine to form mature fibrils (Adamcik and Mezzenga, 2018). A given protein can be induced to form distinct amyloid fibril strains, termed polymorphs, that can differ in terms of core structure, morphology, stability, and cytotoxicity (Tycko, 2015; Adamcik and Mezzenga, 2018). Fibril polymorphism is highly sensitive to environmental or chemical effects; this is perhaps best exemplified by the well-characterized Aβ1−40 peptide that forms distinct polymorphs in response to shaking (striated ribbon fibrils) or not shaking (twisted ribbon fibrils) (Tycko, 2015). The arrangement of water molecules around oligomers and protofilaments during fibrillation could affect polymorphism (Stephens and Kaminski Schierle, 2019)
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