Conformational Switching within Individual Amyloid Fibrils

A key structural component of amyloid fibrils is a highly ordered, crystalline-like cross-β-sheet core. Conformationally different amyloid structures can be formed within the same amino acid sequence. It is generally assumed that individual fibrils consist of conformationally uniform cross-β-structures. Using mammalian recombinant prion protein (PrP), we showed that, contrary to common perception, amyloid is capable of accommodating a significant conformational switching within individual fibrils. The conformational switch occurred when the amino acid sequence of a PrP variant used as a precursor substrate in a fibrillation reaction was not compatible with the strain-specific conformation of the fibrillar template. Despite the mismatch in amino acid sequences between the substrate and template, individual fibrils recruited the heterologous PrP variant; however, the fibril elongation proceeded through a conformational adaptation, resulting in a change in amyloid strain within individual fibrils. This study illustrates the high adaptation potential of amyloid structures and suggests that conformational switching within individual fibrils may account for adaptation of amyloid strains to a heterologous substrate. This work proposes a new mechanistic explanation for the phenomenon of strain conversion and illustrates the direction in evolution of amyloid structures. This study also provides a direct illustration that catalytic activity of self-replicating amyloid structures is not ultimately coupled with their templating effect.The ability to form amyloid structures is considered to be one of the most general properties of a polypeptide backbone (1). Regardless of the specific peptides or proteins involved in fibril formation, all types of amyloid fibrils share a common structural motif that consists of a cross-β-structure (2). Cross-β-structures are comprised of highly ordered, nearly anhydrous, crystalline-like β-sheets stabilized by hydrogen bonding and densely packed side chains (3, 4). Growing evidence indicates that multiple amyloid structures referred to as amyloid strains could be formed within the same amino acid sequence (5–7).Amyloids are capable of self-replicating (8). Self-replicating properties of amyloid fibrils are attributed to the unique arrangement of cross-β-strands that are assembled perpendicular to the fibrillar axis, where β-strands at the growing edge provide a template for recruiting and converting a monomeric precursor. The self-replicating property of the amyloid cross-β-structure consists of two activities: catalytic (i.e. the ability to convert a monomeric precursor into an amyloid state) and templating (i.e. the ability to accurately imprint the strain-specific conformation onto a newly recruited polypeptide). The templating activity is believed to be intimately coupled to the catalytic activity and accounts for the high fidelity of amyloid replication. High fidelity of replication requires identity or high homology between the amino acid sequences of a fibrillar template and a precursor substrate. The species specificity of a template-substrate interaction is believed to account for the species barrier in prion transmission and species specificity of in vitro cross-seeded fibrillation reactions. Local perturbations arising due to mismatches in packing of amino acid side chains within the crystalline-like cross-β-structures could prevent efficient replication of amyloid fibrils.It is generally assumed that individual fibrils are structurally uniform, i.e. maintain the same structure of a cross-β-core throughout the fibrillar length. In the current study, we showed that, contrary to the common perception, amyloid fibrils are capable of accommodating significant conformational switching within individual fibrils. The conformational switch occurred when the amino acid sequence of the precursor substrate was not compatible with the conformation of the template. Despite mismatched amino acid sequences, individual fibrils were able to recruit the heterologous recombinant prion protein (PrP)² variant; however, fibril elongation proceeded through switching to a new conformational state. The implications of these studies are multifold. First, our work illustrates the high adaptation potential of amyloid structures and suggests that the conformational switch accounts for adaptation of amyloid strains to the heterologous substrate. Second, the current studies propose a new molecular explanation for the phenomenon referred to as convergence of strains. Third, this work illustrates the directionality in evolution of amyloid structures, showing that the species-specific amyloid structures (i.e. structures that exist only within a single PrP sequence) can give rise to promiscuous or indiscriminative structures (structures compatible with several PrP variants), but not vice versa. Finally, our studies provide direct illustration that catalytic activity of self-replicating amyloid structures is not ultimately coupled with their templating effect.