Amyloids are protein aggregates displaying a fibrillar morphology that build upon self-assembly of identical sequences. Although amyloids became popular due to their involvement in many human diseases (1), we are now certain that amyloid aggregation is also essential for many vital roles across all kingdoms of life (2). We now also know that amyloids are not just homo-polymers, and that two distinct sequences may co-assemble resulting in hybrid amyloids (3, 4). This concept of hybrid or hetero-amyloid has changed our vision of amyloid aggregation, highlighting the contribution of heterotypic interactions not only in disease but also as a new organizational principle (5).
Despite of the emergence of an increasing number of amyloid structures (6), understanding amyloid assembly requires knowledge not just on the final fibril structures, but also on the monomeric, non-assembled conformations and dynamics. Obtaining this knowledge is a difficult task because of the high propensity of amyloid-forming proteins to self-assemble even at very low concentrations, and the arduousness to distinguish intra- from intermolecular interactions due to the oligomeric nature of amyloids. In this regard, we show that the concept of hetero-amyloid open new avenues to overcome these conventional barriers, paving the way towards an improved understanding of protein-protein recognition during amyloid assembly. Using paradigms of co-assembling proteins, we have identified residues that mediate protein-protein interactions between two different sequences prior to hetero-amyloid association, in the non-assembled states. Analyses of chemical shifts, spin relaxation and paramagnetic relaxation measurements, as well as titration experiments of the two proteins have afforded a detailed view of the interactions between two hetero-amyloid-forming proteins. Key residues in the two proteins that are relevant for hetero-amyloid assembly are also essential for homo-amyloid, self-assembly of the individual sequences when studied in isolation, highlighting the potential of hybrid amyloids to understand fibril assembly events broadly.
Acknowledgements: Funded by the European Union (ERC, 101042403 – BiFOLDOME). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.