A Self-Consistent Molecular Mechanism of β₂-Microglobulin Aggregation

Despite the consensus on the origin of dialysis-related amyloidosis (DRA) being β₂-microglobulin (β₂m) aggregation, the debate on the underlying mechanism persists because of the continuous emergence of β₂m variant- and pH-dependent contradictory results. By characterizing the native monomeric (initiation) and aggregated fibrillar (termination) states of β₂m via a combination of two enhanced sampling approaches, we here propose a mechanism that explains the heterogeneous behavior of wild-type (WT) and pathogenic (V27M and D76N) β₂m variants in physiological and disease-pertinent acidic pH environments. It appears that the higher retainment of monomeric native folds at neutral pH (native-like) distinguishes pathogenic β₂m mutants from the WT (moderate loss). However, at acidic pH, all three variants behave similarly in producing a substantial amount of partially unfolded states (conformational switch, propensity), though with different extents (WT < V27M < D76N). Whereas at the fibrillar end, all β₂m variants display a pH-dependent protofilament separation pathway and a higher protofilament binding affinity (stability) at acidic pH, where the relative order of binding affinity (WT < V27M < D76N) remains consistent with pH modulation. Combining these observations, we conclude that β₂m variants possibly shift from native-like aggregation to conformational switch-initiated fibrillation as the pH is altered from neutral to acidic. The combined propensity-stability approach based on the initiation and termination points of β₂m aggregation not only assists us in deciphering the mechanism but also emphasizes the protagonistic roles of both terminal points in the overall aggregation process.