A Self-Consistent\nMolecular Mechanism of β<sub>2</sub>‑Microglobulin Aggregation

Despite the consensus on the origin of dialysis-related\namyloidosis\n(DRA) being β<sub>2</sub>-microglobulin (β<sub>2</sub>m) aggregation, the debate on the underlying mechanism persists because\nof the continuous emergence of β<sub>2</sub>m variant- and pH-dependent\ncontradictory results. By characterizing the native monomeric (initiation)\nand aggregated fibrillar (termination) states of β<sub>2</sub>m via a combination of two enhanced sampling approaches, we here\npropose a mechanism that explains the heterogeneous behavior of wild-type\n(WT) and pathogenic (V27M and D76N) β<sub>2</sub>m variants\nin physiological and disease-pertinent acidic pH environments. It\nappears that the higher retainment of monomeric native folds at neutral\npH (native-like) distinguishes pathogenic β<sub>2</sub>m mutants\nfrom the WT (moderate loss). However, at acidic pH, all three variants\nbehave similarly in producing a substantial amount of partially unfolded\nstates (conformational switch, propensity), though with different\nextents (WT < V27M < D76N). Whereas at the fibrillar end, all\nβ<sub>2</sub>m variants display a pH-dependent protofilament\nseparation pathway and a higher protofilament binding affinity (stability)\nat acidic pH, where the relative order of binding affinity (WT <\nV27M < D76N) remains consistent with pH modulation. Combining these\nobservations, we conclude that β<sub>2</sub>m variants possibly\nshift from native-like aggregation to conformational switch-initiated\nfibrillation as the pH is altered from neutral to acidic. The combined\npropensity-stability approach based on the initiation and termination\npoints of β<sub>2</sub>m aggregation not only assists us in\ndeciphering the mechanism but also emphasizes the protagonistic roles\nof both terminal points in the overall aggregation process.