Reduction of conformational mobility and aggregation in W60G β₂-microglobulin: Assessment by ₁₅N NMR relaxation

The amyloid pathology associated with long-term haemodialysis is due to the deposition of β₂-microglobulin, the non-polymorphic light chain of class I major histocompatibility complex, that accumulates at bone joints into amyloid fibrils. Several lines of evidence show the relevance of the tryptophan residue at position 60 for the fibrillogenic transition of the protein. A comparative ₁₅N NMR relaxation analysis is presented for wild-type human β₂-microglobulin and W60G β₂- microglobulin, i.e. the mutant with a glycyne replacing the natural tryptophan residue at position 60. The experimental data, collected at 11.4 T and 310 K, were analyzed by means of the reduced spectral density approach. Molecular dynamics (MD) simulations and corresponding thermodynamic integration, together with hydrodynamic calculations were performed to support data interpretation. The analysis results for the mutant protein are consistent with a reduced aggregation with respect to the wild-type counterpart, as a consequence of an increased conformational rigidity probed by either NMR relaxation and MD simulations. Although dynamics in solution is other than fibrillar competence, the assessed properties of the mutant protein can be related with its reduced ability of forming fibrils when seeded in 20% trifluoroethanol.