A partially structured species of β2-microglobulin is significantly populated under physiological conditions and involved in fibrillogenesis

The folding Of beta (2)-microglobulin (beta (2)-M), the protein forming amyloid deposits in dialysis-related amyloidosis, involves formation of a partially folded conformation named I-2, which slowly converts into the native fold, N. Here we show that the partially folded species I-2 can be separated from N by capillary electrophoresis. Data obtained with this technique and analysis of kinetic data obtained with intrinsic fluorescence indicate that the I-2 conformation is populated to similar to 14 +/- 8% at equilibrium under conditions of pH and temperature close to physiological. In the presence of fibrils extracted from patients, the I-2 conformer has a 5-fold higher propensity to aggregate than N, as indicated by the thioflavine T test and light scattering measurements. A mechanism of aggregation of beta (2)-M in vivo involving the association of the preformed fibrils with the fraction of I-2, existing at equilibrium is proposed from these results. The possibility of isolating and quantifying a partially folded conformer of beta (2)-M involved in the amyloidogenesis process provides new opportunities to monitor hemodialytic procedures aimed at the reduction of such species from the pool of circulating beta (2)-m but also to design new pharmaceutical approaches that consider such species as a putative molecular target.