DECIPHERING THE AGGREGATION MECHANISMS IN HUMAN B2-MICROGLOBULIN

Beta2-microglobulin (β2m) is a 99-residue globular protein that represents the light chain of the major histocompatibility complex class I (MHCI). β2m is responsible for two types of human amyloid diseases: dialysis-related amyloidosis (DRA) caused by wt β2m, and hereditary systemic amyloidosis due to the D76N β2m mutant. Two independent projects were carried out during my PhD studies addressing both types of β2m-related amyloidosis: (i) The human cells adopt a sophisticated unfolded protein response (UPR) system for targeting misfolded/aggregated polypeptides. However, the D76N variant, which is unstable and aggregation prone, bypasses the UPR system and reaches the extracellular space forming amyloids. To understand the mechanism(s) that allow the D76N variant to escape the UPR system and to characterize its effect on MHCI, we performed a complete structural and biophysical study on a MHCI bearing the D76N variant. Our results show that MHCI acts as a chaperone that stabilize and hide the amyloidogenic variant from the UPR system, and transfers it to cell membrane, where during its normal turnover, the D76N β2m variant dissociates into blood serum and aggregates causing human pathologies. (ii) The early oligomeric species are responsible for cellular cytotoxicity in amyloidosis. Previous data showed a favourable association interface (between two facing D β-strands) that may involve in β2m oligomerization. To further elucidate its role in aggregation, we created a S-S linked dimer of β2m (DimC33) that interacts via the DD interface. Our data show that DimC33 is highly amyloidogenic compared to wt β2m, suggesting that dimerization through DD interface is instrumental for enhancing its aggregation propensity. Furthermore, DimC33 was co-crystallized in complex with Thioflavin-T (ThT), a well-known amyloid-specific dye, showing unique ThT binding site that may indicate a second key interface involved in β2m oligomerization.