The importance of unstructured termini in the aggregation cascade of beta-2-microglobulin: insights from molecular simulations of D76N mutant

The identification of folding and aggregation intermediate states is important, both from a fundamental standpoint and for the design of new therapies for conformational disorders. Here, we use the single point mutant (D76N) of β2m, the causing agent of a hereditary systemic amyloidosis affecting visceral organs, as a model system to study the aggregation mechanism of β2m using molecular simulations. We present our predictions on the early molecular events triggering the amyloid cascade for the D76N mutant. Folding simulations highlight the existence of an aggregation-prone intermediate called I1 which presents an unstructured C-terminus and of an aggregation-prone intermediate featuring two unstructured termini called I2. Additionally, Monte Carlo docking simulations suggest that both intermediates have high aggregation-propensity. These simulations support an essential role of the termini and of the DE and EF-loops in the dimerization of both intermediates. The relevance of the C-terminus is higher at the acidic pH 5.2 while the N-terminus become more important at pH 6.2. At physiological pH, the DE and EF-loops are the most important regions for dimerization. These predictions rationalize experimental results that support the involvement of Lys-19, Phe-56, Trp-60 and Phe-62 in amyloidogenesis in the wild-type and other model systems of β2m.