The role of the IT-state in D76N ?2-microglobulin amyloid assembly: A crucial intermediate or an innocuous bystander?

ABSTRACT: The D76N variant of human ?2-microglobulin (?2m) is the causative agent of a hereditary amyloid disease. Interestingly, D76N-associated amyloidosis has a distinctive pathology compared with aggregation of WT-?2m, which occurs in dialysis-related amyloidosis. A folding intermediate of WT-?2m, known as the IT-state, which contains a nonnative trans Pro-32, has been shown to be a key precursor of WT-?2m aggregation in vitro However, how a single amino acid substitution enhances the rate of aggregation of D76N-?2m and gives rise to a different amyloid disease remained unclear. Using real-time refolding experiments monitored by CD and NMR, we show that the folding mechanisms of WT- and D76N-?2m are conserved in that both proteins fold slowly via an IT-state that has similar structural properties. Surprisingly, however, direct measurement of the equilibrium population of IT using NMR showed no evidence for an increased population of the IT-state for D76N-?2m, ruling out previous models suggesting that this could explain its enhanced aggregation propensity. Producing a kinetically trapped analog of IT by deleting the N-terminal six amino acids increases the aggregation rate of WT-?2m but slows aggregation of D76N-?2m, supporting the view that although the folding mechanisms of the two proteins are conserved, their aggregation mechanisms differ. The results exclude the IT-state as the origin of the rapid aggregation of D76N-?2m, suggesting that other nonnative states must cause its high aggregation rate. The results highlight how a single substitution at a solvent-exposed site can affect the mechanism of aggregation and the resulting disease.

SUBMITTER: Smith HI 

PROVIDER: S-EPMC7458819 | biostudies-literature | 2020 Aug

REPOSITORIES: biostudies-literature