Membrane-assisted amyloid formation is implicated in human diseases, and many of the aggregating species accelerate amyloid formation and induce cell death. While structures of membrane-associated intermediates would provide tremendous insights into the pathology and aid in the design of compounds to potentially treat the diseases, it has not been feasible to overcome the challenges posed by the cell membrane. Here, we use NMR experimental constraints to solve the structure of a type-2 diabetes related human islet amyloid polypeptide intermediate stabilized in nanodiscs. ROSETTA and MD simulations resulted in a unique β-strand structure distinct from the conventional amyloid β-hairpin and revealed that the nucleating NFGAIL region remains flexible and accessible within this isolated intermediate, suggesting a mechanism by which membrane-associated aggregation may be propagated. The ability of nanodiscs to trap amyloid intermediates as demonstrated could become one of the most powerful approaches to dissect the complicated misfolding pathways of protein aggregation.

Structural studies on membrane-anchored proteins containing a transmembrane (TM) helix have been hampered by difficulties in producing these proteins in a natively folded form. Detergents that are required to solubilize the hydrophobic TM helix usually destabilize the soluble domain. Thus, TM helices are removed for structural studies, which neglects the pivotal role of a membrane on protein function. This work presents a versatile strategy for the production of this protein class attached to phospholipid nanodiscs. By inserting the TM-helix into nanodiscs and a subsequent SortaseA-mediated ligation of the soluble domain, membrane-anchored BclxL could be obtained in a folded conformation. This strategy is suitable for high-resolution structure determination as well as for probing membrane location by NMR. This method will be applicable to a wide range of membrane-anchored proteins and will be useful to decipher their functional role in a native membrane environment.

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