Cyclic N-terminal loop of amylin forms non amyloid fibers.

We report for the first time, to our knowledge, that the N-terminal loop (N_loop) of amylin (islet amyloid polypeptide (IAPP) residues 1-8) forms extremely long and stable non-β-sheet fibers in solution under the same conditions in which human amylin (hIAPP) forms amyloid fibers. This observation applies to the cyclic, oxidized form of the N_loop but not to the linear, reduced form, which does not form fibers. Our findings indicate a potential role of direct N_loop-N_loop interactions in hIAPP aggregation, which has not been previously explored, with important implications for the mechanism of hIAPP amyloid fiber formation, the inhibitory action of IAPP variants, and the competition between ordered and disordered aggregation in peptides of the calcitonin peptide family.