Analysis of the minimal amyloid-forming fragment of the islet amyloid polypeptide. An experimental support for the key role of the phenylalanine residue in amyloid formation

The development of type II diabetes was shown to be associated with the formation of amyloid fibrils consisted of the islet amyloid polypeptide (IAPP or amylin). Recently, a short functional hexapeptide fragment of IAPP (NH ₂-NFGAIL-COOH) was found to form fibrils that are very similar to those formed by the full-length polypeptide. To better understand the specific role of the residues that compose the fragment, we performed a systematic alanine scan of the IAPP "basic amyloidogenic units." Turbidity assay experiments demonstrated that the wild-type peptide and the Asn¹ → Ala and Gly³ → Ala peptides had the highest rate of aggregate formation, whereas the Phe² → Ala peptide did not form any detectable aggregates. Dynamic light-scattering experiments demonstrated that all peptides except the Phe² → Ala form large multimeric structures. Electron microscopy and Congo red staining confirmed that the structures formed by the various peptides are indeed amyloid fibrils. Taken together, the results of our study provide clear experimental evidence for the key role of phenylalanine residue in amyloid formation by IAPP. In contrast, glycine, a residue that was suggested to facilitate amyloid formation in other systems, has only a minor role, if any, in this case. Our results are discussed in the context of the remarkable occurrence of aromatic residues in short functional fragments and potent inhibitors of amyloid-related polypeptides. We hypothesize that π-π interactions may play a significant role in the molecular recognition and self-assembly processes that lead to amyloid formation.