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 Asn 1 3 Ala and Gly 3 3 Ala peptides had the highest rate of aggregate formation, whereas the Phe 2 3 Ala peptide did not form any detectable aggregates. Dynamic light-scattering experiments demonstrated that all peptides except the Phe 2 3 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 -interactions may play a significant role in the molecular recognition and self-assembly processes that lead to amyloid formation.Amyloid fibril formation is a central feature in a variety of unrelated pathological situations. A partial list includes Alzheimer's disease, prion diseases, diabetes mellitus (type II diabetes), familial amyloidosis, and light-chain amyloidosis (for review see Refs. 1-4). Islet amyloids are found in more than 95% of the patients with type II diabetes mellitus and are most likely an important factor in the development of ␤-cells failure (5-8). The islet amyloid fibrils consist predominantly of the islet amyloid polypeptide (IAPP 1 or amylin), a 37-amino acid polypeptide hormone that is produced by pancreatic ␤-cells (5, 9 -15). IAPP plays a central role in glucose homeostasis in its soluble form (16). Although the molecular mechanism of IAPP amyloidogenesis in vivo is not fully understood, the in vitro mechanism has been studied extensively. The 37-amino acid IAPP was shown to form amyloid fibrils in vitro (5,10,11,16,17). These fibrils were shown to be cytotoxic to pancreatic ␤-cell culture and thus are assumed to play a major role in the diabetes mechanism (18, 19). The kinetics of amyloid formation by IAPP as determined by turbidity assay is consistent with a nucleation-dependent mechanism of polymerization (20 -23).Recently, a six-residue peptide fragment of the human IAPP (with the amino acid sequence NFGAIL using the 1-letter code) was shown to form amyloid fibrils that are very similar to those formed by the full-length polypeptid...

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