The minimal amyloid-forming fragment of the islet amyloid polypeptide is a glycolipid-binding domain

Several proteins that interact with cell surface glycolipids share a common fold with a solvent-exposed aromatic residue that stacks onto a sugar ring of the glycolipid (CH-pi stacking interaction). Stacking interactions between aromatic residues (pi-pi stacking) also play a pivotal role in the assembly process, including many cases of amyloid fibril formation. We found a structural similarity between a typical glycolipid-binding domain (the V3 loop of HIV-1 gp120) and the minimal amyloid-forming fragment of the human islet amyloid polypeptide, i.e. the octapeptide core module NFGAILSS. In a monolayer assay at the air-water interface, the NFGAILSS peptide specifically interacted with the glycolipid lactosylceramide. The interaction appears to require an aromatic residue, as NLGAILSS was poorly recognized by lactosylceramide, whereas NYGAILSS behaved like NFGAILSS. In addition, we observed that the full-length human islet amyloid polypeptide (1-37) did interact with a monolayer of lactosylceramide, and that the glycolipid film significantly affected the aggregation process of the peptide. As glycolipid-V3 interactions are efficiently inhibited by suramin, a polyaromatic compound, we investigated the effects of suramin on amyloid formation by human islet amyloid polypeptide. We found that suramin inhibited amyloid fibril formation at low concentrations, but dramatically stimulated the process at high concentrations. Taken together, our results indicate that the minimal amyloid-forming fragment of human islet amyloid polypeptide is a glycolipid-binding domain, and provide further experimental support for the role of aromatic pi-pi and CH-pi stacking interactions in the molecular control of the amyloidogenesis process.