Aggregation-induced conformation changes dictate islet amyloid polypeptide (IAPP) membrane affinity

Islet amyloid polypeptide (IAPP) is a 37 residue intrinsically disordered protein whose aggregation is associated with Type II diabetes. Like most amyloids, it appears that the intermediate aggregates ("oligomers") of IAPP are more toxic than the mature fibrils, and interaction with the cell membrane is likely to be an integral component of the toxicity. Here we probe the membrane affinity and the conformation of the peptide as a function of its aggregation state. We find that the affinity of the peptide for artificial lipid bilayers is more than 15 times higher in the small oligomeric state (hydrodynamic radius ~ 1.6 nm) compared to the monomeric state (hydrodynamic radius ~ 0.7 nm). Binding with RIN-m5F cell membranes also shows qualitatively similar behavior. The monomeric state, as determined by Forster Resonance Energy Transfer, has a much larger end to end distance than the oligomeric state, suggesting conformational change between the monomers and the oligomers. Raman and Infrared spectroscopic measurements show the presence of considerable alpha helical content in the oligomers, whereas the larger aggregates have largely beta sheet character. Therefore, the conformation of the small oligomers is distinct from both the smaller monomers and the larger oligomers, and this is associated with an enhanced membrane affinity. This provides a possible structural basis for the enhanced toxicity of amyloid oligomers. Such change is also reminiscent of amyloid beta, another aggregation prone amyloidogenic peptide, though the nature of the conformational change is quite different in the two cases. We infer that conformational change underlying oligomer formation is a key factor in determining the enhanced membrane affinity of disease causing oligomers, but the toxic "oligomer fold" may not be universal.