Amylin (hIAPP) Aggregates on the Membrane

Abstract

Amylin is a 37 residue intrinsically disordered peptide whose aggregation is associated with Type II diabetes. Small oligomers (n-mers, with n<20) bind strongly to small unilamellar vesicles and are likely to be the key to its toxicity. However, we do not know which n-mer is the key. Here we use Single Molecule Photo-bleaching (smPB) techniques to study the size (‘n’) distribution of the oligomers in physiological solutions (at a sub-nM concentration) as well as on supported anionic lipid bilayers (POPC:POPG:Cholesterol = 1:1:1) exposed to such solutions. Single molecule imaging results indicate that solution state and membrane-bound oligomers mostly consist of monomers to trimers (neglecting any pre-measurement photobleaching). Dimers and trimers have higher membrane affinity than the monomers. This higher membrane affinity is possibly linked to the conformational changes which occur during aggregation. We explore aggregation-associated conformational transitions using fluorescence, vibrational and Solid State NMR (ssNMR) based methods that we have established earlier for amyloid beta. We find that the high membrane affinity species contains a mixture of both alpha-helices and beta-sheets. Our study, therefore, quantifies the structure and membrane interaction of small n-mers of amylin as a function of n, paving the way for size-specific drug-targeting of amylin aggregates.