Role of lysine residue of islet amyloid polypeptide in fibril formation, membrane binding, and inhibitor binding

The aggregation of islet amyloid polypeptide (IAPP) is implicated in the pathogenesis of type 2 diabetes (T2D). In T2D, this peptide aggregates to form amyloid fibrils; the mechanism responsible for islet amyloid formation is unclear. However, it is known that the aggregation propensity of IAPP is highly related to its primary sequence. Several residues have been suggested to be critical in modulating IAPP amyloid formation, but role of the sole lysine residue at position 1 (Lys-1) in IAPP has not been discussed. In our previous study, we found that glycated IAPP can form amyloid faster than normal IAPP and induce normal IAPP to expedite the aggregation process. To gain more insight into the contribution of Lys-1 in the kinetics of fibril formation, we synthesized another two IAPP variants, K1E-IAPP and K1Nle-IAPP, in which the Lys residue was mutated to glutamate and norleucine, respectively. Interestingly, we observed that the negative or neutral charged side chain at this position was preferred for amyloid formation. The findings suggested this residue may take part in the inter- or intra-molecular interaction during IAPP aggregation, even though it was proposed not to be in part of fibril core structure. Our data also revealed that the inhibitory mechanism of some inhibitors for IAPP aggregation require reaction with Lys-1. Modifications of Lys-1, such as protein glycation, may affect the effectiveness of the inhibitory action of some potential drugs in the treatment of amyloidosis.