Effect of Prolines on the Intrinsic Stiffness of Islet Amyloid Polypeptide Variants

Abstract

Human islet amyloid polypeptide (hIAPP) is an intrinsically disordered hormone that is co-secreted with insulin in the beta-cells of the pancreas. The aggregation of hIAPP into insoluble amyloid fibrils is believed to play a causal role in type 2 diabetes. The rodent variant (rIAPP), differing by 6 of 37 amino acids, does not aggregate into amyloid fibrils and inhibits hIAPP amyloid formation. Both these properties have been attributed to rIAPP's three proline mutations (A25P, S28P and S29P). Single proline mutants of hIAPP have also been shown to kinetically inhibit hIAPP fibril formation. Because of their intrinsic dihedral angle preferences, prolines are expected to affect monomer conformational sampling. However, the specific effect of proline substitutions on IAPP structure and dynamics has not yet been explored. Detecting such properties is challenging due to the low molecular weight, fast reconfiguration times, and very low solubility of IAPP peptides. High resolution, time resolved techniques are needed.We use a nanosecond laser spectroscopy technique to measure end-to-end contact formation rates in IAPP mutants. Using this technique, Vaiana et al. showed that rIAPP populates more extended conformations, characterized by larger end-to-end distances, compared to hIAPP. Here we study the effects of proline substitutions in IAPP and characterize them in terms of intrinsic chain stiffness. We find that the three proline mutations alone do not explain rIAPP's increased chain stiffness. Interestingly, early experiments by Green et al. show that mutating the non-proline residues in rIAPP renders it capable of forming amyloids. Together these results suggest that rIAPP's unique chain stiffness is a determinant for its non-amyloidogenic properties. We discuss the reasons for this peptide's unique chain stiffness and the implications of our findings on the effect of prolines in IDPs.