Zinc Inhibits Human Islet Amyloid Polypeptide (IAPP) Amyloidogenesis

Abstract

Human Islet Amyloid Polypeptide (hIAPP) is a highly amyloidogenic protein found in islet cells of patients with type II diabetes. Because hIAPP is highly toxic to beta-cells under certain conditions, it has been proposed that hIAPP is linked to the loss of beta-cells and insulin secretion in type II diabetics. One of the interesting questions surrounding this peptide is how the toxic and aggregation prone hIAPP peptide can be maintained in a safe state at the high concentrations found in the secretory granule where is stored. We show here through a combination of NMR, ITC, CD, and Thioflavin T fluorescence that zinc, which is found at millimolar concentrations in the secretory granule, binds to hIAPP with a Kd of approximately 100 nM and inhibits hIAPP amyloid fibrillogenesis in the micromolar range. NMR spectroscopy shows that zinc interacts with hIAPP through coordination to His18 and a probable cation - pi stacking interaction with Phe15. ITC binding experiments with the rat variant of IAPP, which lacks His18, indicated an additional binding site with approximately 1 mM affinity. The lower affinity binding site was localized to Arg11 by NMR. The binding of zinc also alters the structure of hIAPP, rigidifying the N-terminal region of the protein in a near-helical conformation stabilizing the non-amyloid form of the peptide. The inhibition of the aggregated and toxic forms of hIAPP by zinc provides a possible mechanism between the recent discovery of linkage between deleterious mutations in the SLC30A8 zinc transporter, which transports zinc into the secretory granule, and type II diabetes.