The Structure of Human IAPP Fibrils Reflects Membrane and pH Conditions.

Physiologically relevant in vitro models of amyloid aggregation are essential for linking structural insights to disease pathology. In type 2 diabetes, aggregation of human islet amyloid polypeptide (hIAPP) into fibrils is a hallmark of β-cell dysfunction, yet structural data on ex vivo hIAPP fibrils remain unavailable. Most models use solution-grown fibrils, overlooking membrane interactions and native pH, which underscores the need for more realistic in vitro models. Here, we use solid-state NMR spectroscopy to determine the structure of phospholipid membrane-mediated hIAPP fibrils formed under extracellular (pH 7.4) conditions. These fibrils are homogeneous and adopt an L-shaped protofilament architecture with an extended N-terminal β-strand─a region often unresolved in cryo-EM. The fibril core (N14-L27) adopts the CF1 fold, a conserved β-arch also seen in nonlipidic fibrils, suggesting its relevance in disease. In contrast, fibrils formed at intracellular pH (5.3) are structurally heterogeneous and show distinct structural differences in the C-terminus. hIAPP must exhibit substantial structural plasticity in the membrane environment, transitioning from helical monomers to β-hairpin oligomers and ultimately to β-arch-rich fibrils─transitions that may introduce energy barriers stabilizing toxic intermediates. Our findings provide the first high-resolution structure of membrane-mediated hIAPP fibrils highlighting the need to model aggregation under physiologically relevant conditions.