The protofilament structure of insulin amyloid fibrils

• DOI: 10.1073/pnas.142459399
• Publication Date: 2002-07-28T22:26:28Z
• Authors: José L. Jiménez; Ewan J. Nettleton; Mario Bouchard; Carol V. Robinson; Christopher M. Do...; Helen R. Saibil
• Keywords: Models, Molecular; 0301 basic medicine; Amyloid; 03 medical and health sciences; Macromolecular Substances; Cryoelectron Microscopy; Image Processing, Computer-Assisted; Animals; Insulin; Cattle; In Vitro Techniques; Protein Structure, Secondary

Under solution conditions where the native state is destabilized, the largely helical polypeptide hormone insulin readily aggregates to form amyloid fibrils with a characteristic cross-β structure. However, there is a lack of information relating the 4.8 Å β-strand repeat to the higher order assembly of amyloid fibrils. We have used cryo-electron microscopy (EM), combining single particle analysis and helical reconstruction, to characterize these fibrils and to study the three-dimensional (3D) arrangement of their component protofilaments. Low-resolution 3D structures of fibrils containing 2, 4, and 6 protofilaments reveal a characteristic, compact shape of the insulin protofilament. Considerations of protofilament packing indicate that the cross-β ribbon is composed of relatively flat β-sheets rather than being the highly twisted, β-coil structure previously suggested by analysis of globular protein folds. Comparison of the various fibril structures suggests that very small, local changes in β-sheet twist are important in establishing the long-range coiling of the protofilaments into fibrils of diverse morphology.