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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-beta structure. However, there is a lack of information relating the 4.8 A beta-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-beta ribbon is composed of relatively flat beta-sheets rather than being the highly twisted, beta-coil structure previously suggested by analysis of globular protein folds. Comparison of the various fibril structures suggests that very small, local changes in beta-sheet twist are important in establishing the long-range coiling of the protofilaments into fibrils of diverse morphology.

Original publication

DOI

10.1073/pnas.142459399

Type

Journal article

Journal

Proc Natl Acad Sci U S A

Publication Date

09/07/2002

Volume

99

Pages

9196 - 9201

Keywords

Amyloid, Animals, Cattle, Cryoelectron Microscopy, Image Processing, Computer-Assisted, In Vitro Techniques, Insulin, Macromolecular Substances, Models, Molecular, Protein Structure, Secondary