Amyloid fibers are water-filled nanotubes

^{Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom; and}^{Department of Haematology, University of Cambridge, Wellcome/MRC Building, Cambridge CB2 2XY, United Kingdom}

^{Deceased February 6, 2002.}

^{Inquiries will be handled by}ku.ca.mac@52fra.

Contributed by M. F. Perutz

Accepted 2001 Dec 18.

Abstract

A study of papers on amyloid fibers suggested to us that cylindrical β-sheets are the only structures consistent with some of the x-ray and electron microscope data. We then found that our own 7-year-old and hitherto enigmatic x-ray diagram of poly-l-glutamine fits a cylindrical sheet of 31 Å diameter made of β-strands with 20 residues per helical turn. Successive turns are linked by hydrogen bonds between both the main chain and side chain amides, and side chains point alternately into and out of the cylinder. Fibers of the exon-1 peptide of huntingtin and of the glutamine- and asparagine-rich region of the yeast prion Sup35 give the same underlying x-ray diagrams, which show that they have the same structure. Electron micrographs show that the 100-Å-thick fibers of the Sup35 peptide are ropes made of three protofibrils a little over 30 Å thick. They have a measured mass of 1,450 Da/Å, compared with 1,426 Da/Å for a calculated mass of three protofibrils each with 20 residues per helical turn wound around each other with a helical pitch of 510 Å. Published x-ray diagrams and electron micrographs show that fibers of synuclein, the protein that forms the aggregates of Parkinson disease, consist of single cylindrical β-sheets. Fibers of Alzheimer Aβ fragments and variants are probably made of either two or three concentric cylindrical β-sheets. Our structure of poly-l-glutamine fibers may explain why, in all but one of the neurodegenerative diseases resulting from extension of glutamine repeats, disease occurs when the number of repeats exceeds 37–40. A single helical turn with 20 residues would be unstable, because there is nothing to hold it in place, but two turns with 40 residues are stabilized by the hydrogen bonds between their amides and can act as nuclei for further helical growth. The Aβ peptide of Alzheimer's disease contains 42 residues, the best number for nucleating further growth. All these structures are very stable; the best hope for therapies lies in preventing their growth.

Abstract

In the early 1930s W. T. Astbury discovered that x-ray fiber pictures of wool gave only two reflections, a meridional one at 5.1 Å and an equatorial one at 10 Å. When Astbury stretched wool under steam to twice its length, he got a different pattern, with a meridional reflection at 3.4 Å and equatorial ones at 4.8 and 10 Å. Astbury attributed the meridional reflection to the periodic repeat of the amino acid residues along the length of the polypeptide chains, the 4.-Å reflection to the backbone separation of neighboring main chains, and the 10-Å reflection to the separation of neighboring main chains by their side chains. He called the unstretched wool the α- and the stretched wool the β-pattern. In 1935, Astbury wondered whether stretched polypeptide chains of denatured globular proteins would also give a β-pattern, but he found that the 3.4-Å meridional reflection was now replaced by the 4.8-Å reflection that was no longer on the equator. Astbury called this result the cross-β-pattern, because it implied that the chains ran across rather than along the fiber axes (1). In 1951, Linus Pauling showed that the α-pattern was given by α-helices and the β-pattern by pleated sheets, but the structure underlying the cross-β-pattern has remained enigmatic. Astbury's observation lay dormant until its rediscovery in x-ray diffraction pictures of amyloid fibers brought it to new prominence.

Acknowledgments

We thank Aaron Klug for advice and discussion, Martin Madera for calculating the Fourier transforms in Fig. Fig.1,1, Professor K. C. Holmes and Dr. James Elliott for calculating Fourier transforms of our nanotubes (not shown), Dr. D. Marvin for the fd sample, and the Medical Research Council and the Wellcome Trust for support.

Acknowledgments

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