X-Ray Diffraction Studies of Amyloid Structure

Elucidation of the underlying core structure of amyloid fibrils is essential for understanding the mechanism by which amyloid fibrils are formed and deposited. Conventional methods of X-ray crystallography and NMR cannot be used, since the fibers are insoluble and heterogeneous. X-ray fiber diffraction is one method that has been successfully used to examine the structure of these insoluble fibers. The procedure involves the formation of suitable, ordered amyloid fibrils and characterization (by electron microscopy), partial alignment of fibers, X-ray data collection, data analysis, and finally, model building

Structural characterisation of islet amyloid polypeptide fibrils

Islet amyloid is found in many patients suffering from type 2 diabetes. Amyloid fibrils found deposited in the pancreatic islets are composed of a 37-residue peptide, known as islet amyloid polypeptide (IAPP) (also known as amylin) and are similar to those found in other amyloid diseases. Synthetic IAPP peptide readily forms amyloid fibrils in vitro and this has allowed fibril formation kinetics and the overall morphology of IAPP amyloid to be studied. Here, we use X-ray fibre diffraction, electron microscopy and cryo-electron microscopy to examine the molecular structure of IAPP amyloid fibrils. X-ray diffraction from aligned synthetic amyloid fibrils gave a highly oriented diffraction pattern with layer-lines spaced 4.7 Å apart. Electron diffraction also revealed the characteristic 4.7 Å meridional signal and the position of the reflection could be compared directly to the image of the diffracting unit. Cryo-electron microscopy revealed the strong signal at 4.7 Å that has been previously visualised from a single Aß fibre. Together, these data build up a picture of how the IAPP fibril is held together by hydrogen bonded ß-sheet structure and contribute to the understanding of the generic structure of amyloid fibrils

A simple algorithm locates beta-strands in the amyloid fibril core of alpha-synuclein, Abeta, and tau using the amino acid sequence alone.

Fibrillar inclusions are a characteristic feature of the neuropathology found in the a-synucleinopathies such as Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Familial forms of a-synucleinopathies have also been linked with missense mutations or gene multiplications that result in higher protein expression levels. In order to form these fibrils, the protein, a-synuclein (a-syn), must undergo a process of self-assembly in which its native state is converted from a disordered conformer into a ß-sheet-dominated form. Here, we have developed a novel polypeptide property calculator to locate and quantify relative propensities for ß-strand structure in the sequence of a-syn. The output of the algorithm, in the form of a simple x-y plot, was found to correlate very well with the location of the ß-sheet core in a-syn fibrils. In particular, the plot features three peaks, the largest of which is completely absent for the nonfibrillogenic protein, ß-syn. We also report similar significant correlations for the Alzheimer's disease-related proteins, Aß and tau. A substantial region of a-syn is also of converting from its disordered conformation into a long amphipathic a-helical protein. We have developed the aforementioned algorithm to locate and quantify the a-helical hydrophobic moment in the amino acid sequence of a-syn. As before, the output of the algorithm, in the form of a simple x-y plot, was found to correlate very well with the location of a-helical structure in membrane bilayer-associated a-syn