Proteins which may form wide, insoluble, unbranched filaments possessing a cross-beta sheet quaternary structure, where the beta sheets are oriented perpendicular to the fibre axis. Amyloid fibrils may be involved in abnormal protein depositions, or amyloidosis, such as Alzheimer's or Parkinson's diseases. Some of the best-known native functional amyloidogenic proteins are produced by bacteria, e.g. curli, harpins or chaplins.
