Ubiquitin Is a Component of Paired Helical Filaments in Alzheimer's Disease

Paired helical filaments (PHF), which constitute a distinct type of pathological neuronal fiber, are the principal constituent of neurofibrillary tangles that occur in the brain of patients with Alzheimer's disease. Their insolubility in sodium dodecyl sulfate and urea has prevented the analysis of their subunit composition by gel electrophoresis. A monoclonal antibody (DF2) was isolated that specifically labeled PHF at both the light and electron microscopic levels. It labeled a small polypeptide (5 kilodaltons) that was shown to be ubiquitin in immunoblots of the soluble fraction of brain homogenates. To obtain direct evidence that ubiquitin is a component of PHF, PHF were treated with concentrated formic acid and digested with lysylendopeptidase; ubiquitin-derived peptides were then identified by reversed-phase high-performance liquid chromatography. Two fragments in the PHF digest were identified as derived from ubiquitin by protein sequencing. This procedure should make possible definitive identification of other PHF components.