Characterization of tau protein present in microtubules and paired helical filaments of Alzheimer's disease patient's brain.

Two proteins immunologically related to porcine tau protein are found in the brain of Alzheimer's disease patients. One is bound to microtubules and, after isolation by co-polymerization with tubulin, shows a size and tryptic peptide map, similar to the microtubule-associated tau protein, present in the brain of non-demented patients. The other tau-related protein is present as the major protein of a purified fraction of paired helical filaments. The paired helical filament-associated protein shows smaller molecular weight (33,000) than microtubule-associated tau; however, this 33,000 mol. wt protein reacts with a monospecific anti-tau antibody and with an antibody to a 19-amino acid peptide corresponding to amino acids 228-246 of human tau. Furthermore, the 33,000 mol. wt protein and the tau protein have similar tryptic peptide maps. These results suggest that the paired helical filament protein is a modified form of the microtubule-associated tau protein.