A modified form of microtubule-associated tau protein is the main component of paired helical filaments.

Paired helical filaments, which are present in the brain of Alzheimer's disease patients have been isolated and characterized. Treatment of the filaments with reducing agents and detergents extracts several proteins from these structures. The remaining filaments are composed mainly of a protein with molecular weight of 33 kDa suggesting that this protein is the core component of these filaments. Peptide mapping using trypsin and endoproteinase Arg-C revealed that the 33 kDa protein, was a modified form of tau protein present in normal human brain.