Polypeptide composition of paired helical filaments

Immunocytochemical studies using antibodies to cytoskeletal proteins have provided conflicting data on the components of paired helical filaments (PHF), due solely to immunological cross-reactivities. To avoid such ambiguity, we developed a protein chemical approach to the identification of the PHF components. After treatment with formic acid, PHF were digested with lysylendopeptidase and the resultant peptides were separated by HPLC. All major peaks were analysed for their amino acid compositions and sequences. From the PHF digest, proteolytic fragments of ubiquitin, tau and beta protein were sequenced. Ubiquitin in PHF appears to be in a conjugated form, while its target protein remains unidentified. Tau is integrated into PHF at the site of its carboxyl third. The presence of beta protein fragments is best interpreted as being due to contamination of amyloid filaments in the PHF preparation. Thus, ubiquitin and tau are the two definite components of PHF.