Characterization of antisera to paired helical filaments and tau. Implication for the extent of tau tightly bound to paired helical filaments.

The two antisera to paired helical filaments (PHF) and tau, which stained neurofibrillary tangles similarly in tissue section, were characterized. Anti-PHF reacted exclusively with the carboxyl terminal portion of tau, whereas antitau reacted with the amino terminal portion. In the blots of PHF prepared in Sarkosyl (Sarkosyl-PHF), anti-PHF stained an extensive smear from the top to the bottom of the gel, while antitau did not stain this kind of smear but labeled a small amount of intact tau that was bound to Sarkosyl-PHF. An LDS- or SDS-insoluble residue of Sarkosyl-PHF was labeled with anti-PHF, but not with antitau. Antitau-defined epitopes were susceptible to protease, whereas anti-PHF-defined epitopes were unaffected. Based on these observations, we conclude that Sarkosyl-PHF were composed largely of the carboxyl terminal region of tau and accompanied by a small amount of intact tau on their surfaces.