Highly purified and SDS-soluble paired helical filaments (PHFs) were immunogold labeled and immunoblotted with antibodies to tau: Tau 14 (N-terminal half), AH-1 (microtubule-binding domain), and Tau 46 (C-terminal end). The main component of PHFs was modified tau of 68, 64, and 60 kd, also called A68 or PHF-tau. Trypsin digestion reduced the maximum width of PHFs by 10%-20%, increased aggregation of filaments, and abolished the binding of Tau 14, but had no effect on the binding of AH-1. The smallest tau-reactive tryptic fragments were 13 and 7-8 kd, positive with AH-1, and negative with Tau 46. Our results and the model of Crowther and Wischik suggest that by self-association and anti-parallel arrangement of the microtubule-binding domains, PHF-tau forms the backbone of PHFs.
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