MICROTUBULE-ASSOCIATED PROTEIN-TAU - ABNORMAL PHOSPHORYLATION OF A NON-PAIRED HELICAL FILAMENT POOL IN ALZHEIMER-DISEASE

The major protein subunit of the paired helical filaments (PHF) of Alz heimer disease (AD) is the microtubule-associated protein tau. Tau is a family of phosphopolypeptides that are abnormally phosphorylated in PHF. In this study, a non-PHF pool of tau abnormally phosphorylated at Ser-199/202, and tau not phosphorylated at this site (AD P-tau and AD tau, respectively) were isolated from the 27,000 x g to 200,000 x g f raction of AD brain homogenate by extraction in 8 M urea, followed by dialysis against Tris buffer. AD P-tau and AD tau were further purifie d and separated from each other by acid precipitation, glial fibrillar y acidic protein affinity chromatography, and phosphocellulose chromat ography. The resulting AD P-tau and AD tau preparations were free of c ytoskeletal proteins, ubiquitin, and beta-amyloid peptide. Immunochemi cal and morphological analysis of AD P-tau preparations revealed that most of the protein was of non-PHF origin. The AD P-tau was about 3-4- fold (approximately 8 mol P0(4)/mol protein, M(r) 41,318) more phospho rylated than cytosolic tau from AD and control brains. Unlike PHF, the AD P-tau lacked ubiquitin. In AD brain the levels of cytosolic tau we re about half of those in control aged cases. These findings suggest t hat the abnormal phosphorylation of tau in AD occurs in the cytosol.