AMINO-ACID-RESIDUES 226-240 OF TAU, WHICH ENCOMPASS THE FIRST LYS-SER-PRO SITE OF TAU, ARE PARTIALLY PHOSPHORYLATED IN ALZHEIMER PAIRED HELICAL FILAMENT-TAU

A synthetic peptide corresponding to residues 226-240 (E9 peptide) of human tau, which contains an Lys-Ser-Pro motif, was used to raise a po lyclonal antibody. The antibody, E9, was 10-fold less reactive with ph ospho-E9 peptide than with native E9 peptide. E9 antibody was used to study the extent of phosphorylation in a modified form of tau (PHF-tau ) that is found in Alzheimer's disease brain and is incorporated into paired helical filaments (PHFs). E9 immunolabeled Alzheimer's disease neurofibrillary tangles and abnormal neurites in brain sections intens ely, with increased immunoreactivity detected after pretreatment of se ctions with phosphatase. On immunoblots and ELISA, E9 reacted with PHF -tau and recombinant human tau but not with the high and middle molecu lar weight neurofilament proteins. Phosphatase treatment of PHF-tau im proved the E9 immunoreactivity by 30-50%. Dephosphorylated high but no t middle molecular weight neurofilament protein became reactive with E 9. These results indicate that <50% of the PHF-tau is phosphorylated i n the subregion corresponding to residues 226-240 of tau and suggest t hat the phosphorylation of this region may not be essential for PHF fo rmation.