The State of Phosphorylation of Normal Adult Brain τ, Fetal τ, and τ from Alzheimer Paired Helical Filaments at Amino Acid Residue Ser 262

Antibody Ab262 was raised against a synthetic τ peptide (SKIGSTENLK, amino acids 258–267 of τ, termed Ser 262 peptide). The antibody was more reactive with Ser 262 peptide and unphosphorylated τ than a related phosphopeptide [SKIGS(P)TENLK, termed P‐Ser 262 peptide] and τ phosphorylated by a partially purified kinase, glycogen synthase kinase (GSK) 3β. Ab262 reacted poorly with a peptide having the sequence DRVQSKIGSLD (amino acids 348–358). Treatment of P‐Ser 262 peptide or GSK 3β phosphorylated τ with alkaline phosphatase increased Ab262 immunoreactivity, indicating that Ab262 is a reagent useful for studying τ phosphorylation at the Ser 262 residue. The Ab262 immunoreactivity was detected in τ from normal brains and Alzheimer paired helical filament (PHF‐τ) and in PHFs. Alkaline phosphatase treatment had no effect on the Ab262 immunoreactivity of normal τ and PHF‐τ but altered the Tau‐1 and PHF‐1 immunoreactivities. τ proteins from rat brains at 3 and 8 h postmortem exhibited 5 and 19%, respectively, more Ab262 immunoreactivity than τ from fresh tissues. In comparison, rat τ at 8 h postmortem was 40% more immunoreactive with Tau‐1. The results suggest that Ser 262 is not a major phosphorylation site in vivo. Moreover, there is little or no difference between PHF‐τ and normal τ in the extent of phosphorylation at Ser 262 .