Characterization of Two Distinct Monoclonal Antibodies to Paired Helical Filaments: Further Evidence for Fetal‐Type Phosphorylation of the τ in Paired Helical Filaments

Abstract

Two monoclonal antibodies C5 and M4 raised against Sarkosyl‐insoluble paired helical filaments (PHF) specifically labeled fetal τ, but hardly labeled normal adult τ. C5 immunoreactivity was eliminated by alkaline phosphatase treatment at 37°C, whereas M4 reactivity could be removed only by the treatment at 67°C. Epitope analysis showed that C5 and M4 recognition sites are in residues 386–406 and 198–250, respectively, according to the numbering of the longest human τ isoform. Thus, the phosphorylation sites are located in the amino‐ and carboxyl‐terminal portions of the microtubule‐binding region. These two well‐characterized monoclonals should be valuable in the identification of a protein kinase(s) that converts normal τ into PHF‐τ.