Identification and localization of a τ peptide to paired helical filaments of Alzheimer disease

Abstract

Amino acid sequencing of a CNBr digest of the τ protein isolated from bovine brain revealed an amino acid sequence of 17 residues, Pro-Gly-Leu-Lys-Glu-Ser-Pro-Leu-Gln-Ile-Gly-Ala-Ala-Pro-Gly-Leu-Lys, which we call peptide I, with heterogeneity at position 11 of glycine (peptide Ia) and proline (peptide Ib); peptide I showed no homology with the previously reported cDNA-derived mouse and human τ sequences. Antisera raised to synthetic peptides corresponding to peptides Ia and Ib labeled all the bovine τ polypeptides recognized by other monoclonal and polyclonal antibodies to bovine τ. Antisera to peptide Ib did not label any mouse τ polypeptides; however, an anti-Ia antiserum labeled two of the four mouse τ polypeptides. Antisera to both peptides labeled paired helical filaments (PHF) as neurofibrillary tangles, plaque neurites, and neuropil threads in Alzheimer disease brain and PHF polypeptides on immunoblots. Immunostaining with anti-Ia antisera of PHF in tissue sections and PHF polypeptides, but not bovine τ, on immunoblots was markedly increased when pretreated with alkaline phosphatase. These studies suggest that (i) the amino acid sequences of some isoforms of τ peptide might be different from that predicted from cDNAs, (ii) a τ peptide that is absent in the predicted sequences is present in PHF in Alzheimer disease, and (iii) τ in PHF is abnormally phosphorylated.