Identification of 3- and 4-repeat tau isoforms within the PHF in Alzheimer's disease.

The microtubule associated protein tau is incorporated into the pronase resistant core of the paired helical filament (PHF) in such a way that the repeat region is protected from proteases, but can be released as a major 12 kDa species from the PHF core by formic acid treatment and by boiling in SDS. This fragment retains the ability to aggregate in the presence of SDS. Detailed sequence analysis of the 12 kDa species shows that it consists of a mixture of peptides derived from the repeat region of 3- and 4-repeat tau isoforms comigrating as a single electrophoretic band. However, the 4-repeat isoforms released from the core lack either the first or the last repeat. The pronase-protected region of tau within the PHF core is therefore restricted to three repeats, regardless of isoform. The alignment of cleavage sites at homologous positions within tandem repeats after protease treatment indicates that the tau-core association is precisely constrained by the tandem repeat structure of the tau molecule.