Tau-related protein present in paired helical filaments has a decreased tubulin binding capacity as compared with microtubule-associated protein tau.

We have isolated, after exhaustive detergent treatments, a 33 kDa tau-related protein isolated from paired helical filaments from Alzheimer's disease patient brains. The N-terminal sequence of the 33 kDa protein begins at residue 71 of the sequence described for human fetal tau protein. This truncated form of tau is not the consequence of the translation of a tau RNA lacking a region at its 5' end, as measured by primer extension analyses, suggesting that the 33 kDa protein must be generated by proteolysis of previously synthesized tau. This tau-related protein has only one blocked cysteine residue and also has a decreased tubulin binding capacity as compared with that of tau protein.