Role of the carboxy terminus

We have shown that various truncations of the tau molecule at its carboxy end enhance polymerization in vitro, implying that the tail normally inhibits polymerization (Post-translational modifications). We have also demonstrated that caspases create such a truncation by cleavage at Asp421 and that this enhances filament assembly (Caspase cleavage). In order to gain a better understanding of how tau’s carboxy-terminus achieves its inhibitory effect, we synthesized a “tail” peptide corresponding to the carboxy-terminal residues 422-441 that are removed from the parent molecule by caspase cleavage. Then we tested the influence of this peptide on the polymerization of tau and various truncation mutants.

We found that the tail peptide inhibited the polymerization of wild-type tau and several truncation mutants, including the one truncated at the caspase cleavage site as shown below. This confirms the inhibitory nature of residues 422-441 and additionally implies that binding of the tail peptide to some portion of the remaining molecule is responsible for this effect.

Analysis of which truncation mutants were and were not affected by the peptide implicated residues 321-375, part of third and fourth microtubule binding repeats, as the site of interaction with the carboxy-terminal tail.

The tail peptide is predicted to form an alpha helix, and we confirmed this tendency in circular dichroism experiments. Moreover, the predicted helix is amphipathic; that is, one side contains hydrophobic amino acids, the other side hydrophilic ones. We hypothesized that this structure would be an important factor in the interaction of the peptide with its binding site. Accordingly, we synthesized a “scrambled” peptide having the same amino acids in a different order, but one that still had strong helix-forming potential. This peptide was effective in inhibiting polymerization, whereas a different scrambled peptide with no helix-forming propensity was not, confirming the hypothesis. 

We conclude from these findings that the carboxy-terminus of tau is capable of forming an amphipathic alpha helix that interacts with the microtubule binding repeat region to inhibit polymerization. This provides insight into the mechanism by which caspase cleavage may induce tau filament formation in Alzheimer’s disease (Caspase Cleavage).

This work is described in our publication (Berry, et al., 2003).