The Microtubule-Associated Protein Tau Cross-Links to Two Distinct Sites on Each α and β Tubulin Monomer via Separate Domains

The interaction between tubulin subunits and microtubule-associated proteins (MAPs) such as tau is fundamental for microtubule structure and function. Previous work has suggested that the “microtubule binding domain” of tau (composed of three or four imperfect 18-amino acid repeats, separated by 13- or 14-amino acid inter-repeat regions) can bind to the C-terminal ends of both α and β tubulin monomers. Here, using covalent cross-linking strategies, we demonstrate that there are two distinct tau cross-linking sites (designated as “C-terminal” and “internal”) on each α and β tubulin monomer. The C-terminal tau cross-linking site is located within the 12 C-terminal amino acids of both α and β tubulin, while the internal tau cross-linking site is located within the C-terminal one-third of α and β tubulin but not within the last 12 amino acids. In addition, we show that tau cross-links to the C-terminal site via its repeat 1 and/or the R1−R2 inter-repeat. The cross-linking of tau to the internal site is mediated by some subset of its other repeat units. Integrating these and earlier data with the 3.7 Å resolution model of the αβ tubulin dimer recently presented by E. Nogales et al. [(1998), Nature 391, 199−203], we propose a new model for the tau−microtubule interaction.