FUNCTIONAL INTERACTIONS BETWEEN THE PROLINE-RICH AND REPEAT REGIONS OF TAU-ENHANCE MICROTUBULE-BINDING AND ASSEMBLY

Tau is a neuronal microtubule-associated protein that promotes microtu bule assembly, stability, and bundling in axons. Two distinct regions of tau are important for the tau-microtubule interaction, a relatively well-characterized ''repeat region'' in the carboxyl terminus (contai ning either three or four imperfect 18-amino acid repeats separated by 13- or 14-amino acid long inter-repeats) and a more centrally located , relatively poorly characterized proline-rich region. By using amino- terminal truncation analyses of tau, we have localized the microtubule binding activity of the proline-rich region to Lys(215)-Asn(246) and identified a small sequence within this region, (215)KKVAVVR(221), tha t exerts a strong influence on microtubule binding and assembly in bot h three- and four-repeat tau isoforms. Site-directed mutagenesis exper iments indicate that these capabilities are derived largely from Lys(2 15)/Lys(216) and Arg(221). In marked contrast to synthetic peptides co rresponding to the repeat region, peptides corresponding to Lys(215)-A sn(236) and Lys(215)-Thr(222) alone possess little or no ability to pr omote microtubule assembly, and the peptide Lys(215)-Thr(222) does not effectively suppress in vitro microtubule dynamics. However, combinin g the proline-rich region sequences (Lys(215)-Asn(246)) with their adj acent repeat region sequences within a single peptide (Lys(215) Lys(27 2)) enhances microtubule assembly by 10-fold, suggesting intramolecula r interactions between the proline-rich and repeat regions. Structural complexity in this region of tau also is suggested by sequential amin o-terminal deletions through the proline-rich and repeat regions, whic h reveal an unusual pattern of loss and gain of function. Thus, these data lead to a model in which efficient microtubule binding and assemb ly activities by tau require intramolecular interactions between its r epeat and proline-rich regions. This model, invoking structural comple xity for the microtubule-bound conformation of tau, is fundamentally d ifferent from previous models of tau structure and function, which vie wed tau as a simple linear array of independently acting tubulin-bindi ng sites.