Differences in microtubule binding and self-association abilities of bovine brain tau isoforms

We have fractionated tau isoforms by elution at increasing pH values using iron-chelated affinity chromatography, which discriminates between isoforms phosphorylated to different extents. Microtubule-associated tau elutes from the column at a pH gradient narrower than that of total brain tau. Neither under-phosphorylated nor highly phosphorylated isoforms are found in the microtubule-associated tau protein preparation. This indicates that phosphorylation at certain sites is needed for tau binding to microtubules, whereas phosphorylation at some other sites may prevent the association. The self-association ability of the different tau isoforms has also been analyzed. Tau isoforms containing three tubulin binding motifs form covalently bound dimers more efficiently than tau isoforms containing four motifs. This dimer-forming ability is notably diminished in the presence of a reducing agent, as determined by SDS-polyacrylamide gel electrophoresis, thus suggesting the involvement of cysteine residues. Additionally, tau forms larger aggregates, as detected by gel permeation chromatography, which are solubilized by SDS and cannot, therefore, be observed by SDS-polyacrylamide gel electrophoresis. These tau aggregates are observed even in the presence of reducing agents. These results support the idea that other regions in the tau molecule, besides the Cys-containing tubulin binding region, also contribute to tau self-association. Tau dimerization and aggregation may be prior steps to the formation of paired helical filaments.