Pseudo-phosphorylation of tau at Ser202 and Thr205 affects tau filament formation

The microtubule-associated protein tau aggregates into insoluble filaments in numerous neurodegenerative diseases, most common of which is Alzheimer's disease. Tau aggregation in Alzheimer's disease appears to follow a continuum from soluble monomer to an end point of insoluble extracellular tangles with a strong correlation between the amount of fibrillar tau and dementia. The phosphorylation of amino acids S202 and T205 in the tau molecule is recognized by the phosphorylation-specific monoclonal antibody, AT8, and has been observed by a number of researchers to be an early step in the progression of monomer to filaments. In addition, these amino acids are located in a proline-rich region containing a set of five phosphorylation sites (one being S202), that when phosphorylated, were reported to alter several properties of tau, including filament formation. Considering these observations, we have investigated the role of S202 and T205 phosphorylation in the in vitro polymerization of tau. Pseudo-phosphorylation mutants were constructed by site-directed mutagenesis in which amino acids S202 and T205 were changed to negatively charged glutamic acids mimicking post-translational phosphorylation. These pseudo-phosphorylated, mutant tau proteins were then assayed in vitro for changes in structure, polymerization into filaments, and microtubule binding. Phosphorylation at the AT8 site does not appear to influence either SDS-resistant structure nor microtubule binding. However, in regard to filament formation, phosphorylation at S202 appears to enhance polymerization; and phosphorylation at both sites not only enhances polymerization but also makes filament formation more sensitive to small changes in tau concentration.