Characterization of the Aggregation-Prone Ensemble of Tau in the Presence of Polyphosphates

Abstract

Tau is a microtubule associated protein implicated in Alzheimer's disease through its aggregation and deposition as neurofibrillary tangles. Tau is both an intrinsically disordered protein and it aggregates very slowly in vitro making it challenging to study the mechanism of aggregation. Typically highly anionic molecular aggregation inducers, such as heparin, are used to induce aggregation in vitro, although these molecules are unlikely to have much physiological relevance. Recently, polyphosphates, linear chain of phosphate residues found in all prokaryotic and eukaryotic cells, have been proposed to act as a universal scaffold for the initiation of aggregation of amyloidogenic proteins. Polyphosphates have been shown to both increase the rate of amyloid fiber formation and to stabilize the fibers. Here, we use single molecule FRET to characterize conformational changes in tau in the presence of polyphosphates. The low concentrations (pM) of protein utilized in single molecule measurements allow for the population of aggregation-prone conformations, while inhibiting aggregation. In the presence of polyphosphates, tau undergoes domain specific conformational changes in which there is an overall expansion of the protein with a concomitant compaction of the microtubule binding region. Conservation of conformational shifts in the presence of different aggregation inducers will provide a model of the consensus conformational features that potentiate tau for aggregation in disease.