The Role of the Lipid Bilayer in Tau Aggregation

Tau is a microtubule associated protein whose aggregation is implicated in a number of neurodegenerative diseases. We investigate the mechanism by which anionic lipid vesicles induce aggregation of tau in vitro using K18, a fragment of tau corresponding to the four repeats of the microtubule binding domain. Our results show that aggregation occurs when the amount of K18 bound to the lipid bilayer exceeds a critical surface density. The ratio of protein/lipid at the critical aggregation concentration is pH-dependent, as is the binding affinity. At low pH, where the protein binds with high affinity, the critical surface density is independent both of total lipid concentration as well as the fraction of anionic lipid present in the bilayer. Furthermore, the aggregates consist of both protein and vesicles and bind the β-sheet specific dye, Thioflavin T, in the manner characteristic of pathological aggregates. Our results suggest that the lipid bilayer facilitates protein-protein interactions both by screening charges on the protein and by increasing the local protein concentration, resulting in rapid aggregation. Because anionic lipids are abundant in cellular membranes, these findings contribute to understanding tau-lipid bilayer interactions that may be relevant to disease pathology

Identification of an Aggregation-Prone Structure of Tau

The aggregation and deposition of normally soluble proteins is the hallmark of several devastating neurodegenerative disorders. For proteins such as tau in Alzheimer’s disease and α-synuclein in Parkinson’s disease, aggregation involves a transition from an intrinsically disordered monomer to a highly structured fiber. While understanding the role of these proteins in neurodegeneration requires elucidation of the structural basis of self-association, the conformational heterogeneity of disordered proteins makes their structural characterization inherently challenging. Here we use single molecule Förster resonance energy transfer to measure the conformational ensemble of tau in the absence and presence of heparin to identify critical conformational changes relevant to the initiation of aggregation. We find that different domains of tau display distinct conformational properties that are strongly correlated with their degree of disorder and that may relate to their roles in aggregation. Moreover, we observe that heparin binding induces a distinct two-state structural transition in tau characterized by a loss of long-range contacts and a concomitant compaction of the microtubule binding domain. Our results describe a conformational intermediate of tau that precedes the formation of aggregates and could serve as a target for tau-focused therapeutics

Tau mutants bind tubulin heterodimers with enhanced affinity

Tau is a microtubule binding protein that forms pathological aggregates in the brain in Alzheimer's disease and other tauopathies. Disease etiology is thought to arise from loss of native interactions between tau and microtubules, as well as from gain of toxicity tied to tau aggregation, although neither mechanism is well understood. Here we investigate the link between function and disease using disease-associated and disease-motivated mutants of tau. We find that mutations to highly conserved proline residues in repeats 2 and 3 of the microtubule binding domain have differential effects on tau binding to tubulin and the capacity of tau to enhance tubulin polymerization. Notably, mutations to these residues result in an increased affinity for tubulin dimers while having a negligible effect on binding to stabilized microtubules. We measure conformational changes in tau on binding to tubulin that provide a structural framework for the observed altered affinity and function. Additionally, we find that these mutations do not necessarily enhance aggregation, which could have important implications for tau therapeutic strategies that focus solely on searching for tau aggregation inhibitors. We propose a model that describes tau binding to tubulin dimers and a mechanism by which disease-relevant alterations to tau impact its function. Together, these results draw attention to the interaction between tau and free tubulin as playing an important role in mechanisms of tau pathology.</p

The Conformational Ensembles of a-Synuclein and Tau: Combining Single-Molecule FRET and Simulations

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