Liquid–Liquid Phase Separation of Tau Protein Is Encoded at the Monomeric Level

Abstract

Liquid–liquid phase separation (LLPS) is involved in both physiological and pathological processes. The intrinsically disordered protein Tau and its K18 construct can undergo LLPS in a distinct temperature-dependent manner, and the LLPS of Tau protein can initiate Tau aggregation. However, the underlying mechanism driving Tau LLPS remains largely elusive. To understand the temperature-dependent LLPS behavior of Tau at the monomeric level, we explored the conformational ensemble of Tau at different temperatures by performing all-atom replica-exchange molecular dynamic simulation on K18 monomer with an accumulated simulation time of 26.4 μs. Our simulation demonstrates that the compactness, β-structure propensity, and intramolecular interaction of K18 monomer exhibit nonlinear temperature-dependent behavior. 295DNI­KHV300/326GNI­HHK331/337VEV­KSE342 make significant contributions to the temperature dependence of the β propensity of K18 monomer, while the two fibril-nucleating cores display relatively high β propensity at all temperatures. At a specific temperature, K18 monomer adopts the most collapsed state with exposed sites for both persistent and transient interactions. Given that more collapsed polypeptide chains were reported to be more prone to phase separate, our results suggest that K18 monomer inherently possesses conformational characteristics favoring LLPS. Our simulation predicts the importance of 295DNI­KHV300/326GNI­HHK331/337VEV­KSE342 to the temperature-dependent conformational properties of K18, which is corroborated by CD spectra, turbidity assays, and DIC microscopy. Taken together, we offer a computational and experimental approach to comprehend the structural basis for LLPS by amyloidal building blocks.

Atomic Level Investigations of Early Aggregation of Tau43 in Water I. Conformational Propensity of Monomeric Tau43

“…The Tau43 domain in the solution NMR htau40 investigation has increased β‐sheet propensity, 13 also recent reports show β‐populated domains in different diseased Tau isoforms 57,58 . However, Tau43 structural ensemble, within currently simulated timescales, are intrinsically unstructured with transient structural propensity, in qualitative agreement with several studies, 11,28,34,59–61 where relatively compact, transiently structured, yet disordered conformations have been predicted for monomeric and oligomeric conformational ensembles of Tau isoforms. Together, these indicate the current monomeric conformational ensemble to be a relatively compact yet unstructured metastable state in the early aggregation process of Tau43, which will be further underscored with corresponding insights from structural parameters.…”

“…Also, PHF6 is found to be the most hydrophobic, and conformationally most extended, in spite of displaying least conformational fluctuations, due to plausibly being shielded by contiguous segments from aqueous environment (Supporting Information Figure SI 5, Figure SI 6; Figures 6 and 7). Recent computational studies on K18, have found the PHF6 to display enhanced beta‐probability at all temperatures, unlike most of the other aggregate‐prone segments 61 . Our results further underscore the mechanistic insights as well as importance of PHF6 in the early aggregation process of Tau43.…”

“…To the extent of our knowledge, this is the first theoretical study on the Tau43 isoform. Previous experimental and computational studies have also underscored the fact that important domains and isoforms of Tau to be compact within ambient conditions 10,59,61 . Our results also reveal the Tau43 monomer to be compact with mainly random coil, and occasionally helix and β‐sheet components, similar to the secondary structure population of other intrinsically disordered proteins, Aβ42 in particular.…”

Molecular Organization of the Early Stages of Nucleosome Phase Separation Visualized by Cryo-Electron Tomography

“…S2 and S4A). Interestingly, a similar non-monotonic behavior with temperature has been described for tau protein condensates (Dong et al, 2021).…”

Phospho-Tau and Chromatin Landscapes in Early and Late Alzheimer’s Disease

“…The C-terminus contains a basic microtubule-binding domain (MTBD), which is predominantly positively charged, followed by a negatively charged terminal region. The electrostatic interactions between domains and tau’s IDP nature allow the formation of condensed liquid tau droplets, both in vitro and in vivo [ 47 , 48 , 49 , 50 , 51 ]. Tau interacts with nuclear components such as nucleoli and nuclear speckles, as well as with cytoplasmic bodies such as stress granules and P granules [ 52 , 53 , 54 , 55 , 56 , 57 ].…”