Entry created on 1 July 2019 (Revision 1.0) Annotator: Éva Schád
Basic protein information
Accession P10636-8
Common name Tau-F, Tau
Gene MAPT
Organism Homo sapiens
Uniprot name Isoform Tau-F of Microtubule-associated protein tau
Basic LLPS information
Organelle cytoplasmic microtubule; condensed compartments of microtubule bundling
Type of experimental evidence
Protein region(s) mediating LLPS
244
-
368
Tau/MAP repeats
Based on the experimental results of the following publication: 28819146
Molecular features viewer
PDB structures
View on PDBe
Extended LLPS information
Functional description
Non-centrosomal microtubule bundles play important roles in cellular organization and function. The concentration of tubulin into a condensed, liquid-like compartment composed of the unstructured neuronal protein tau is sufficient to nucleate microtubule bundles.Under conditions of molecular crowding, tau forms liquid-like drops. Tubulin partitions into these drops, where it nucleates and drives the formation of microtubule bundles. These bundles deform the drops and remain enclosed by diffusible tau molecules, exhibiting a liquid-like behavior. (PMID:28877466) Alternative splicing of Tau can regulate the formation of Tau-containing membrane-less compartments. Phosphorylation of Tau repeats promotes liquid–liquid phase separation at cellular protein conditions. Liquid droplets formed by the positively charged microtubule-binding domain of Tau undergo coacervation with negatively charged molecules to promote amyloid formation. LLPS promotes Tau fibrillization in the presence of heparin (polyanion) (PMID:28819146). Tau complexes with RNA to form droplets. Uniquely, the pool of RNAs to which tau binds in living cells are tRNAs. The LLPS process is directly and sensitively tuned by salt concentration and temperature, implying it is modulated by both electrostatic interactions between the involved protein and nucleic acid constituents, as well as net changes in entropy. Despite the high protein concentration within the complex coacervate phase, tau is locally freely tumbling and capable of diffusing through the droplet interior. However, prolonged residency within the droplet state can result in the emergence of detectable β-sheet structures. Thus the droplet state can incubate tau and predispose the protein toward the formation of insoluble fibrils (PMID:28683104). Liquid demixing of tau does not require phosphorylation. Tau LLPS is driven by attractive electrostatic intermolecular interactions between the negatively charged N-terminal and positively charged middle/C-terminal domains of the protein, with hydrophobic interactions playing a surprisingly small role. (PMID: 31097543) In Alzheimer's disease, tau is predominantly acetylated at K174, K274, K280, and K281 residues. The acetylation of K274-tau is linked with memory loss and dementia. Acetylation mimicking mutation at K274 (K→Q) residue of tau strongly reduces the ability of tau to bind to tubulin and also to polymerize tubulin and strongly decreases the critical concentration for the liquid-liquid phase separation of tau. (PMID: 31036717)
Literature supporting the LLPS: 29472250, 28683104, 28819146, 28877466, 29734651, 30950394, 30068389, 31097543, 31260737, 31036717, 31456657
Functional class of membraneless organelle: activation/nucleation/signal amplification/bioreactor
Binding partners (at biological protein concentrations)
1) tRNA 2) heparin 3) tubulin
Type of RNA(s) required/used for the LLPS at biological protein concentrations
other type of RNA: tRNA
Molecular interaction types contributing to LLPS
linear oligomerization/self-association (PMID:28683104) formation of amyloid-like/cross-beta/kinked/stacked beta-sheet structures (PMID:28819146) protein-RNA interaction (PMID:28683104) electrostatic (cation-anion) interaction (PMID:31097543)
Determinants of phase separation and droplet properties
1) protein concentration of Tau 2) phosphorylation state 3) alternative splicing 4) salt concentration 5) temperature 6) crowding agent concentration 7) modification state
Membrane cluster No
Partner-dependent No
RNA-dependent No
PTM required No
Domain-motif interactions No
Discrete oligomerization No
Regulation and disease
Post-translational modifications affecting LLPS
Position Residue PTM Effect Reference Modifying enzyme Notes
Isoforms known to affect LLPS
Isoform Effect Reference
All known isoforms containing sequence changes in the LLPS region(s)
Position type Isoform names from UniProt
Disease mutations affecting LLPS
Mutation dbSNP Disease OMIM Effect Reference Notes
Experimental information
Experimental techniques applied to prove/investigate LLPS
To exclude the influence of intramolecular and intermolecular cross-linking through Tau’s two native cysteine residues, C291 and C322, in vitro turbidity measurements were performed in the presence of tris(2-carboxyethyl)phosphine (TCEP), mimicking the reducing environment inside neurons. Changes in solution turbidity can arise from liquid–liquid demixing/LLPS, but also from formation of other types of aggregates. To support the presence of a liquid phase separated state of the repeat domain of Tau, differential interference contrast (DIC) microscopy were performed. To demonstrate the presence of tau proteins in the liquid droplets, confocal microscopy of fluorescently labeled protein was used and at 37 °C, but not at 5 °C, fluorescent droplets were observed. To dissect the consequences of LLPS on the molecular properties of the repeat domain of Tau, NMR spectroscopy was used. The NMR data suggest that the protein stays largely disordered within liquid droplets, in agreement with the overall low content of regular secondary structure observed by CD spectroscopy. NMR measurements using attached paramagnetic nitroxide tag to the two native cysteines demonstrate that LLPS of Tau repeats results in a tight molecular mesh of amyloid-promoting elements. (PMID:28819146) Bright-field and fluorescence microscopy show that tau/tau-EGFP form drops in vitro in the presence of crowding agents. Fusion of tau droplets was visualized using dual-trap optical tweezers and Internal rearrangement of tau drops was monitored using fluorescence recovery after photo-bleaching (FRAP). (PMID:28877466) In vitro tau-RNA binding were detected using gel shift mobility assay and tau LLPS in the presence of RNA was investigated with light and confocal microscopy images of fluorescence-labeled proteins. Light microscopy images show that tau-RNA droplets form a complex coacervate phase. In vivo experiments show that tRNA transfection accumulates sarkosyl insoluble tau in human-induced pluripotent stem cell (hiPSC) derived neurons. (PMID:28683104) In vitro, upon addition to non-phosphorylated tau441 of polyethylene glycol (PEG), the volume-excluding polymer frequently used to mimic ntracellular crowding, a rapid increase in sample turbidity was observed, strongly suggesting LLPS. The decreased tendency of tau441 to form liquid droplets at increasing salt concentrations - confirmed by fluorescence microscopy using Alexa488-labeled tau441 - strongly suggests that LLPS is at least partly driven by attractive electrostatic interactions. (PMID: 31097543). EFhd2 alters tau liquid phase behavior in a calcium and coiled-coil domain dependent manner. Co-incubation of EFhd2 and tau in the absence of calcium leads to the formation of solid-like structures containing both proteins, while in the presence of calcium these two proteins phase separate together into liquid droplets. EFhd2's coiled-coil domain is necessary to alter tau's liquid phase separation, indicating that protein-protein interaction is required (PMID:31456657).
Experimental observations supporting the liquid material state of the condensate
rheological traits (PMID:28819146, PMID:28877466) morphological traits (PMID:28819146, PMID:28877466, PMID:28683104) dynamic movement/reorganization of molecules within the droplet (PMID:28819146, PMID:28877466) temperature-dependence (PMID:28819146, PMID:28683104) reversibility of formation and dissolution (PMID:28683104)