Protein Semisynthesis Provides Access to Tau Disease-Associated Post-translational Modifications (PTMs) and Paves the Way to Deciphering the Tau PTM Code in Health and Diseased States

Protein desulfurization and deselenization

Methods enabling the dechalcogenation of thiols or selenols have been investigated and developed for a long time in fields of research as diverse as the study of prebiotic chemistry, the engineering of fuel processing techniques, the study of biomolecule structures and function or the chemical synthesis of biomolecules. The dechalcogenation of thiol or selenol amino acids is nowadays a particularly flourishing area of research for being a pillar of modern chemical protein synthesis, when used in combination with thiol or selenol-based chemoselective peptide ligation chemistries. This review offers a comprehensive and scholarly overview of the field, emphasizing emerging trends and providing a detailed and critical mechanistic discussion of the dechalcogenation methods developed so far. Taking advantage of recently published reports, it also clarifies some unexpected desulfurization reactions that were observed in the past and for which no explanation was provided at the time. Additionally, the review includes a discussion on principal desulfurization methods within the framework of newly introduced green chemistry metrics and toolkits, providing a well-rounded exploration of the subject.

An Uncommon Phosphorylation Mode Regulates the Activity and Protein Interactions of N-Acetylglucosamine Kinase

While the function of protein phosphorylation in eukaryotic cell signaling is well established, the role of a closely related modification, protein pyrophosphorylation, is just starting to surface. A recent study has identified several targets of endogenous protein pyrophosphorylation in mammalian cell lines, including N-acetylglucosamine kinase (NAGK). Here, a detailed functional analysis of NAGK phosphorylation and pyrophosphorylation on serine 76 (S76) has been conducted. This analysis was enabled by using amber codon suppression to obtain phosphorylated pS76-NAGK, which was subsequently converted to site-specifically pyrophosphorylated NAGK (ppS76-NAGK) with a phosphorimidazolide reagent. A significant reduction in GlcNAc kinase activity was observed upon phosphorylation and near-complete inactivation upon pyrophosphorylation. The formation of ppS76-NAGK proceeded via an ATP-dependent autocatalytic process, and once formed, ppS76-NAGK displayed notable stability toward dephosphorylation in mammalian cell lysates. Proteomic examination unveiled a distinct set of protein-protein interactions for ppS76-NAGK, suggesting an alternative function, independent of its kinase activity. Overall, a significant regulatory role of pyrophosphorylation on NAGK activity was uncovered, providing a strong incentive to investigate the influence of this unusual phosphorylation mode on other kinases.

Azole reagents enabled ligation of peptide acyl pyrazoles for chemical protein synthesis

Native chemical ligation (NCL) has been playing an increasingly important role in chemical protein synthesis (CPS). More efficient ligation methods that circumvent the requirement of a peptidyl thioester and thiol additive-which allow the following desulfurization or refolding in one pot-are urgently needed for the synthesis of more complex protein targets and in large quantities. Herein, we discover that the weak acyl donor peptidyl N-acyl pyrazole can be activated by azole reagents like 3-methylpyrazole or imidazole to facilitate its ligation directly with an N-terminal cysteine peptide. As it requires no thioester or thiol additive, this ligation strategy can be conveniently combined with metal-free desulfurization (MFD) or oxidative protein folding to allow various one-pot protocols. The utility and generality of the strategy are showcased by the total synthesis of ubiquitin via an N-to-C sequential ligation-MFD strategy, the semi-synthesis of the copper protein azurin, and the efficient assembly of a sulfated hirudin variant and the cyclotide kalata B1, all in a one-pot fashion.

Molecular Insights into the Differential Effects of Acetylation on the Aggregation of Tau Microtubule-Binding Repeats

Aggregation of tau protein into intracellular fibrillary inclusions is characterized as the hallmark of tauopathies, including Alzheimer's disease and chronic traumatic encephalopathy. The microtubule-binding (MTB) domain of tau, containing either three or four repeats with sequence similarities, plays an important role in determining tau's aggregation. Previous studies have reported that abnormal acetylation of lysine residues displays a distinct effect on the formation of pathological tau aggregates. However, the underlying molecular mechanism remains mostly elusive. In this study, we performed extensive replica exchange molecular dynamics (REMD) simulations of 144 μs in total to systematically investigate the dimerization of four tau MTB repeats and explore the impacts of Lys280 (K280) or Lys321 (K321) acetylation on the conformational ensembles of the R2 or R3 dimer. Our results show that R3 is the most prone to aggregation among the four repeats, followed by R2 and R4, while R1 displays the weakest aggregation propensity with a disordered structure. Acetylation of K280 could promote the aggregation of R2 peptides by increasing the formation of β-sheet structures and strengthening the interchain interaction. However, K321 acetylation decreases the β-sheet content of the R3 dimer, reduces the ability of R3 peptides to form long β-strands, and promotes the stable helix structure formation. The salt bridge and Y310-Y310 π-π stacking interactions of the R3 dimer are greatly weakened by K321 acetylation, resulting in the inhibition of dimerization. This study uncovers the structural ensembles of tau MTB repeats and provides mechanistic insights into the influences of acetylation on tau aggregation, which may deepen the understanding of the pathogenesis of tauopathies.

Hyperacetylation mimetics within the tau filament core inhibits prion-like propagation of misfolded tau

Acetylation of key Lysine residues characterizes aggregates of the microtubule-associated protein tau constituting the neuropathological hallmark of many neurodegenerative diseases, such as Alzheimer's disease (AD) and Progressive Supranuclear Palsy (PSP). This has led to the idea that acetylation influences tau aggregation. Using a HEK293 cell-based aggregation assay, we tested whether acetylation-mimicking substitutions (K→Q) on five AD-associated acetyl-modified sites (AcK-311, 353, 369, 370, 375) influenced its propensity to aggregate when exposed to tau seeds derived from two clinically distinctive diseases - AD and PSP. In combination, the presence of 5K→Q sites ablated tau aggregation induced by seeds from both AD and PSP patients, indicating that acetylation within the filament core domain of tau could have an inhibitory effect on seed-mediated aggregation. We had previously identified that a phosphorylation-mimetic on Ser305 (S→E) abrogated tau aggregation by seeds from AD patients, without affecting seeding by PSP patients. Combining the S305→E to the 5K→Q acetyl-modified sites, we found that this tau could now be seeded only by PSP patients, but not by AD patients, confirming Ser305 as a critical determinant of strain-specific tau seeding. On the other hand, acetylation-nullifying substitutions (K→R or K→A) on these same Lys sites did not alter tau seeding abilities compared to the parental tau construct. Notably, the combined acetylation-nullifying Alanine substitutions on these 5 Lys sites resulted in spontaneous self-aggregation, with the filaments resembling amorphous deposits. All together, we demonstrate that cooperative acetyl-occupancy in the tau filament core influences seeded propagation of misfolded tau as well as drives self-aggregation.

Post-Translational Modifications Control Phase Transitions of Tau

The self-assembly of Tau(297-391) into filaments, which mirror the structures observed in Alzheimer's disease (AD) brains, raises questions about the role of AD-specific post-translational modifications (PTMs) in the formation of paired helical filaments (PHFs). To investigate this, we developed a synthetic approach to produce Tau(291-391) featuring N-acetyllysine, phosphoserine, phosphotyrosine, and N-glycosylation at positions commonly modified in post-mortem AD brains, thus facilitating the study of their roles in Tau pathology. Using transmission electron microscopy (TEM), cryo-electron microscopy (cryo-EM), and a range of optical microscopy techniques, we discovered that these modifications generally hinder the in vitro assembly of Tau into PHFs. Interestingly, while acetylation's effect on Tau assembly displayed variability, either promoting or inhibiting phase transitions in the context of cofactor free aggregation, heparin-induced aggregation, and RNA-mediated liquid-liquid phase separation (LLPS), phosphorylation uniformly mitigated these processes. Our observations suggest that PTMs, particularly those situated outside the fibril's rigid core are pivotal in the nucleation of PHFs. Moreover, in scenarios involving heparin-induced aggregation leading to the formation of heterogeneous aggregates, most AD-specific PTMs, except for K311, appeared to decelerate the aggregation process. The impact of acetylation on RNA-induced LLPS was notably site-dependent, exhibiting both facilitative and inhibitory effects, whereas phosphorylation consistently reduced LLPS across all proteoforms examined. These insights underscore the complex interplay between site-specific PTMs and environmental factors in modulating Tau aggregation kinetics, enhancing our understanding of the molecular underpinnings of Tau pathology in AD and highlighting the critical role of PTMs located outside the ordered filament core in driving the self-assembly of Tau into PHF structures.

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Molecular Mechanism of Tau Misfolding and Aggregation: Insights from Molecular Dynamics Simulation

Tau dysfunction has a close association with many neurodegenerative diseases, which are collectively referred to as tauopathies. Neurofibrillary tangles (NFTs) formed by misfolding and aggregation of tau are the main pathological process of tauopathy. Therefore, uncovering the misfolding and aggregation mechanism of tau protein will help to reveal the pathogenic mechanism of tauopathies. Molecular dynamics (MD) simulation is well suited for studying the dynamic process of protein structure changes. It provides detailed information on protein structure changes over time at the atomic resolution. At the same time, MD simulation can also simulate various conditions conveniently. Based on these advantages, MD simulations are widely used to study conformational transition problems such as protein misfolding and aggregation. Here, we summarized the structural features of tau, the factors affecting its misfolding and aggregation, and the applications of MD simulations in the study of tau misfolding and aggregation.

Deciphering the Role of the Ser-Phosphorylation Pattern on the DNA-Binding Activity of Max Transcription Factor Using Chemical Protein Synthesis

The chemical synthesis of site-specifically modified transcription factors (TFs) is a powerful method to investigate how post-translational modifications (PTMs) influence TF-DNA interactions and impact gene expression. Among these TFs, Max plays a pivotal role in controlling the expression of 15 % of the genome. The activity of Max is regulated by PTMs; Ser-phosphorylation at the N-terminus is considered one of the key regulatory mechanisms. In this study, we developed a practical synthetic strategy to prepare homogeneous full-length Max for the first time, to explore the impact of Max phosphorylation. We prepared a focused library of eight Max variants, with distinct modification patterns, including mono-phosphorylated, and doubly phosphorylated analogues at Ser2/Ser11 as well as fluorescently labeled variants through native chemical ligation. Through comprehensive DNA binding analyses, we discovered that the phosphorylation position plays a crucial role in the DNA-binding activity of Max. Furthermore, in vitro high-throughput analysis using DNA microarrays revealed that the N-terminus phosphorylation pattern does not interfere with the DNA sequence specificity of Max. Our work provides insights into the regulatory role of Max's phosphorylation on the DNA interactions and sequence specificity, shedding light on how PTMs influence TF function.

Chemical Synthesis of Microtubule-Associated Protein Tau

Deposits of the microtubule-associated protein Tau (MAPT) serve as a hallmark of neurodegenerative diseases known as tauopathies. Numerous studies have demonstrated that in diseases such as Alzheimer's disease (AD), Tau undergoes extensive remodeling. The attachment of post-translational modifications distributed throughout the entire sequence of the protein correlates with clinical presentation. A systematic examination of these protein alterations can shed light on their roles in both healthy and diseased states. However, the ability to access these modifications in the entire protein chain is limited as Tau can only be produced recombinantly or through semisynthesis. In this article, we describe the first chemical synthesis of the longest 2N4R isoform of Tau, consisting of 441 amino acids. The 2N4R Tau was divided into 3 major segments and a total of 11 fragments, all of which were prepared via solid-phase peptide synthesis. The successful chemical strategy has relied on the strategic use of two cysteine sites (C291 and C322) for the native chemical ligations (NCLs). This was combined with modern preparative protein chemistries, such as mercaptothreonine ligation (T205), diselenide-selenoester ligation (D358), and mutations of mercaptoamino acids into native residues via homogeneous radical desulfurization (A40, A77, A119, A157, A246, and A390). The successful completion of the synthesis has established a robust and scalable route to the native protein in multimilligram quantities and high purity. In broader terms, the presented strategy can be applied to the preparation of other shorter isoforms of Tau as well as to introduce all post-translational modifications that are characteristic of tauopathies such as AD.

Disease-associated patterns of acetylation stabilize tau fibril formation

Assembly of tau into beta-sheet-rich amyloids dictates the pathology of a diversity of diseases. Lysine acetylation has been proposed to drive tau amyloid assembly, but no direct mechanism has emerged. Using tau fragments, we identify patterns of acetylation that flank amyloidogenic motifs on the tau fragments that promote rapid fibril assembly. We determined a 3.9 Å cryo-EM amyloid fibril structure assembled from an acetylated tau fragment uncovering how lysine acetylation can mediate gain-of-function interactions. Comparison of the structure to an ex vivo tauopathy fibril reveals regions of structural similarity. Finally, we show that fibrils encoding disease-associated patterns of acetylation are active in cell-based tau aggregation assays. Our data uncover the dual role of lysine residues in limiting tau aggregation while their acetylation leads to stabilizing pro-aggregation interactions. Design of tau sequence with specific acetylation patterns may lead to controllable tau aggregation to direct folding of tau into distinct amyloid folds.

Phosphorylation of a Cleaved Tau Proteoform at a Single Residue Inhibits Binding to the E3 Ubiquitin Ligase, CHIP

Microtubule-associated protein tau (MAPT/tau) accumulates in a family of neurodegenerative diseases, including Alzheimer's disease (AD). In disease, tau is aberrantly modified by post-translational modifications (PTMs), including hyper-phosphorylation. However, it is often unclear which of these PTMs contribute to tau's accumulation or what mechanisms might be involved. To explore these questions, we focused on a cleaved proteoform of tau (tauC3), which selectively accumulates in AD and was recently shown to be degraded by its direct binding to the E3 ubiquitin ligase, CHIP. Here, we find that phosphorylation of tauC3 at a single residue, pS416, is sufficient to block its interaction with CHIP. A co-crystal structure of CHIP bound to the C-terminus of tauC3 revealed the mechanism of this clash and allowed design of a mutation (CHIPD134A) that partially restores binding and turnover of pS416 tauC3. We find that pS416 is produced by the known AD-associated kinase, MARK2/Par-1b, providing a potential link to disease. In further support of this idea, an antibody against pS416 co-localizes with tauC3 in degenerative neurons within the hippocampus of AD patients. Together, these studies suggest a discrete molecular mechanism for how phosphorylation at a specific site contributes to accumulation of an important tau proteoform.

Fully co-factor-free ClearTau platform produces seeding-competent Tau fibrils for reconstructing pathological Tau aggregates

Tau protein fibrillization is implicated in the pathogenesis of several neurodegenerative diseases collectively known as Tauopathies. For decades, investigating Tau fibrillization in vitro has required the addition of polyanions or other co-factors to induce its misfolding and aggregation, with heparin being the most commonly used. However, heparin-induced Tau fibrils exhibit high morphological heterogeneity and a striking structural divergence from Tau fibrils isolated from Tauopathies patients' brains at ultra- and macro-structural levels. To address these limitations, we developed a quick, cheap, and effective method for producing completely co-factor-free fibrils from all full-length Tau isoforms and mixtures thereof. We show that Tau fibrils generated using this ClearTau method - ClearTau fibrils - exhibit amyloid-like features, possess seeding activity in biosensor cells and hiPSC-derived neurons, retain RNA-binding capacity, and have morphological properties and structures more reminiscent of the properties of the brain-derived Tau fibrils. We present the proof-of-concept implementation of the ClearTau platform for screening Tau aggregation-modifying compounds. We demonstrate that these advances open opportunities to investigate the pathophysiology of disease-relevant Tau aggregates and will facilitate the development of Tau pathology-targeting and modifying therapies and PET tracers that can distinguish between different Tauopathies.

Chemical Synthesis of Bioactive Proteins

Nature has developed a plethora of protein machinery to operate and maintain nearly every task of cellular life. These processes are tightly regulated via post-expression modifications-transformations that modulate intracellular protein synthesis, folding, and activation. Methods to prepare homogeneously and precisely modified proteins are essential to probe their function and design new bioactive modalities. Synthetic chemistry has contributed remarkably to protein science by allowing the preparation of novel biomacromolecules that are often challenging or impractical to prepare via common biological means. The ability to chemically build and precisely modify proteins has enabled the production of new molecules with novel physicochemical properties and programmed activity for biomedical research, diagnostic, and therapeutic applications. This minireview summarizes recent developments in chemical protein synthesis to produce bioactive proteins, with emphasis on novel analogs with promising in vitro and in vivo activity.

The Involvement of Post-Translational Modifications in Regulating the Development and Progression of Alzheimer's Disease

Post-translational modifications (PTMs) have been recently reported to be involved in the development and progression of Alzheimer's disease (AD). In detail, PTMs include phosphorylation, glycation, acetylation, sumoylation, ubiquitination, methylation, nitration, and truncation, which are associated with pathological functions of AD-related proteins, such as β-amyloid (Aβ), β-site APP-cleavage enzyme 1 (BACE1), and tau protein. In particular, the roles of aberrant PTMs in the trafficking, cleavage, and degradation of AD-associated proteins, leading to the cognitive decline of the disease, are summarized under AD conditions. By summarizing these research progress, the gaps will be filled between PMTs and AD, which will facilitate the discovery of potential biomarkers, leading to the establishment of novel clinical intervention methods against AD.

Network of hotspot interactions cluster tau amyloid folds

Cryogenic electron microscopy has revealed unprecedented molecular insight into the conformations of β-sheet-rich protein amyloids linked to neurodegenerative diseases. It remains unknown how a protein can adopt a diversity of folds and form multiple distinct fibrillar structures. Here we develop an in silico alanine scan method to estimate the relative energetic contribution of each amino acid in an amyloid assembly. We apply our method to twenty-seven ex vivo and in vitro fibril structural polymorphs of the microtubule-associated protein tau. We uncover networks of energetically important interactions involving amyloid-forming motifs that stabilize the different fibril folds. We evaluate our predictions in cellular and in vitro aggregation assays. Using a machine learning approach, we classify the structures based on residue energetics to identify distinguishing and unifying features. Our energetic profiling suggests that minimal sequence elements control the stability of tau fibrils, allowing future design of protein sequences that fold into unique structures.

Disease-associated patterns of acetylation stabilize tau fibril formation

Assembly of the microtubule-associated protein into tauopathy fibril conformations dictates the pathology of a diversity of diseases. Recent cryogenic Electron Microscopy (cryo-EM) structures have uncovered distinct fibril conformations in different tauopathies but it remains unknown how these structures fold from a single protein sequence. It has been proposed that post-translational modifications may drive tau assembly but no direct mechanism for how modifications drive assembly has emerged. Leveraging established aggregation-regulating tau fragments that are normally inert, we tested the effect of chemical modification of lysines with acetyl groups on tau fragment conversion into amyloid aggregates. We identify specific patterns of acetylation that flank amyloidogenic motifs on the tau fragments that drive rapid fibril assembly. To understand how this pattern of acetylation may drive assembly, we determined a 3.9 Å cryo-EM structure of an amyloid fibril assembled from an acetylated tau fragment. The structure uncovers how lysine acetylation patterns mediate gain-of-function interactions to promote amyloid assembly. Comparison of the structure to an ex vivo tau fibril conformation from Pick's Disease reveals regions of high structural similarity. Finally, we show that our lysine- acetylated sequences exhibit fibril assembly activity in cell-based tau aggregation assays. Our data uncover the dual role of lysine residues in limiting aggregation while their acetylation leads to stabilizing pro-aggregation interactions. Design of tau sequence with specific acetylation patterns may lead to controllable tau aggregation to direct folding of tau into distinct folds.

Efficient Multiple Domain Ligation for Proteins Using Asparaginyl Endopeptidase by Selection of Appropriate Ligation Sites Based on Steric Hindrance

Three domain fragments of a multi-domain protein, ER-60, were ligated in two short linker regions using asparaginyl endopeptidase not involving denaturation. To identify appropriate ligation sites, by selecting several potential ligation sites with fewer mutations around two short linker regions, their ligation efficiencies and the functions of the ligated ER-60s were examined experimentally. To evaluate the dependence of ligation efficiencies on the ligation sites computationally, steric hinderances around the sites for the ligation were calculated through molecular dynamics simulations. Utilizing the steric hindrance, a site-dependent ligation potential index was introduced as reproducing the experimental ligation efficiency. Referring to this index, the reconstruction of ER-60 was succeeded by the ligation of the three domains for the first time. In addition, the new ligation potential index well-worked for application to other domain ligations. Therefore, the index may serve as a more time-effective tool for multi-site ligations.

Chemical tools for study and modulation of biomolecular phase transitions

Biomolecular phase transitions play an important role in organizing cellular processes in space and time. Methods and tools for studying these transitions, and the intrinsically disordered proteins (IDPs) that often drive them, are typically less developed than tools for studying their folded protein counterparts. In this perspective, we assess the current landscape of chemical tools for studying IDPs, with a specific focus on protein liquid-liquid phase separation (LLPS). We highlight methodologies that enable imaging and spectroscopic studies of these systems, including site-specific labeling with small molecules and the diverse range of capabilities offered by inteins and protein semisynthesis. We discuss strategies for introducing post-translational modifications that are central to IDP and LLPS function and regulation. We also investigate the nascent field of noncovalent small-molecule modulators of LLPS. We hope that this review of the state-of-the-art in chemical tools for interrogating IDPs and LLPS, along with an associated perspective on areas of unmet need, can serve as a valuable and timely resource for these rapidly expanding fields of study.

The emerging role of autophagy and mitophagy in tauopathies: From pathogenesis to translational implications in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease, affecting more than 55 million individuals worldwide in 2021. In addition to the "amyloid hypothesis," an increasing number of studies have demonstrated that phosphorylated tau plays an important role in AD pathogenesis. Both soluble tau oligomers and insoluble tau aggregates in the brain can induce structural and functional neuronal damage through multiple pathways, eventually leading to memory deficits and neurodegeneration. Autophagy is an important cellular response to various stress stimuli and can generally be categorized into non-selective and selective autophagy. Recent studies have indicated that both types of autophagy are involved in AD pathology. Among the several subtypes of selective autophagy, mitophagy, which mediates the selective removal of mitochondria, has attracted increasing attention because dysfunctional mitochondria have been suggested to contribute to tauopathies. In this review, we summarize the latest findings on the bidirectional association between abnormal tau proteins and defective autophagy, as well as mitophagy, which might constitute a vicious cycle in the induction of neurodegeneration. Neuroinflammation, another important feature in the pathogenesis and progression of AD, has been shown to crosstalk with autophagy and mitophagy. Additionally, we comprehensively discuss the relationship between neuroinflammation, autophagy, and mitophagy. By elucidating the underlying molecular mechanisms governing these pathologies, we highlight novel therapeutic strategies targeting autophagy, mitophagy and neuroinflammation, such as those using rapamycin, urolithin, spermidine, curcumin, nicotinamide, and actinonin, for the prevention and treatment of AD.

Ligation Embedding Aggregation Disruptor Strategy Enables the Chemical Synthesis of PD-1 Immunoglobulin and Extracellular Domains

Chemical synthesis of proteins with aggregable or colloidal peptide segments presents a formidable task, as such peptides prove to be difficult for both solid-phase peptide synthesis and peptide ligation. To address this issue, we have developed ligation embedding aggregation disruptor (LEAD) as an effective strategy for the chemical synthesis of difficult-to-obtain proteins. The N,O/S-benzylidene acetals generated from Ser/Thr ligation and Cys/Pen ligation are found to effectively disrupt peptide aggregation, and they can be carried for sequential ligations toward protein synthesis. The effectiveness and generality of this strategy have been demonstrated with total syntheses of programmed cell death protein 1 immunoglobulin like V-type domain and extracellular domain.

Posttranslational Chemical Mutagenesis Methods to Insert Posttranslational Modifications into Recombinant Proteins

Posttranslational modifications (PTMs) dramatically expand the functional diversity of the proteome. The precise addition and removal of PTMs appears to modulate protein structure and function and control key regulatory processes in living systems. Deciphering how particular PTMs affect protein activity is a current frontier in biology and medicine. The large number of PTMs which can appear in several distinct positions, states, and combinations makes preparing such complex analogs using conventional biological and chemical tools challenging. Strategies to access homogeneous and precisely modified proteins with desired PTMs at selected sites and in feasible quantities are critical to interpreting their molecular code. Here, we summarize recent advances in posttranslational chemical mutagenesis and late-stage functionalization chemistry to transfer novel PTM mimicry into recombinant proteins with emphasis on novel transformations.

Deciphering the Structure and Formation of Amyloids in Neurodegenerative Diseases With Chemical Biology Tools

Protein aggregation into highly ordered, regularly repeated cross-β sheet structures called amyloid fibrils is closely associated to human disorders such as neurodegenerative diseases including Alzheimer's and Parkinson's diseases, or systemic diseases like type II diabetes. Yet, in some cases, such as the HET-s prion, amyloids have biological functions. High-resolution structures of amyloids fibrils from cryo-electron microscopy have very recently highlighted their ultrastructural organization and polymorphisms. However, the molecular mechanisms and the role of co-factors (posttranslational modifications, non-proteinaceous components and other proteins) acting on the fibril formation are still poorly understood. Whether amyloid fibrils play a toxic or protective role in the pathogenesis of neurodegenerative diseases remains to be elucidated. Furthermore, such aberrant protein-protein interactions challenge the search of small-molecule drugs or immunotherapy approaches targeting amyloid formation. In this review, we describe how chemical biology tools contribute to new insights on the mode of action of amyloidogenic proteins and peptides, defining their structural signature and aggregation pathways by capturing their molecular details and conformational heterogeneity. Challenging the imagination of scientists, this constantly expanding field provides crucial tools to unravel mechanistic detail of amyloid formation such as semisynthetic proteins and small-molecule sensors of conformational changes and/or aggregation. Protein engineering methods and bioorthogonal chemistry for the introduction of protein chemical modifications are additional fruitful strategies to tackle the challenge of understanding amyloid formation.

The Role of Post-Translational Modifications on the Structure and Function of Tau Protein

Involving addition of chemical groups or protein units to specific residues of the target protein, post-translational modifications (PTMs) alter the charge, hydrophobicity, and conformation of a protein, which in tune influences protein function, protein - protein interaction, and protein aggregation. While the occurrence of PTMs is dynamic and subject to regulations, conformational disorder of the target protein facilitates PTMs. The microtubule-associated protein tau is a typical intrinsically disordered protein that undergoes a variety of PTMs including phosphorylation, acetylation, ubiquitination, methylation, and oxidation. Accumulated evidence shows that these PTMs play a critical role in regulating tau-microtubule interaction, tau localization, tau degradation and aggregation, and reinforces the correlation between tau PTMs and pathogenesis of neurodegenerative disease. Here, we review tau PTMs with an emphasis on their influence on tau structure. With available biophysical characterization results, we describe how PTMs induce conformational changes in tau monomer and regulate tau aggregation. Compared to functional analysis of tau PTMs, biophysical characterization of tau PTMs is lagging. While it is challenging, characterizing the specific effects of PTMs on tau conformation and interaction is indispensable to unravel the tau PTM code.

Unraveling the Influence of K280 Acetylation on the Conformational Features of Tau Core Fragment: A Molecular Dynamics Simulation Study

Abnormal aggregation of the microtubule-associated protein Tau is closely associated with tauopathies, including Alzheimer's disease and chronic traumatic encephalopathy. The hexapeptide 275VQIINK280 (PHF6*), a fibril-nucleating core motif of Tau, has been shown to play a vital role in the aggregation of Tau. Mounting experiment evidence demonstrated the acetylation of a single-lysine residue K280 in the PHF6* was a critical event for the formation of pathological Tau amyloid deposits. However, the underlying mechanisms by which K280 acetylation affects Tau aggregation at the atomic level remain elusive. In this work, we performed replica exchange molecular dynamics simulations to investigate the influence of acetylation of K280 on the aggregation of PHF6*. Our simulations show that acetylation of K280 not only enhances the self-assembly capability of PHF6* peptides but also increases the β-sheet structure propensity of the PHF6*. The inter-molecular interactions among PHF6* peptides are strengthened by the acetylation of K280, resulting in an increased ordered β-sheet-rich conformations of the PHF6* assemblies along with a decrease of the structural diversity. The residue-pairwise contact frequency analysis shows that K280 acetylation increases the interactions among the hydrophobic chemical groups from PHF6* peptides, which promotes the aggregation of PHF6*. This study offers mechanistic insights into the effects of acetylation on the aggregation of PHF6*, which will be helpful for an in-depth understanding of the relationship between acetylation and Tau aggregation at the molecular level.

Cryo-EM structures of τ filaments from human brain

Electron cryo-microscopy (cryo-EM) has made it possible to determine near-atomic structures of τ filaments from human brain. Previous work had shown that the cores of paired helical and straight filaments of Alzheimer's disease are made of two identical, but differently arranged C-shaped protofilaments. In recent years, cryo-EM has shown that the Alzheimer τ fold is 79 amino acids long. Five of the eight β-strands give rise to two antiparallel β-sheets, with the other three forming a β-helix. High-affinity binding sites of positron emission tomography ligand APN-1607 (PM-PBB3) are in the β-helix region. The Alzheimer fold contrasts with the 94 amino acid-long Pick fold, which is J-shaped and comprises nine β-strands that give rise to four antiparallel β-sheets, in the absence of a β-helix. Chronic traumatic encephalopathy τ fold is similar to the Alzheimer fold, but differs in the β-helix region, which is larger and contains a non-proteinaceous density that is probably hydrophobic. These folds are mostly two-layered. By contrast, the 107 amino acid τ fold of the 4R tauopathy corticobasal degeneration is four-layered and comprises 11 β-strands. It contains an internal, probably hydrophilic, density that is surrounded by τ. The τ folds described here share the presence of microtubule-binding repeats 3 and 4, as well as 10-13 amino acids after repeat 4.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

Chemical Synthesis of a Full-Length G-Protein-Coupled Receptor β2-Adrenergic Receptor with Defined Modification Patterns at the C-Terminus

The β₂-adrenergic receptor (β₂AR) is a G-protein-coupled receptor (GPCR) that responds to the hormone adrenaline and is an important drug target in the context of respiratory diseases, including asthma. β₂AR function can be regulated by post-translational modifications such as phosphorylation and ubiquitination at the C-terminus, but access to the full-length β₂AR with well-defined and homogeneous modification patterns critical for biochemical and biophysical studies remains challenging. Here, we report a practical synthesis of differentially modified, full-length β₂AR based on a combined native chemical ligation (NCL) and sortase ligation strategy. An array of homogeneous samples of full-length β₂ARs with distinct modification patterns, including a full-length β₂AR bearing both monoubiquitination and octaphosphorylation modifications, were successfully prepared for the first time. Using these homogeneously modified full-length β₂AR receptors, we found that different phosphorylation patterns mediate different interactions with β-arrestin1 as reflected in different agonist binding affinities. Our experiments also indicated that ubiquitination can further modulate interactions between β₂AR and β-arrestin1. Access to full-length β₂AR with well-defined and homogeneous modification patterns at the C-terminus opens a door to further in-depth mechanistic studies into the structure and dynamics of β₂AR complexes with downstream transducer proteins, including G proteins, arrestins, and GPCR kinases.

To target Tau pathologies, we must embrace and reconstruct their complexities

The accumulation of hyperphosphorylated fibrillar Tau aggregates in the brain is one of the defining hallmarks of Tauopathy diseases, including Alzheimer's disease. However, the primary events or molecules responsible for initiation of the pathological Tau aggregation and spreading remain unknown. The discovery of heparin as an effective inducer of Tau aggregation in vitro was instrumental to enabling different lines of research into the role of Tau aggregation in the pathogenesis of Tauopathies. However, recent proteomics and cryogenic electron microscopy (cryo-EM) studies have revealed that heparin-induced Tau fibrils generated in vitro do not reproduce the biochemical and ultrastructural properties of disease-associated brain-derived Tau fibrils. These observations demand that we reassess our current approaches for investigating the mechanisms underpinning Tau aggregation and pathology formation. Our review article presents an up-to-date survey and analyses of 1) the evolution of our understanding of the interactions between Tau and heparin, 2) the various structural and mechanistic models of the heparin-induced Tau aggregation, 3) the similarities and differences between brain-derived and heparin-induced Tau fibrils; and 4) emerging concepts on the biochemical and structural determinants underpinning Tau pathological heterogeneity in Tauopathies. Our analyses identify specific knowledge gaps and call for 1) embracing the complexities of Tau pathologies; 2) reassessment of current approaches to investigate, model and reproduce pathological Tau aggregation as it occurs in the brain; 3) more research towards a better understanding of the naturally-occurring cofactor molecules that are associated with Tau brain pathology initiation and propagation; and 4) developing improved approaches for in vitro production of the Tau aggregates and fibrils that recapitulate and/or amplify the biochemical and structural complexity and diversity of pathological Tau in Tauopathies. This will result in better and more relevant tools, assays, and mechanistic models, which could significantly improve translational research and the development of drugs and antibodies that have higher chances for success in the clinic.

Current directions in tau research: Highlights from Tau 2020

Studies supporting a strong association between tau deposition and neuronal loss, neurodegeneration, and cognitive decline have heightened the allure of tau and tau-related mechanisms as therapeutic targets. In February 2020, leading tau experts from around the world convened for the first-ever Tau2020 Global Conference in Washington, DC, co-organized and cosponsored by the Rainwater Charitable Foundation, the Alzheimer's Association, and CurePSP. Representing academia, industry, government, and the philanthropic sector, presenters and attendees discussed recent advances and current directions in tau research. The meeting provided a unique opportunity to move tau research forward by fostering global partnerships among academia, industry, and other stakeholders and by providing support for new drug discovery programs, groundbreaking research, and emerging tau researchers. The meeting also provided an opportunity for experts to present critical research-advancing tools and insights that are now rapidly accelerating the pace of tau research.

Post-translational modifications on RNA-binding proteins: accelerators, brakes, or passengers in neurodegeneration?

RNA-binding proteins (RBPs) are critical players in RNA expression and metabolism, thus, the proper regulation of this class of proteins is critical for cellular health. Regulation of RBPs often occurs through post-translational modifications (PTMs), which allow the cell to quickly and efficiently respond to cellular and environmental stimuli. PTMs have recently emerged as important regulators of RBPs implicated in neurodegenerative disorders, in particular amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we summarize how disease-associated PTMs influence the biophysical properties, molecular interactions, subcellular localization, and function of ALS/FTD-linked RBPs, such as FUS and TDP-43. We will discuss how PTMs are believed to play pathological, protective, or ambiguous roles in these neurodegenerative disorders.

Biophysical properties of a tau seed

Pathogenesis of tauopathies involves conversion of tau monomer into pathological tau conformers that serve as templates to recruit native tau into growing assemblies. Small soluble tau seeds have been proposed to drive pathological tau assembly in vitro, in cells and in vivo. We have previously described the isolation of monomeric pathogenic tau seeds derived from recombinant samples and tauopathy tissues but in-depth biophysical characterization of these species has not been done. Here we describe a chromatographic method to isolate recombinant soluble tau seeds derived from heparin treatment. We used biochemical and biophysical approaches to show that the seeds are predominantly monomeric and have the capacity to nucleate aggregation of inert forms of tau in vitro and in cells. Finally, we used crosslinking mass spectrometry to identify the topological changes in tau as it converts from an inert state to a pathogenic seed. Future studies will reveal the relationship between soluble seeds and structural polymorphs derived from tauopathies to help diagnose and develop therapeutics targeting specific tauopathies.

Consequences of post-translational modifications on amyloid proteins as revealed by protein semisynthesis

Alterations to the global levels of certain types of post-translational modifications (PTMs) are commonly observed in neurodegenerative diseases. The net influence of these PTM changes to the progression of these diseases can be deduced from cellular and animal studies. However, at the molecular level, how one PTM influences a given protein is not uniform and cannot be easily generalized from systemic observations, thus requiring protein-specific interrogations. Given that protein aggregation is a shared pathological hallmark in neurodegeneration, it is important to understand how these PTMs affect the behavior of amyloid-forming proteins. For this purpose, protein semisynthesis techniques, largely via native chemical and expressed protein ligation, have been widely used. These approaches have thus far led to our increased understanding of the site-specific consequences of certain PTMs to amyloidogenic proteins' endogenous function, their propensity for aggregation, and the structural variations these PTMs induce toward the aggregates formed.

Rethinking protein aggregation and drug discovery in neurodegenerative diseases: Why we need to embrace complexity?

More than a century has passed since pathological protein aggregates were first identified in the brains of patients with neurodegenerative diseases (NDDs). Yet, we still do not have effective therapies to treat or slow the progression of these devastating diseases or diagnostics for early detection and monitoring disease progression. Herein, I reflect on recent findings that are challenging traditional views about the composition, ultrastructural properties, and diversity of protein pathologies in the brain, their mechanisms of formation and how we investigate and model pathological aggregation processes in the laboratory today. This article is an invitation to embrace the complexity of proteinopathies as an essential step to understanding the molecular mechanisms underpinning NDDs and to advance translational research and drug discovery in NDDs.

Peptide Hydrazides as Thioester Equivalents for the Chemical Synthesis of Proteins

The chemical synthesis of proteins allows for the precise control of structural information at the atomic level, overcoming the limits of protein expression. Peptide hydrazides are widely used as thioester equivalents in the total chemical synthesis and semisynthesis of proteins as they can be easily prepared using standard solid phase peptide synthesis (SPPS) and recombinant peptide techniques. Via treatment with NaNO₂ and subsequent thiolysis, peptide hydrazides can be rapidly converted to peptide thioesters, which then selectively react with recombinant protein containing an N-terminal cysteine (Cys) to form a native peptide bond, thereby linking the two peptide segments without isolating any intermediates.

Ion channel engineering using protein trans-splicing

Conventional site-directed mutagenesis and genetic code expansion approaches have been instrumental in providing detailed functional and pharmacological insight into membrane proteins such as ion channels. Recently, this has increasingly been complemented by semi-synthetic strategies, in which part of the protein is generated synthetically. This means a vast range of chemical modifications, including non-canonical amino acids (ncAA), backbone modifications, chemical handles, fluorescent or spectroscopic labels and any combination of these can be incorporated. Among these approaches, protein trans-splicing (PTS) is particularly promising for protein reconstitution in live cells. It relies on one or more split inteins, which can spontaneously and covalently link flanking peptide or protein sequences. Here, we describe the use of PTS and its variant tandem PTS (tPTS) in semi-synthesis of ion channels in Xenopus laevis oocytes to incorporate ncAAs, post-translational modifications or metabolically stable mimics thereof. This strategy has the potential to expand the type and number of modifications in ion channel research.

Facile Installation of Post-translational Modifications on the Tau Protein via Chemical Mutagenesis

Post-translational modifications of proteins are ubiquitous in living organisms, as they enable an accurate control of the interactions of these macromolecules. For mechanistic studies, it would be highly advantageous to be able to produce in vitro post-translationally modified proteins with site-specificity. Here, we demonstrate one facile way to achieve this goal through the use of post-translational chemical mutagenesis. We illustrate this approach by performing site-specific phosphorylation and methylation of tau, a protein that stabilizes microtubules and whose aggregation is closely linked with Alzheimer's disease. We then verify the effects of the post-translational modifications on the ability of tau to control microtubule polymerization, revealing in particular an unexpected role for phosphorylation at S199, which is outside the microtubule-binding region of tau. These results show how the chemical mutagenesis approach that we present enables the systematic analysis of site-specific post-translational modifications of a key protein involved in the pathogenesis of Alzheimer's disease.

The Lys 280 → Gln mutation mimicking disease-linked acetylation of Lys 280 in tau extends the structural core of fibrils and modulates their catalytic properties

Amyloid fibrillar aggregates isolated from the brains of patients with neurodegenerative diseases invariably have post-translational modifications (PTMs). The roles that PTMs play in modulating the structures and polymorphism of amyloid aggregates, and hence their ability to catalyze the conversion of monomeric protein to their fibrillar structure is, however, poorly understood. This is particularly true in the case of tau aggregates, where specific folds of fibrillar tau have been implicated in specific tauopathies. Several PTMs, including acetylation at Lys 280, increase aggregation of tau in the brain, and increase neurodegeneration. In this study, tau-K18 K280Q, in which the Lys 280 → Gln mutation is used to mimic acetylation at Lys 280, is shown, using HX-MS measurements, to form fibrils with a structural core that is longer than that of tau-K18 fibrils. Measurements of critical concentrations show that the binding affinity of monomeric tau-K18 for its fibrillar counterpart is only marginally more than that of monomeric tau-K18 K280Q for its fibrillar counterpart. Quantitative analysis of the kinetics of seeded aggregation, using a simple Michaelis-Menten-like model, in which the monomer first binds and then undergoes conformational conversion to β-strand, shows that the fibrils of tau-K18 K280Q convert monomeric protein more slowly than do fibrils of tau-K18. In contrast, monomeric tau-K18 K280Q is converted faster to fibrils than is monomeric tau-K18. Thus, the effect of Lys 280 acetylation on tau aggregate propagation in brain cells is expected to depend on the amount of acetylated tau present, and on whether the propagating seed is acetylated at Lys 280 or not.

Tau Post-translational Modifications: Dynamic Transformers of Tau Function, Degradation, and Aggregation

Post-translational modifications (PTMs) on tau have long been recognized as affecting protein function and contributing to neurodegeneration. The explosion of information on potential and observed PTMs on tau provides an opportunity to better understand these modifications in the context of tau homeostasis, which becomes perturbed with aging and disease. Prevailing views regard tau as a protein that undergoes abnormal phosphorylation prior to its accumulation into the toxic aggregates implicated in Alzheimer's disease (AD) and other tauopathies. However, the phosphorylation of tau may, in fact, represent part of the normal but interrupted function and catabolism of the protein. In addition to phosphorylation, tau undergoes another forms of post-translational modification including (but not limited to), acetylation, ubiquitination, glycation, glycosylation, SUMOylation, methylation, oxidation, and nitration. A holistic appreciation of how these PTMs regulate tau during health and are potentially hijacked in disease remains elusive. Recent studies have reinforced the idea that PTMs play a critical role in tau localization, protein-protein interactions, maintenance of levels, and modifying aggregate structure. These studies also provide tantalizing clues into the possibility that neurons actively choose how tau is post-translationally modified, in potentially competitive and combinatorial ways, to achieve broad, cellular programs commensurate with the distinctive environmental conditions found during development, aging, stress, and disease. Here, we review tau PTMs and describe what is currently known about their functional impacts. In addition, we classify these PTMs from the perspectives of protein localization, electrostatics, and stability, which all contribute to normal tau function and homeostasis. Finally, we assess the potential impact of tau PTMs on tau solubility and aggregation. Tau occupies an undoubtedly important position in the biology of neurodegenerative diseases. This review aims to provide an integrated perspective of how post-translational modifications actively, purposefully, and dynamically remodel tau function, clearance, and aggregation. In doing so, we hope to enable a more comprehensive understanding of tau PTMs that will positively impact future studies.

Chemical Synthesis of PDZ Domains

Developments in chemical protein synthesis have enabled the generation of tailor-made proteins including incorporation of many types of modifications into proteins, enhancing our ability to control site-specificity of protein posttranslational modifications (PTMs), modify protein backbones and introduce photocrosslinking probes. For PDZ (postsynaptic density protein, disks large, zonula occludens) protein domains, expressed protein ligation (EPL) has been employed to introduce analogs of cognate amino acids, amide-to-ester bond mutations, and phosphorylations in the study of PDZ domain-mediated protein-protein interactions (PPIs). Here, we present protocols for EPL of PDZ domains focusing on phosphorylation and amide-to-ester modifications in the PDZ domain proteins.

Tau Protein and Frontotemporal Dementias

Filamentous inclusions of tau protein are found in cases of inherited and sporadic frontotemporal dementias (FTDs). Mutations in MAPT, the tau gene, cause approximately 5% of cases of FTD. They proved that dysfunction of tau protein is sufficient to cause neurodegeneration and dementia. Clinically and pathologically, cases with MAPT mutations can resemble sporadic diseases, such as Pick's disease, globular glial tauopathy, progressive supranuclear palsy and corticobasal degeneration. The structures of tau filaments from Pick's disease and corticobasal degeneration, determined by electron cryo-microscopy, revealed the presence of specific tau folds in each disease, with no inter-individual variation. The same was true of chronic traumatic encephalopathy.

PupStruct: Prediction of Pupylated Lysine Residues Using Structural Properties of Amino Acids

Post-translational modification (PTM) is a critical biological reaction which adds to the diversification of the proteome. With numerous known modifications being studied, pupylation has gained focus in the scientific community due to its significant role in regulating biological processes. The traditional experimental practice to detect pupylation sites proved to be expensive and requires a lot of time and resources. Thus, there have been many computational predictors developed to challenge this issue. However, performance is still limited. In this study, we propose another computational method, named PupStruct, which uses the structural information of amino acids with a radial basis kernel function Support Vector Machine (SVM) to predict pupylated lysine residues. We compared PupStruct with three state-of-the-art predictors from the literature where PupStruct has validated a significant improvement in performance over them with statistical metrics such as sensitivity (0.9234), specificity (0.9359), accuracy (0.9296), precision (0.9349), and Mathew’s correlation coefficient (0.8616) on a benchmark dataset.

Chemical Protein Synthesis in Medicinal Chemistry

Although the majority of proteins used for biomedical research are produced using living systems such as bacteria, biological means for producing proteins can be advantageously complemented by protein semisynthesis or total chemical synthesis. The latter approach is particularly useful when the proteins to be produced are toxic for the expression system or show unusual features that cannot be easily programmed in living organisms. The aim of this review is to provide a wide overview of the use of chemical protein synthesis in medicinal chemistry with a special focus on the production of post-translationally modified proteins and backbone cyclized proteins.

Prion-Like Propagation Mechanisms in Tauopathies and Traumatic Brain Injury: Challenges and Prospects

The accumulation of tau protein in the form of filamentous aggregates is a hallmark of many neurodegenerative diseases such as Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE). These dementias share traumatic brain injury (TBI) as a prominent risk factor. Tau aggregates can transfer between cells and tissues in a "prion-like" manner, where they initiate the templated misfolding of normal tau molecules. This enables the spread of tau pathology to distinct parts of the brain. The evidence that tauopathies spread via prion-like mechanisms is considerable, but work detailing the mechanisms of spread has mostly used in vitro platforms that cannot fully reveal the tissue-level vectors or etiology of progression. We review these issues and then briefly use TBI and CTE as a case study to illustrate aspects of tauopathy that warrant further attention in vivo. These include seizures and sleep/wake disturbances, emphasizing the urgent need for improved animal models. Dissecting these mechanisms of tauopathy progression continues to provide fresh inspiration for the design of diagnostic and therapeutic approaches.

Deciphering protein post-translational modifications using chemical biology tools

Proteins carry out a wide variety of catalytic, regulatory, signalling and structural functions in living systems. Following their assembly on ribosomes and throughout their lifetimes, most eukaryotic proteins are modified by post-translational modifications; small functional groups and complex biomolecules are conjugated to amino acid side chains or termini, and the protein backbone is cleaved, spliced or cyclized, to name just a few examples. These modifications modulate protein activity, structure, location and interactions, and, thereby, control many core biological processes. Aberrant post-translational modifications are markers of cellular stress or malfunction and are implicated in several diseases. Therefore, gaining an understanding of which proteins are modified, at which sites and the resulting biological consequences is an important but complex challenge requiring interdisciplinary approaches. One of the key challenges is accessing precisely modified proteins to assign functional consequences to specific modifications. Chemical biologists have developed a versatile set of tools for accessing specifically modified proteins by applying robust chemistries to biological molecules and developing strategies for synthesizing and ligating proteins. This Review provides an overview of these tools, with selected recent examples of how they have been applied to decipher the roles of a variety of protein post-translational modifications. Relative advantages and disadvantages of each of the techniques are discussed, highlighting examples where they are used in combination and have the potential to address new frontiers in understanding complex biological processes.

Tau proteinopathies and the prion concept

The ordered assembly of a small number of proteins into amyloid filaments is central to age-related neurodegenerative diseases. Tau is the most commonly affected of these proteins. In sporadic diseases, assemblies of tau form in a stochastic manner in certain brain regions, from where they appear to spread in a deterministic way, giving rise to disease symptoms. Over the past decade, multiple lines of evidence have shown that assembled tau behaves like a prion. More recently, electron cryo-microscopy of tau filaments has shown that distinct conformers are present in different diseases, with no inter-individual variation for a given disease.

The Chemical Biology of Reversible Lysine Post-translational Modifications

Lysine (Lys) residues in proteins undergo a wide range of reversible post-translational modifications (PTMs), which can regulate enzyme activities, chromatin structure, protein-protein interactions, protein stability, and cellular localization. Here we discuss the "writers," "erasers," and "readers" of some of the common protein Lys PTMs and summarize examples of their major biological impacts. We also review chemical biology approaches, from small-molecule probes to protein chemistry technologies, that have helped to delineate Lys PTM functions and show promise for a diverse set of biomedical applications.

Phosphorylation of the overlooked tyrosine 310 regulates the structure, aggregation, and microtubule- and lipid-binding properties of Tau

The microtubule-associated protein Tau is implicated in the pathogenesis of several neurodegenerative disorders, including Alzheimer's disease. Increasing evidence suggests that post-translational modifications play critical roles in regulating Tau's normal functions and its pathogenic properties in tauopathies. Very little is known about how phosphorylation of tyrosine residues influences the structure, aggregation, and microtubule- and lipid-binding properties of Tau. Here, we sought to determine the relative contributions of phosphorylation of one or several of the five tyrosine residues in Tau (Tyr-18, -29, -197, -310, and -394) to the regulation of its biophysical, aggregation, and functional properties. We used a combination of site-specific mutagenesis and in vitro phosphorylation by c-Abl kinase to generate Tau species phosphorylated at all five tyrosine residues, all tyrosine residues except Tyr-310 or Tyr-394 (pTau-Y310F and pTau-Y394F, respectively) and Tau phosphorylated only at Tyr-310 or Tyr-394 (4F/pTyr-310 or 4F/pTyr-394). We observed that phosphorylation of all five tyrosine residues, multiple N-terminal tyrosine residues (Tyr-18, -29, and -197), or specific phosphorylation only at residue Tyr-310 abolishes Tau aggregation and inhibits its microtubule- and lipid-binding properties. NMR experiments indicated that these effects are mediated by a local decrease in β-sheet propensity of Tau's PHF6 domain. Our findings underscore Tyr-310 phosphorylation has a unique role in the regulation of Tau aggregation, microtubule, and lipid interactions. These results also highlight the importance of conducting further studies to elucidate the role of Tyr-310 in the regulation of Tau's normal functions and pathogenic properties.