The kinase MST4 limits inflammatory responses through direct phosphorylation of the adaptor TRAF6.

Immune responses need to be tightly controlled to avoid excessive inflammation and prevent unwanted host damage. Here we report that germinal center kinase MST4 responded dynamically to bacterial infection and acted as a negative regulator of inflammation. We found that MST4 directly interacted with and phosphorylated the adaptor TRAF6 to prevent its oligomerization and autoubiquitination. Accordingly, MST4 did not inhibit lipopolysaccharide-induced cytokine production in Traf6(-/-) embryonic fibroblasts transfected to express a mutant form of TRAF6 that cannot be phosphorylated at positions 463 and 486 (with substitution of alanine for threonine at those positions). Upon developing septic shock, mice in which MST4 was knocked down showed exacerbated inflammation and reduced survival, whereas heterozygous deletion of Traf6 (Traf6(+/-)) alleviated such deleterious effects. Our findings reveal a mechanism by which TRAF6 is regulated and highlight a role for MST4 in limiting inflammatory responses.

The TOR-EIN2 axis mediates nuclear signalling to modulate plant growth.

The evolutionarily conserved target of rapamycin (TOR) kinase acts as a master regulator that coordinates cell proliferation and growth by integrating nutrient, energy, hormone and stress signals in all eukaryotes1,2. Research has focused mainly on TOR-regulated translation, but how TOR orchestrates the global transcriptional network remains unclear. Here we identify ethylene-insensitive protein 2 (EIN2), a central integrator3-5 that shuttles between the cytoplasm and the nucleus, as a direct substrate of TOR in Arabidopsis thaliana. Glucose-activated TOR kinase directly phosphorylates EIN2 to prevent its nuclear localization. Notably, the rapid global transcriptional reprogramming that is directed by glucose-TOR signalling is largely compromised in the ein2-5 mutant, and EIN2 negatively regulates the expression of a wide range of target genes of glucose-activated TOR that are involved in DNA replication, cell wall and lipid synthesis and various secondary metabolic pathways. Chemical, cellular and genetic analyses reveal that cell elongation and proliferation processes that are controlled by the glucose-TOR-EIN2 axis are decoupled from canonical ethylene-CTR1-EIN2 signalling, and mediated by different phosphorylation sites. Our findings reveal a molecular mechanism by which a central signalling hub is shared but differentially modulated by diverse signalling pathways using distinct phosphorylation codes that can be specified by upstream protein kinases.

Structural Snapshots of 26S Proteasome Reveal Tetraubiquitin-Induced Conformations.

The 26S proteasome is the ATP-dependent protease responsible for regulating the proteome of eukaryotic cells through degradation of mainly ubiquitin-tagged substrates. In order to understand how proteasome responds to ubiquitin signal, we resolved an ensemble of cryo-EM structures of proteasome in the presence of K48-Ub4, with three of them resolved at near-atomic resolution. We identified a conformation with stabilized ubiquitin receptors and a previously unreported orientation of the lid, assigned as a Ub-accepted state C1-b. We determined another structure C3-b with localized K48-Ub4 to the toroid region of Rpn1, assigned as a substrate-processing state. Our structures indicate that tetraUb induced conformational changes in proteasome could initiate substrate degradation. We also propose a CP gate-opening mechanism involving the propagation of the motion of the lid to the gate through the Rpn6-α2 interaction. Our results enabled us to put forward a model of a functional cycle for proteasomes induced by tetraUb and nucleotide.

NRDE2 negatively regulates exosome functions by inhibiting MTR4 recruitment and exosome interaction.

The exosome functions in the degradation of diverse RNA species, yet how it is negatively regulated remains largely unknown. Here, we show that NRDE2 forms a 1:1 complex with MTR4, a nuclear exosome cofactor critical for exosome recruitment, via a conserved MTR4-interacting domain (MID). Unexpectedly, NRDE2 mainly localizes in nuclear speckles, where it inhibits MTR4 recruitment and RNA degradation, and thereby ensures efficient mRNA nuclear export. Structural and biochemical data revealed that NRDE2 interacts with MTR4's key residues, locks MTR4 in a closed conformation, and inhibits MTR4 interaction with the exosome as well as proteins important for MTR4 recruitment, such as the cap-binding complex (CBC) and ZFC3H1. Functionally, MID deletion results in the loss of self-renewal of mouse embryonic stem cells. Together, our data pinpoint NRDE2 as a nuclear exosome negative regulator that ensures mRNA stability and nuclear export.

Structural basis for catalysis and substrate specificity of human ACAT1.

As members of the membrane-bound O-acyltransferase (MBOAT) enzyme family, acyl-coenzyme A:cholesterol acyltransferases (ACATs) catalyse the transfer of an acyl group from acyl-coenzyme A to cholesterol to generate cholesteryl ester, the primary form in which cholesterol is stored in cells and transported in plasma1. ACATs have gained attention as potential drug targets for the treatment of diseases such as atherosclerosis, Alzheimer's disease and cancer2-7. Here we present the cryo-electron microscopy structure of human ACAT1 as a dimer of dimers. Each protomer consists of nine transmembrane segments, which enclose a cytosolic tunnel and a transmembrane tunnel that converge at the predicted catalytic site. Evidence from structure-guided mutational analyses suggests that acyl-coenzyme A enters the active site through the cytosolic tunnel, whereas cholesterol may enter from the side through the transmembrane tunnel. This structural and biochemical characterization helps to rationalize the preference of ACAT1 for unsaturated acyl chains, and provides insight into the catalytic mechanism of enzymes within the MBOAT family8.

mTORC1 Phosphorylates Acetyltransferase p300 to Regulate Autophagy and Lipogenesis.

Acetylation is increasingly recognized as one of the major post-translational mechanisms for the regulation of multiple cellular functions in mammalian cells. Acetyltransferase p300, which acetylates histone and non-histone proteins, has been intensively studied in its role in cell growth and metabolism. However, the mechanism underlying the activation of p300 in cells remains largely unknown. Here, we identify the homeostatic sensor mTORC1 as a direct activator of p300. Activated mTORC1 interacts with p300 and phosphorylates p300 at 4 serine residues in the C-terminal domain. Mechanistically, phosphorylation of p300 by mTORC1 prevents the catalytic HAT domain from binding to the RING domain, thereby eliminating intra-molecular inhibition. Functionally, mTORC1-dependent phosphorylation of p300 suppresses cell-starvation-induced autophagy and activates cell lipogenesis. These results uncover p300 as a direct target of mTORC1 and suggest that the mTORC1-p300 pathway plays a pivotal role in cell metabolism by coordinately controlling cell anabolism and catabolism.

VPS34 Acetylation Controls Its Lipid Kinase Activity and the Initiation of Canonical and Non-canonical Autophagy.

The class III phosphoinositide 3-kinase VPS34 plays a key role in the regulation of vesicular trafficking and macroautophagy. So far, we know little about the molecular mechanism of VPS34 activation besides its interaction with regulatory proteins to form complexes. Here, we report that VPS34 is specifically acetylated by the acetyltransferase p300, and p300-mediated acetylation represses VPS34 activity. Acetylation at K771 directly diminishes the affinity of VPS34 for its substrate PI, while acetylation at K29 hinders the VPS34-Beclin 1 core complex formation. Inactivation of p300 induces VPS34 deacetylation, PI3P production, and autophagy, even in AMPK-/-, TSC2-/-, or ULK1-/- cells. In fasting mice, liver autophagy correlates well with p300 inactivation/VPS34 deacetylation, which facilitates the clearance of lipid droplets in hepatocytes. Thus, p300-dependent VPS34 acetylation/deacetylation is the physiological key to VPS34 activation, which controls the initiation of canonical autophagy and of non-canonical autophagy in which the upstream kinases of VPS34 can be bypassed.

Cholesterol Modification of Smoothened Is Required for Hedgehog Signaling.

Hedgehog (Hh) has been known as the only cholesterol-modified morphogen playing pivotal roles in development and tumorigenesis. A major unsolved question is how Hh signaling regulates the activity of Smoothened (SMO). Here, we performed an unbiased biochemical screen and identified that SMO was covalently modified by cholesterol on the Asp95 (D95) residue through an ester bond. This modification was inhibited by Patched-1 (Ptch1) but enhanced by Hh. The SMO(D95N) mutation, which could not be cholesterol modified, was refractory to Hh-stimulated ciliary localization and failed to activate downstream signaling. Furthermore, homozygous SmoD99N/D99N (the equivalent residue in mouse) knockin mice were embryonic lethal with severe cardiac defects, phenocopying the Smo-/- mice. Together, the results of our study suggest that Hh signaling transduces to SMO through modulating its cholesterylation and provides a therapeutic opportunity to treat Hh-pathway-related cancers by targeting SMO cholesterylation.

Assessing base-resolution DNA mechanics on the genome scale.

Intrinsic DNA properties including bending play a crucial role in diverse biological systems. A recent advance in a high-throughput technology called loop-seq makes it possible to determine the bendability of hundred thousand 50-bp DNA duplexes in one experiment. However, it's still challenging to assess base-resolution sequence bendability in large genomes such as human, which requires thousands of such experiments. Here, we introduce 'BendNet'-a deep neural network to predict the intrinsic DNA bending at base-resolution by using loop-seq results in yeast as training data. BendNet can predict the DNA bendability of any given sequence from different species with high accuracy. To explore the utility of BendNet, we applied it to the human genome and observed DNA bendability is associated with chromatin features and disease risk regions involving transcription/enhancer regulation, DNA replication, transcription factor binding and extrachromosomal circular DNA generation. These findings expand our understanding on DNA mechanics and its association with transcription regulation in mammals. Lastly, we built a comprehensive resource of genomic DNA bendability profiles for 307 species by applying BendNet, and provided an online tool to assess the bendability of user-specified DNA sequences (http://www.dnabendnet.com/).

Protein Tyrosine Amination: Detection, Imaging, and Chemoproteomic Profiling with Synthetic Probes.

Protein tyrosine nitration (PTN) by oxidative and nitrative stress is a well-known post-translational modification that plays a role in the initiation and progression of various diseases. Despite being recognized as a stable modification for decades, recent studies have suggested the existence of a reduction in PTN, leading to the formation of 3-aminotyrosine (3AT) and potential denitration processes. However, the vital functions of 3AT-containing proteins are still unclear due to the lack of selective probes that directly target the protein tyrosine amination. Here, we report a novel approach to label and enrich 3AT-containing proteins with synthetic salicylaldehyde (SAL)-based probes: SALc-FL with a fluorophore and SALc-Yn with an alkyne tag. These probes exhibit high selectivity and efficiency in labeling and can be used in cell lysates and live cells. More importantly, SALc-Yn offers versatility when integrated into multiple platforms by enabling proteome-wide quantitative profiling of cell nitration dynamics. Using SALc-Yn, 355 proteins were labeled, enriched, and identified to carry the 3AT modification in oxidatively stressed RAW264.7 cells. These findings provide compelling evidence supporting the involvement of 3AT as a critical intermediate in nitrated protein turnover. Moreover, our probes serve as powerful tools to investigate protein nitration and denitration processes, and the identification of 3AT-containing proteins contributes to our understanding of PTN dynamics and its implications in cellular redox biology.

Unconventional tonicity-regulated nuclear trafficking of NFAT5 mediated by KPNB1, XPOT and RUVBL2.

NFAT5 is the only known mammalian tonicity-responsive transcription factor with an essential role in cellular adaptation to hypertonic stress. It is also implicated in diverse physiological and pathological processes. NFAT5 activity is tightly regulated by extracellular tonicity, but the underlying mechanisms remain elusive. Here, we demonstrate that NFAT5 enters the nucleus via the nuclear pore complex. We found that NFAT5 utilizes a unique nuclear localization signal (NFAT5-NLS) for nuclear import. siRNA screening revealed that only karyopherin ß1 (KPNB1), but not karyopherin α, is responsible for the nuclear import of NFAT5 via direct interaction with the NFAT5-NLS. Proteomics analysis and siRNA screening further revealed that nuclear export of NFAT5 under hypotonicity is driven by exportin-T (XPOT), where the process requires RuvB-like AAA-type ATPase 2 (RUVBL2) as an indispensable chaperone. Our findings have identified an unconventional tonicity-dependent nucleocytoplasmic trafficking pathway for NFAT5 that represents a critical step in orchestrating rapid cellular adaptation to change in extracellular tonicity. These findings offer an opportunity for the development of novel NFAT5 targeting strategies that are potentially useful for the treatment of diseases associated with NFAT5 dysregulation.

Novel Proteoform Discovery by Precise Semi-De Novo Sequencing of Novel Junction Peptides.

Alternative splicing allows a small number of human genes to encode large amounts of proteoforms that play essential roles in normal and disease physiology. Some low-abundance proteoforms may remain undiscovered due to limited detection and analysis capabilities. Peptides coencoded by novel exons and annotated exons separated by introns are called novel junction peptides, which are the key to identifying novel proteoforms. Traditional de novo sequencing does not take into account the specificity in the composition of the novel junction peptide and is therefore not as accurate. We first developed a novel de novo sequencing algorithm, CNovo, which outperformed the mainstream PEAKS and Novor in all six test sets. We then built on CNovo to develop a semi-de novo sequencing algorithm, SpliceNovo, specifically for identifying novel junction peptides. SpliceNovo identifies junction peptides with much higher accuracy than CNovo, CJunction, PEAKS, and Novor. Of course, it is also possible to replace the built-in CNovo in SpliceNovo with other more accurate de novo sequencing algorithms to further improve its performance. We also successfully identified and validated two novel proteoforms of the human EIF4G1 and ELAVL1 genes by SpliceNovo. Our results significantly improve the ability to discover novel proteoforms through de novo sequencing.

AMPK-Dependent Phosphorylation of GAPDH Triggers Sirt1 Activation and Is Necessary for Autophagy upon Glucose Starvation.

Eukaryotes initiate autophagy to cope with the lack of external nutrients, which requires the activation of the nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase Sirtuin 1 (Sirt1). However, the mechanisms underlying the starvation-induced Sirt1 activation for autophagy initiation remain unclear. Here, we demonstrate that glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a conventional glycolytic enzyme, is a critical mediator of AMP-activated protein kinase (AMPK)-driven Sirt1 activation. Under glucose starvation, but not amino acid starvation, cytoplasmic GAPDH is phosphorylated on Ser122 by activated AMPK. This causes GAPDH to redistribute into the nucleus. Inside the nucleus, GAPDH interacts directly with Sirt1, displacing Sirt1's repressor and causing Sirt1 to become activated. Preventing this shift of GAPDH abolishes Sirt1 activation and autophagy, while enhancing it, through overexpression of nuclear-localized GAPDH, increases Sirt1 activation and autophagy. GAPDH is thus a pivotal and central regulator of autophagy under glucose deficiency, undergoing AMPK-dependent phosphorylation and nuclear translocation to activate Sirt1 deacetylase activity.

Bax inhibitor 1 is a γ-secretase-independent presenilin-binding protein.

Presenilin is the catalytic subunit of γ-secretase, a four-component intramembrane protease responsible for the generation of ß-amyloid (Aß) peptides. Over 200 Alzheimer's disease-related mutations have been identified in presenilin 1 (PS1) and PS2. Here, we report that Bax-inhibitor 1 (BI1), an evolutionarily conserved transmembrane protein, stably associates with PS1. BI1 specifically interacts with PS1 in isolation, but not with PS1 in the context of an assembled γ-secretase. The PS1-BI1 complex exhibits no apparent proteolytic activity, as judged by the inability to produce Aß40 and Aß42 from the substrate APP-C99. At an equimolar concentration, BI1 has no impact on the proteolytic activity of γ-secretase; at a 200-fold molar excess, BI1 reduces γ-secretase activity nearly by half. Our biochemical study identified BI1 as a PS1-interacting protein, suggesting additional functions of PS1 beyond its involvement in γ-secretase.

Proteogenomics Integrating Novel Junction Peptide Identification Strategy Discovers Three Novel Protein Isoforms of Human NHSL1 and EEF1B2.

In eukaryotes, alternative pre-mRNA splicing allows a single gene to encode different protein isoforms that function in many biological processes, and they are used as biomarkers or therapeutic targets for diseases. Although protein isoforms in the human genome are well annotated, we speculate that some low-abundance protein isoforms may still be under-annotated because most genes have a primary coding product and alternative protein isoforms tend to be under-expressed. A peptide coencoded by a novel exon and an annotated exon separated by an intron is known as a novel junction peptide. In the absence of known transcripts and homologous proteins, traditional whole-genome six-frame translation-based proteogenomics cannot identify novel junction peptides, and it cannot capture novel alternative splice sites. In this article, we first propose a strategy and tool for identifying novel junction peptides, called CJunction, which we then integrate into a proteogenomics process specifically designed for novel protein isoform discovery and apply to the analysis of a deep-coverage HeLa mass spectrometry data set with identifier PXD004452 in ProteomeXchange. We succeeded in identifying and validating three novel protein isoforms of two functionally important genes, NHSL1 (causative gene of Nance-Horan syndrome) and EEF1B2 (translation elongation factor), which validate our hypothesis. These novel protein isoforms have significant sequence differences from the annotated gene-coding products introduced by the novel N-terminal, suggesting that they may play importantly different functions.

Proteomic Analysis Identifies Prolonged Disturbances in Pathways Related to Cholesterol Metabolism and Myocardium Function in the COVID-19 Recovery Stage.

The COVID-19 pandemic has become a worldwide health crisis. So far, most studies have focused on the epidemiology and pathogenesis of this infectious disease. Little attention has been given to the disease sequelae in patients recovering from COVID-19, and nothing is known about the mechanisms underlying these sequelae. Herein, we profiled the serum proteome of a cohort of COVID-19 patients in the disease onset and recovery stages. Based on the close integration of our proteomic analysis with clinical data, we propose that COVID-19 is associated with prolonged disorders in cholesterol metabolism and myocardium, even in the recovery stage. We identify potential biomarkers for these disorders. Moreover, severely affected patients presented more serious disturbances in these pathways. Our findings potentially support clinical decision-making to improve the prognosis and treatment of patients.

c-Jun NH2 -Terminal Protein Kinase Phosphorylates the Nrf2-ECH Homology 6 Domain of Nuclear Factor Erythroid 2-Related Factor 2 and Downregulates Cytoprotective Genes in Acetaminophen-Induced Liver Injury in Mice.

BACKGROUND AND AIMS: Acetaminophen (APAP) overdose induces severe liver injury and hepatic failure. While the activation of c-Jun NH2 -terminal kinase (JNK) has been implicated as a mechanism in APAP-induced liver injury, the hepatic defense system controlled by nuclear factor erythroid 2-related factor 2 (Nrf2) plays a central role in the mitigation of APAP toxicity. However, the link between the two signaling pathways in APAP-induced liver injury (AILI) remains unclear. APPROACH AND RESULTS: In this study, we demonstrated that the activation of JNK in mouse liver following exposure to APAP was correlated with the phosphorylation of Nrf2 and down-regulation of the antioxidant response element (ARE)-driven genes, NAD(P)H:quinone dehydrogenase 1, glutathione S-transferase α3, glutathione S-transferase M1, glutathione S-transferase M5, and aldo-keto reductase 1C. The JNK inhibitor, SP600125, or knockdown of JNK by infection of adenovirus expressing JNK small interfering RNA, ameliorated the APAP induced liver toxicity, and inhibited the phosphorylation of Nrf2 and down-regulation of detoxifying enzymes by stabilizing the transcription factor. Mechanistically, JNK antagonized Nrf2- and ARE-driven gene expression in a Kelch-like ECH-associated protein 1-independent manner. Biochemical analysis revealed that phosphorylated JNK (P-JNK) directly interacted with the Nrf2-ECH homology (Neh) 1 domain of Nrf2 and phosphorylated the serine-aspartate-serine motif 1 (SDS1) region in the Neh6 domain of Nrf2. CONCLUSIONS: Mass spectrometric analysis identified serine 335 in the SDS1 region of mNrf2 as the major phosphorylation site for modulation of Nrf2 ubiquitylation by P-JNK. This study demonstrates that Nrf2 is a target of P-JNK in AILI. Our finding may provide a strategy for the treatment of AILI.

A non-canonical role of the p97 complex in RIG-I antiviral signaling.

RIG-I is a well-studied sensor of viral RNA that plays a key role in innate immunity. p97 regulates a variety of cellular events such as protein quality control, membrane reassembly, DNA repair, and the cell cycle. Here, we report a new role for p97 with Npl4-Ufd1 as its cofactor in reducing antiviral innate immune responses by facilitating proteasomal degradation of RIG-I. The p97 complex is able to directly bind both non-ubiquitinated RIG-I and the E3 ligase RNF125, promoting K48-linked ubiquitination of RIG-I at residue K181. Viral infection significantly strengthens the interaction between RIG-I and the p97 complex by a conformational change of RIG-I that exposes the CARDs and through K63-linked ubiquitination of these CARDs. Disruption of the p97 complex enhances RIG-I antiviral signaling. Consistently, administration of compounds targeting p97 ATPase activity was shown to inhibit viral replication and protect mice from vesicular stomatitis virus (VSV) infection. Overall, our study uncovered a previously unrecognized role for the p97 complex in protein ubiquitination and revealed the p97 complex as a potential drug target in antiviral therapy.

Nanoliter-Scale Oil-Air-Droplet Chip-Based Single Cell Proteomic Analysis.

Single cell proteomic analysis provides crucial information on cellular heterogeneity in biological systems. Herein, we describe a nanoliter-scale oil-air-droplet (OAD) chip for achieving multistep complex sample pretreatment and injection for single cell proteomic analysis in the shotgun mode. By using miniaturized stationary droplet microreaction and manipulation techniques, our system allows all sample pretreatment and injection procedures to be performed in a nanoliter-scale droplet with minimum sample loss and a high sample injection efficiency (>99%), thus substantially increasing the analytical sensitivity for single cell samples. We applied the present system in the proteomic analysis of 100 ± 10, 50 ± 5, 10, and 1 HeLa cell(s), and protein IDs of 1360, 612, 192, and 51 were identified, respectively. The OAD chip-based system was further applied in single mouse oocyte analysis, with 355 protein IDs identified at the single oocyte level, which demonstrated its special advantages of high enrichment of sequence coverage, hydrophobic proteins, and enzymatic digestion efficiency over the traditional in-tube system.

HTLV-1 Tax Functions as a Ubiquitin E3 Ligase for Direct IKK Activation via Synthesis of Mixed-Linkage Polyubiquitin Chains.

The HTLV-1 oncoprotein Tax plays a key role in CD4+ T cell transformation by promoting cell proliferation and survival, mainly through permanent activation of the NK-κB pathway and induction of many NF-κB target genes. Elucidating the underlying molecular mechanism is therefore critical in understanding HTLV-1-mediated transformation. Current studies have suggested multiple but controversial mechanisms regarding Tax-induced IKK activation mainly due to blending of primary Tax-induced IKK activation events and secondary IKK activation events induced by cytokines secreted by the primary Tax-induced IKK-NF-κB activation events. We reconstituted Tax-stimulated IKK activation in a cell-free system to dissect the essential cellular components for primary IKK activation by Tax and studied the underlying biochemical mechanism. We found that Tax is a putative E3 ubiquitin ligase, which, together with UbcH2, UhcH5c, or UbcH7, catalyzes the assembly of free mixed-linkage polyubiquitin chains. These free mixed-linkage polyubiquitin chains are then responsible for direct IKK activation by binding to the NEMO subunit of IKK. Our studies revealed the biochemical function of Tax in the process of IKK activation, which utilizes the minimal cellular ubiquitination components for NF-κB activation.