The structure of TAX1BP1 UBZ1+2 provides insight into target specificity and adaptability

Mechanisms of mitochondrial reorganization

The cytoplasm of eukaryotes is dynamically zoned by membrane-bound and membraneless organelles. Cytoplasmic zoning allows various biochemical reactions to take place at the right time and place. Mitochondrion is a membrane-bound organelle that provides a zone for intracellular energy production and metabolism of lipids and iron. A key feature of mitochondria is their high dynamics: mitochondria constantly undergo fusion and fission, and excess or damaged mitochondria are selectively eliminated by mitophagy. Therefore, mitochondria are appropriate model systems to understand dynamic cytoplasmic zoning by membrane organelles. In this review, we summarize the molecular mechanisms of mitochondrial fusion and fission as well as mitophagy unveiled through studies using yeast and mammalian models.

Structural view on autophagosome formation

Autophagy is a conserved intracellular degradation system in eukaryotes, involving the sequestration of degradation targets into autophagosomes, which are subsequently delivered to lysosomes (or vacuoles in yeasts and plants) for degradation. In budding yeast, starvation-induced autophagosome formation relies on approximately 20 core Atg proteins, grouped into six functional categories: the Atg1/ULK complex, the phosphatidylinositol-3 kinase complex, the Atg9 transmembrane protein, the Atg2-Atg18/WIPI complex, the Atg8 lipidation system, and the Atg12-Atg5 conjugation system. Additionally, selective autophagy requires cargo receptors and other factors, including a fission factor, for specific sequestration. This review covers the 30-year history of structural studies on core Atg proteins and factors involved in selective autophagy, examining X-ray crystallography, NMR, and cryo-EM techniques. The molecular mechanisms of autophagy are explored based on protein structures, and future directions in the structural biology of autophagy are discussed, considering the advancements in the era of AlphaFold.

Decreased TAX1BP1 participates in systemic lupus erythematosus by regulating monocyte/macrophage function

Systemic lupus erythematosus (SLE) involves disorders of innate and adaptive immune pathways. Tax1-binding protein 1 (TAX1BP1) modulates the production of antibodies in B cells and the T-cell cycle by regulating the NF-κB signaling pathway. However, the potential association of TAX1BP1 with SLE and its role in monocytes/macrophages have not been fully elucidated. In this study, we utilized whole-exome sequencing (WES) in combination with Sanger sequencing and identified 16 gene mutations, including in TAX1BP1, in an SLE family. TAX1BP1 protein expression with western blotting detection was reduced in SLE patients and correlated with disease activity negatively. Furthermore, RNA sequencing and 4D Label-Free Phosphoproteomic analysis were employed to characterize the transcriptome and phosphoproteome profiles in THP-1 and THP-1-differentiated M1 macrophages with TAX1BP1 knockdown. Silencing of TAX1BP1 in THP-1 and THP-1-differentiated M1 macrophages led to an increase in cluster of differentiation 80 (CD80) expression and differential changes in CD14 and CD16 expression, as assessed by flow cytometry. Additionally, western blot analysis showed that knockdown of TAX1BP1 led to a reduction in TRAF6 and p-p65 in THP-1-differentiated macrophages, with or without lipopolysaccharide (LPS) or tumor necrosis factor (TNF)-α stimulation. Taken together, our findings suggest that TAX1BP1 participates in SLE activity by regulating antigen presentation in monocytes and inflammatory responses in M1 macrophages.

The exploitation of host autophagy and ubiquitin machinery by Mycobacterium tuberculosis in shaping immune responses and host defense during infection

Intracellular pathogens have evolved various efficient molecular armaments to subvert innate defenses. Cellular ubiquitination, a normal physiological process to maintain homeostasis, is emerging one such exploited mechanism. Ubiquitin (Ub), a small protein modifier, is conjugated to diverse protein substrates to regulate many functions. Structurally diverse linkages of poly-Ub to target proteins allow enormous functional diversity with specificity being governed by evolutionarily conserved enzymes (E3-Ub ligases). The Ub-binding domain (UBD) and LC3-interacting region (LIR) are critical features of macroautophagy/autophagy receptors that recognize Ub-conjugated on protein substrates. Emerging evidence suggests that E3-Ub ligases unexpectedly protect against intracellular pathogens by tagging poly-Ub on their surfaces and targeting them to phagophores. Two E3-Ub ligases, PRKN and SMURF1, provide immunity against Mycobacterium tuberculosis (M. tb). Both enzymes conjugate K63 and K48-linked poly-Ub to M. tb for successful delivery to phagophores. Intriguingly, M. tb exploits virulence factors to effectively dampen host-directed autophagy utilizing diverse mechanisms. Autophagy receptors contain LIR-motifs that interact with conserved Atg8-family proteins to modulate phagophore biogenesis and fusion to the lysosome. Intracellular pathogens have evolved a vast repertoire of virulence effectors to subdue host-immunity via hijacking the host ubiquitination process. This review highlights the xenophagy-mediated clearance of M. tb involving host E3-Ub ligases and counter-strategy of autophagy inhibition by M. tb using virulence factors. The role of Ub-binding receptors and their mode of autophagy regulation is also explained. We also discuss the co-opting and utilization of the host Ub system by M. tb for its survival and virulence.Abbreviations: APC: anaphase promoting complex/cyclosome; ATG5: autophagy related 5; BCG: bacille Calmette-Guerin; C2: Ca²⁺-binding motif; CALCOCO2: calcium binding and coiled-coil domain 2; CUE: coupling of ubiquitin conjugation to ER degradation domains; DUB: deubiquitinating enzyme; GABARAP: GABA type A receptor-associated protein; HECT: homologous to the E6-AP carboxyl terminus; IBR: in-between-ring fingers; IFN: interferon; IL1B: interleukin 1 beta; KEAP1: kelch like ECH associated protein 1; LAMP1: lysosomal associated membrane protein 1; LGALS: galectin; LIR: LC3-interacting region; MAPK11/p38: mitogen-activated protein kinase 11; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAP3K7/TAK1: mitogen-activated protein kinase kinase kinase 7; MAPK8/JNK: mitogen-activated protein kinase 8; MHC-II: major histocompatibility complex-II; MTOR: mechanistic target of rapamycin kinase; NBR1: NBR1 autophagy cargo receptor; NFKB1/p50: nuclear factor kappa B subunit 1; OPTN: optineurin; PB1: phox and bem 1; PE/PPE: proline-glutamic acid/proline-proline-glutamic acid; PknG: serine/threonine-protein kinase PknG; PRKN: parkin RBR E3 ubiquitin protein ligase; RBR: RING-in between RING; RING: really interesting new gene; RNF166: RING finger protein 166; ROS: reactive oxygen species; SMURF1: SMAD specific E3 ubiquitin protein ligase 1; SQSTM1: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TAX1BP1: Tax1 binding protein 1; TBK1: TANK binding kinase 1; TNF: tumor necrosis factor; TRAF6: TNF receptor associated factor 6; Ub: ubiquitin; UBA: ubiquitin-associated; UBAN: ubiquitin-binding domain in ABIN proteins and NEMO; UBD: ubiquitin-binding domain; UBL: ubiquitin-like; ULK1: unc-51 like autophagy activating kinase 1.

Multifaceted roles of TAX1BP1 in autophagy

TAX1BP1 is a selective macroautophagy/autophagy receptor that plays a central role in host defense to pathogens and in regulating the innate immune system. TAX1BP1 facilitates the xenophagic clearance of pathogenic bacteria such as Salmonella typhimurium and Mycobacterium tuberculosis and regulates TLR3 (toll-like receptor 3)-TLR4 and DDX58/RIG-I-like receptor (RLR) signaling by targeting TICAM1 and MAVS for autophagic degradation respectively. In addition to these canonical autophagy receptor functions, TAX1BP1 can also exert multiple accessory functions that influence the biogenesis and maturation of autophagosomes. In this review, we will discuss and integrate recent findings related to the autophagy function of TAX1BP1 and highlight outstanding questions regarding its functions in autophagy and regulation of innate immunity and host defense.Abbreviations: ATG: autophagy related; CALCOCO: calcium binding and coiled-coil domain; CC: coiled-coil; CHUK/IKKα: conserved helix-loop-helix ubiquitous kinase; CLIR: noncanonical LC3-interacting region; GABARAP: gamma-aminobutyric acid receptor associated protein; HTLV-1: human T-lymphotropic virus 1; IFN: interferon; IL1B/IL1β: interleukin 1 beta; LIR: LC3-interacting region; LPS: lipopolysaccharide; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAPK/JNK: mitogen-activated protein kinase; mATG8: mammalian Atg8 homolog; MAVS: mitochondrial antiviral signaling protein; MEF: mouse embryonic fibroblast; MTB: Mycobacterium tuberculosis; MYD88: myeloid differentiation primary response gene 88; NBR1: NBR1, autophagy cargo receptor; NFKB/NF-κB: nuclear factor of kappa light polypeptide gene enhancer in B cells; OPTN: optineurin; Poly(I:C): polyinosinic:polycytidylic acid; PTM: post-translational modification; RB1CC1: RB1-inducible coiled-coil 1; RIPK: receptor (TNFRSF)-interacting serine-threonine kinase; RLR: DDX58/RIG-I-like receptor; RSV: respiratory syncytia virus; SKICH: SKIP carboxyl homology; SLR: SQSTM1 like receptor; SQSTM1: sequestosome 1; TAX1BP1: Tax1 (human T cell leukemia virus type I) binding protein 1; TBK1: TANK-binding kinase 1; TICAM1: toll-like receptor adaptor molecule 1; TLR: toll-like receptor; TNF: tumor necrosis factor; TNFAIP3: TNF alpha induced protein 3; TNFR: tumor necrosis factor receptor; TOM1: target of myb1 trafficking protein; TRAF: TNF receptor-associated factor; TRIM32: tripartite motif-containing 32; UBD: ubiquitin binding domain; ZF: zinc finger.

Depletion of TAX1BP1 Amplifies Innate Immune Responses during Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is the main cause of acute respiratory infections in young children and also has a major impact on the elderly and immunocompromised people. In the absence of a vaccine or efficient treatment, a better understanding of RSV interactions with the host antiviral response during infection is needed. Previous studies revealed that cytoplasmic inclusion bodies (IBs), where viral replication and transcription occur, could play a major role in the control of innate immunity during infection by recruiting cellular proteins involved in the host antiviral response. We recently showed that the morphogenesis of IBs relies on a liquid-liquid-phase separation mechanism depending on the interaction between viral nucleoprotein (N) and phosphoprotein (P). These scaffold proteins are expected to play a central role in the recruitment of cellular proteins to IBs. Here, we performed a yeast two-hybrid screen using RSV N protein as bait and identified the cellular protein TAX1BP1 as a potential partner of this viral protein. This interaction was validated by pulldown and immunoprecipitation assays. We showed that TAX1BP1 suppression has only a limited impact on RSV infection in cell cultures. However, RSV replication is decreased in TAX1BP1-deficient (TAX1BP1 knockout [TAX1BP1KO]) mice, whereas the production of inflammatory and antiviral cytokines is enhanced. In vitro infection of wild-type or TAX1BP1KO alveolar macrophages confirmed that the innate immune response to RSV infection is enhanced in the absence of TAX1BP1. Altogether, our results suggest that RSV could hijack TAX1BP1 to restrain the host immune response during infection. IMPORTANCE Respiratory syncytial virus (RSV), which is the leading cause of lower respiratory tract illness in infants, remains a medical problem in the absence of a vaccine or efficient treatment. This virus is also recognized as a main pathogen in the elderly and immunocompromised people, and the occurrence of coinfections (with other respiratory viruses and bacteria) amplifies the risks of developing respiratory distress. In this context, a better understanding of the pathogenesis associated with viral respiratory infections, which depends on both viral replication and the host immune response, is needed. The present study reveals that the cellular protein TAX1BP1, which interacts with the RSV nucleoprotein N, participates in the control of the innate immune response during RSV infection, suggesting that the N-TAX1BP1 interaction represents a new target for the development of antivirals.

Oligomerization of Selective Autophagy Receptors for the Targeting and Degradation of Protein Aggregates

The selective targeting and disposal of solid protein aggregates are essential for cells to maintain protein homoeostasis. Autophagy receptors including p62, NBR1, Cue5/TOLLIP (CUET), and Tax1-binding protein 1 (TAX1BP1) proteins function in selective autophagy by targeting ubiquitinated aggregates through ubiquitin-binding domains. Here, we summarize previous beliefs and recent findings on selective receptors in aggregate autophagy. Since there are many reviews on selective autophagy receptors, we focus on their oligomerization, which enables receptors to function as pathway determinants and promotes phase separation.

Dynamic post-translational modification profiling of Mycobacterium tuberculosis-infected primary macrophages

Macrophages are highly plastic cells with critical roles in immunity, cancer, and tissue homeostasis, but how these distinct cellular fates are triggered by environmental cues is poorly understood. To uncover how primary murine macrophages respond to bacterial pathogens, we globally assessed changes in post-translational modifications of proteins during infection with Mycobacterium tuberculosis, a notorious intracellular pathogen. We identified hundreds of dynamically regulated phosphorylation and ubiquitylation sites, indicating that dramatic remodeling of multiple host pathways, both expected and unexpected, occurred during infection. Most of these cellular changes were not captured by mRNA profiling, and included activation of ubiquitin-mediated autophagy, an evolutionarily ancient cellular antimicrobial system. This analysis also revealed that a particular autophagy receptor, TAX1BP1, mediates clearance of ubiquitylated Mtb and targets bacteria to LC3-positive phagophores. These studies provide a new resource for understanding how macrophages shape their proteome to meet the challenge of infection.

On the helical arrangements of protein molecules

Helical structures are prevalent in biology. In the PDB, there are many examples where protein molecules are helically arranged, not only according to strict crystallographic screw axes but also according to approximate noncrystallographic screws. The preponderance of such screws is rather striking as helical arrangements in crystals must preserve an integer number of subunits per turn, while intuition and simple packing arguments would seem to favor fractional helices. The article provides insights into such questions, based on stereochemistry, trigonometry, and topology, and illustrates the findings with concrete PDB structures. Updated statistics of Sohncke space groups in the PDB are also presented.

A novel mode of ubiquitin recognition by the ubiquitin-binding zinc finger domain of WRNIP1

The ubiquitin-binding zinc finger (UBZ) is a type of zinc-coordinating β-β-α fold domain found mainly in proteins involved in DNA repair and transcriptional regulation. Here, we report the crystal structure of the UBZ domain of Y-family DNA polymerase (pol) η and the crystal structure of the complex between the UBZ domain of Werner helicase-interacting protein 1 (WRNIP1) and ubiquitin, crystallized using the GFP fusion technique. In contrast to the pol η UBZ, which has been proposed to bind ubiquitin via its C-terminal α-helix, ubiquitin binds to a novel surface of WRNIP1 UBZ composed of the first β-strand and the C-terminal α-helix. In addition, we report the structure of the tandem UBZ domains of Tax1-binding protein 1 (TAX1BP1) and show that the second UBZ of TAX1BP1 binds ubiquitin, presumably in a manner similar to that of WRNIP1 UBZ. We propose that UBZ domains can be divided into at least two different types in terms of the ubiquitin-binding surfaces: the pol η type and the WRNIP1 type.

DATABASE

Structural data are available in the Protein Data Bank under accession numbers 3WUP (pol η UBZ), 3VHS (WRNIP1 UBZ), 3VHT (GFP-WRNIP1/ubiquitin), 4Z4K (TAX1BP1 UBZ1 + 2), and 4Z4M (TAX1BP1 UBZ2).

Structural basis for ubiquitin recognition by ubiquitin-binding zinc finger of FAAP20

Several ubiquitin-binding zinc fingers (UBZs) have been reported to preferentially bind K63-linked ubiquitin chains. In particular, the UBZ domain of FAAP20 (FAAP20-UBZ), a member of the Fanconi anemia core complex, seems to recognize K63-linked ubiquitin chains, in order to recruit the complex to DNA interstrand crosslinks and mediate DNA repair. By contrast, it is reported that the attachment of a single ubiquitin to Rev1, a translesion DNA polymerase, increases binding of Rev1 to FAAP20. To clarify the specificity of FAAP20-UBZ, we determined the crystal structure of FAAP20-UBZ in complex with K63-linked diubiquitin at 1.9 Å resolution. In this structure, FAAP20-UBZ interacts only with one of the two ubiquitin moieties. Consistently, binding assays using surface plasmon resonance spectrometry showed that FAAP20-UBZ binds ubiquitin and M1-, K48- and K63-linked diubiquitin chains with similar affinities. Residues in the vicinity of Ala168 within the α-helix and the C-terminal Trp180 interact with the canonical Ile44-centered hydrophobic patch of ubiquitin. Asp164 within the α-helix and the C-terminal loop mediate a hydrogen bond network, which reinforces ubiquitin-binding of FAAP20-UBZ. Mutations of the ubiquitin-interacting residues disrupted binding to ubiquitin in vitro and abolished the accumulation of FAAP20 to DNA damage sites in vivo. Finally, structural comparison among FAAP20-UBZ, WRNIP1-UBZ and RAD18-UBZ revealed distinct modes of ubiquitin binding. UBZ family proteins could be divided into at least three classes, according to their ubiquitin-binding modes.

Ubiquitin recognition by FAAP20 expands the complex interface beyond the canonical UBZ domain

FAAP20 is an integral component of the Fanconi anemia core complex that mediates the repair of DNA interstrand crosslinks. The ubiquitin-binding capacity of the FAAP20 UBZ is required for recruitment of the Fanconi anemia complex to interstrand DNA crosslink sites and for interaction with the translesion synthesis machinery. Although the UBZ–ubiquitin interaction is thought to be exclusively encapsulated within the ββα module of UBZ, we show that the FAAP20–ubiquitin interaction extends beyond such a canonical zinc-finger motif. Instead, ubiquitin binding by FAAP20 is accompanied by transforming a disordered tail C-terminal to the UBZ of FAAP20 into a rigid, extended β-loop that latches onto the complex interface of the FAAP20 UBZ and ubiquitin, with the invariant C-terminal tryptophan emanating toward I44^Ub for enhanced binding specificity and affinity. Substitution of the C-terminal tryptophan with alanine in FAAP20 not only abolishes FAAP20–ubiquitin binding in vitro, but also causes profound cellular hypersensitivity to DNA interstrand crosslink lesions in vivo, highlighting the indispensable role of the C-terminal tail of FAAP20, beyond the compact zinc finger module, toward ubiquitin recognition and Fanconi anemia complex-mediated DNA interstrand crosslink repair.

Crystallographic and modelling studies suggest that the SKICH domains from different protein families share a common Ig-like fold but harbour substantial structural variations

TAX1BP1 is a pleiotropic multi-domain protein involved in many important biological processes such as signal transduction, cell growth and apoptosis, transcriptional coactivation, membrane trafficking, neurotransmission and autophagy. The N-terminus of TAX1BP1 contains a SKICH domain implicated in autophagy. SKICH domains are also present in four other proteins including NDP52, CALCOCO1, SKIP and PIPP. The SKICH domains of SKIP and PIPP mediate plasma membrane localisation. The functions of the SKICH domains of NDP52 and CALCOCO1 are not known. Here we report the crystal structure of the TAX1BP1 SKICH domain, which has an Ig-like fold similar to the NDP52 SKICH domain. Extensive pairwise and clustered aromatic π-stacking interactions are present in the TAX1BP1 SKICH domain. The aromatic residues mediating these interactions can be classified into four groups with varying degrees of conservation among different protein families. The interactions mediated by highly conserved residues are found in the interior and one outward face of the Ig-like β-barrel, representing common structural features of the SKICH domains. Three TAX1BP1-specific pairwise interactions locate in the loop regions, each augmented by a proline-aromatic interaction. The three proline-aromatic clusters are linked together by more generic hydrophobic interactions, forming a unique hydrophobic surface at one end of the TAX1BP1 SKICH domain. The structures and homologous models of SKICH domains from different proteins reveal substantial differences in electrostatic surface properties of the domains. Together with existing biochemical data, results from the structural study suggest that an intact SKICH domain is required for the autophagy function of TAX1BP1.