Masato Akutsu

Specific Recognition of Linear Ubiquitin Chains by NEMO Is Important for NF-κB Activation

Activation of nuclear factor-kappaB (NF-kappaB), a key mediator of inducible transcription in immunity, requires binding of NF-kappaB essential modulator (NEMO) to ubiquitinated substrates. Here, we report that the UBAN (ubiquitin binding in ABIN and NEMO) motif of NEMO selectively binds linear (head-to-tail) ubiquitin chains. Crystal structures of the UBAN motif revealed a parallel coiled-coil dimer that formed a heterotetrameric complex with two linear diubiquitin molecules. The UBAN dimer contacted all four ubiquitin moieties, and the integrity of each binding site was required for efficient NF-kappaB activation. Binding occurred via a surface on the proximal ubiquitin moiety and the canonical Ile44 surface on the distal one, thereby providing specificity for linear chain recognition. Residues of NEMO involved in binding linear ubiquitin chains are required for NF-kappaB activation by TNF-alpha and other agonists, providing an explanation for the detrimental effect of NEMO mutations in patients suffering from X-linked ectodermal dysplasia and immunodeficiency.

Regulation of endoplasmic reticulum turnover by selective autophagy

The endoplasmic reticulum (ER) is the largest intracellular endomembrane system, enabling protein and lipid synthesis, ion homeostasis, quality control of newly synthesized proteins and organelle communication. Constant ER turnover and modulation is needed to meet different cellular requirements and autophagy has an important role in this process. However, its underlying regulatory mechanisms remain unexplained. Here we show that members of the FAM134 reticulon protein family are ER-resident receptors that bind to autophagy modifiers LC3 and GABARAP, and facilitate ER degradation by autophagy (‘ER-phagy’). Downregulation of FAM134B protein in human cells causes an expansion of the ER, while FAM134B overexpression results in ER fragmentation and lysosomal degradation. Mutant FAM134B proteins that cause sensory neuropathy in humans are unable to act as ER-phagy receptors. Consistently, disruption of Fam134b in mice causes expansion of the ER, inhibits ER turnover, sensitizes cells to stress-induced apoptotic cell death and leads to degeneration of sensory neurons. Therefore, selective ER-phagy via FAM134 proteins is indispensable for mammalian cell homeostasis and controls ER morphology and turnover in mice and humans.

OTU Deubiquitinases Reveal Mechanisms of Linkage Specificity and Enable Ubiquitin Chain Restriction Analysis

Summary Sixteen ovarian tumor (OTU) family deubiquitinases (DUBs) exist in humans, and most members regulate cell-signaling cascades. Several OTU DUBs were reported to be ubiquitin (Ub) chain linkage specific, but comprehensive analyses are missing, and the underlying mechanisms of linkage specificity are unclear. Using Ub chains of all eight linkage types, we reveal that most human OTU enzymes are linkage specific, preferring one, two, or a defined subset of linkage types, including unstudied atypical Ub chains. Biochemical analysis and five crystal structures of OTU DUBs with or without Ub substrates reveal four mechanisms of linkage specificity. Additional Ub-binding domains, the ubiquitinated sequence in the substrate, and defined S1’ and S2 Ub-binding sites on the OTU domain enable OTU DUBs to distinguish linkage types. We introduce Ub chain restriction analysis, in which OTU DUBs are used as restriction enzymes to reveal linkage type and the relative abundance of Ub chains on substrates.

LC3C, Bound Selectively by a Noncanonical LIR Motif in NDP52, Is Required for Antibacterial Autophagy

Summary Autophagy protects cellular homeostasis by capturing cytosolic components and invading pathogens for lysosomal degradation. Autophagy receptors target cargo to autophagy by binding ATG8 on autophagosomal membranes. The expansion of the ATG8 family in higher eukaryotes suggests that specific interactions with autophagy receptors facilitate differential cargo handling. However, selective interactors of ATG8 orthologs are unknown. Here we show that the selectivity of the autophagy receptor NDP52 for LC3C is crucial for innate immunity since cells lacking either protein cannot protect their cytoplasm against Salmonella. LC3C is required for antibacterial autophagy because in its absence the remaining ATG8 orthologs do not support efficient antibacterial autophagy. Structural analysis revealed that the selectivity of NDP52 for LC3C is conferred by a noncanonical LIR, in which lack of an aromatic residue is balanced by LC3C-specific interactions. Our report illustrates that specificity in the interaction between autophagy receptors and autophagy machinery is of functional importance to execute selective autophagy.

Two-sided ubiquitin binding explains specificity of the TAB2 NZF domain

The protein kinase TAK1 is activated by binding to Lys63 (K63)-linked ubiquitin chains through its subunit TAB2. Here we analyze crystal structures of the TAB2 NZF domain bound to Lys63-linked di- and triubiquitin, revealing that TAB2 binds adjacent ubiquitin moieties via two distinct binding sites. The conformational constraints imposed by TAB2 on a Lys63 dimer cannot be adopted by linear chains, explaining why TAK1 cannot be activated by linear ubiquitination events.

Rab GTPase-Activating Proteins in Autophagy: Regulation of Endocytic and Autophagy Pathways by Direct Binding to Human ATG8 Modifiers

ABSTRACT Autophagy is an evolutionarily conserved degradation pathway characterized by dynamic rearrangement of membranes that sequester cytoplasm, protein aggregates, organelles, and pathogens for delivery to the vacuole and lysosome, respectively. The ability of autophagosomal membranes to act selectively toward specific cargo is dependent on the small ubiquitin-like modifier ATG8/LC3 and the LC3-interacting region (LIR) present in autophagy receptors. Here, we describe a comprehensive protein-protein interaction analysis of TBC (Tre2, Bub2, and Cdc16) domain-containing Rab GTPase-activating proteins (GAPs) as potential autophagy adaptors. We identified 14 TBC domain-containing Rab GAPs that bind directly to ATG8 modifiers and that colocalize with LC3-positive autophagy membranes in cells. Intriguingly, one of our screening hits, TBC1D5, contains two LIR motifs. The N-terminal LIR was critical for interaction with the retromer complex and transport of cargo. Direct binding of the retromer component VPS29 to TBC1D5 could be titrated out by LC3, indicating a molecular switch between endosomes and autophagy. Moreover, TBC1D5 could bridge the endosome and autophagosome via its C-terminal LIR motif. During starvation-induced autophagy, TBC1D5 was relocalized from endosomal localization to the LC3-positive autophagosomes. We propose that LC3-interacting Rab GAPs are implicated in the reprogramming of the endocytic trafficking events under starvation-induced autophagy.

Polyubiquitin binding and cross‐reactivity in the USP domain deubiquitinase USP21

Modification of proteins by ubiquitin (Ub) and Ub‐like (Ubl) modifiers regulates a variety of cellular functions. The ability of Ub to form chains of eight structurally and functionally distinct types adds further complexity to the system. Ub‐specific proteases (USPs) hydrolyse polyUb chains, and some have been suggested to be cross‐reactive with Ubl modifiers, such as neural precursor cell expressed, developmentally downregulated 8 (NEDD8) and interferon‐stimulated gene 15 (ISG15). Here, we report that USP21 cleaves Ub polymers, and with reduced activity also targets ISG15, but is inactive against NEDD8. A crystal structure of USP21 in complex with linear diUb aldehyde shows how USP21 interacts with polyUb through a previously unidentified second Ub‐ and ISG15‐binding surface on the USP domain core. We also rationalize the inability of USP21 to target NEDD8 and identify differences that allow USPs to distinguish between structurally related modifications.

Molecular basis for ubiquitin and ISG15 cross-reactivity in viral ovarian tumor domains.

Crimean Congo hemorrhagic fever virus (CCHFV) is a deadly human pathogen that evades innate immune responses by efficiently interfering with antiviral signaling pathways mediated by NF-κB, IRF3, and IFNα/β. These pathways rely on protein ubiquitination for their activation, and one outcome is the modification of proteins with the ubiquitin (Ub)-like modifier interferon-stimulated gene (ISG)15. CCHFV and related viruses encode a deubiquitinase (DUB) of the ovarian tumor (OTU) family, which unlike eukaryotic OTU DUBs also targets ISG15 modifications. Here we characterized the viral OTU domain of CCHFV (vOTU) biochemically and structurally, revealing that it hydrolyzes four out of six tested Ub linkages, but lacks activity against linear and K29-linked Ub chains. vOTU cleaved Ub and ISG15 with similar kinetics, and we were able to understand vOTU cross-reactivity at the molecular level from crystal structures of vOTU in complex with Ub and ISG15. An N-terminal extension in vOTU not present in eukaryotic OTU binds to the hydrophobic Ile44 patch of Ub, which results in a dramatically different Ub orientation compared to a eukaryotic OTU–Ub complex. The C-terminal Ub-like fold of ISG15 (ISG15-C) adopts an equivalent binding orientation. Interestingly, ISG15-C contains an additional second hydrophobic surface that is specifically contacted by vOTU. These subtle differences in Ub/ISG15 binding allowed the design of vOTU variants specific for either Ub or ISG15, which will be useful tools to understand the relative contribution of ubiquitination vs. ISGylation in viral infection. Furthermore, the crystal structures will allow structure-based design of antiviral agents targeting this enzyme.

An ankyrin-repeat ubiquitin-binding domain determines TRABID's specificity for atypical ubiquitin chains.

Eight different types of ubiquitin linkages are present in eukaryotic cells that regulate diverse biological processes. Proteins that mediate specific assembly and disassembly of atypical Lys6, Lys27, Lys29 and Lys33 linkages are mainly unknown. We here reveal how the human ovarian tumor (OTU) domain deubiquitinase (DUB) TRABID specifically hydrolyzes both Lys29- and Lys33-linked diubiquitin. A crystal structure of the extended catalytic domain reveals an unpredicted ankyrin repeat domain that precedes an A20-like catalytic core. NMR analysis identifies the ankyrin domain as a new ubiquitin-binding fold, which we have termed AnkUBD, and DUB assays in vitro and in vivo show that this domain is crucial for TRABID efficiency and linkage specificity. Our data are consistent with AnkUBD functioning as an enzymatic S1′ ubiquitin-binding site, which orients a ubiquitin chain so that Lys29 and Lys33 linkages are cleaved preferentially.

Ubiquitin chain diversity at a glance

ABSTRACT Ubiquitin plays an essential role in modulating protein functions, and deregulation of the ubiquitin system leads to the development of multiple human diseases. Owing to its molecular features, ubiquitin can form various homo- and heterotypic polymers on substrate proteins, thereby provoking distinct cellular responses. The concept of multifaceted ubiquitin chains encoding different functions has been substantiated in recent years. It has been established that all possible ubiquitin linkage types are utilized for chain assembly and propagation of specific signals in vivo. In addition, branched ubiquitin chains and phosphorylated ubiquitin molecules have been put under the spotlight recently. The development of novel technologies has provided detailed insights into the structure and function of previously poorly understood ubiquitin signals. In this Cell Science at a Glance article and accompanying poster, we provide an update on the complexity of ubiquitin chains and their physiological relevance. Summary: In this Cell Science at a Glance article and accompanying poster, an update is provided on the complexity of ubiquitin chains and their physiological relevance.