Basis of the H2AK119 specificity of the Polycomb repressive deubiquitinase.

Repression of gene expression by protein complexes of the Polycomb group is a fundamental mechanism that governs embryonic development and cell-type specification1-3. The Polycomb repressive deubiquitinase (PR-DUB) complex removes the ubiquitin moiety from monoubiquitinated histone H2A K119 (H2AK119ub1) on the nucleosome4, counteracting the ubiquitin E3 ligase activity of Polycomb repressive complex 1 (PRC1)5 to facilitate the correct silencing of genes by Polycomb proteins and safeguard active genes from inadvertent silencing by PRC1 (refs. 6-9). The intricate biological function of PR-DUB requires accurate targeting of H2AK119ub1, but PR-DUB can deubiquitinate monoubiquitinated free histones and peptide substrates indiscriminately; the basis for its exquisite nucleosome-dependent substrate specificity therefore remains unclear. Here we report the cryo-electron microscopy structure of human PR-DUB, composed of BAP1 and ASXL1, in complex with the chromatosome. We find that ASXL1 directs the binding of the positively charged C-terminal extension of BAP1 to nucleosomal DNA and histones H3-H4 near the dyad, an addition to its role in forming the ubiquitin-binding cleft. Furthermore, a conserved loop segment of the catalytic domain of BAP1 is situated near the H2A-H2B acidic patch. This distinct nucleosome-binding mode displaces the C-terminal tail of H2A from the nucleosome surface, and endows PR-DUB with the specificity for H2AK119ub1.

UbiBrowser 2.0: a comprehensive resource for proteome-wide known and predicted ubiquitin ligase/deubiquitinase-substrate interactions in eukaryotic species.

As an important post-translational modification, ubiquitination mediates ∼80% of protein degradation in eukaryotes. The degree of protein ubiquitination is tightly determined by the delicate balance between specific ubiquitin ligase (E3)-mediated ubiquitination and deubiquitinase-mediated deubiquitination. In 2017, we developed UbiBrowser 1.0, which is an integrated database for predicted human proteome-wide E3-substrate interactions. Here, to meet the urgent requirement of proteome-wide E3/deubiquitinase-substrate interactions (ESIs/DSIs) in multiple organisms, we updated UbiBrowser to version 2.0 (http://ubibrowser.ncpsb.org.cn). Using an improved protocol, we collected 4068/967 known ESIs/DSIs by manual curation, and we predicted about 2.2 million highly confident ESIs/DSIs in 39 organisms, with >210-fold increase in total data volume. In addition, we made several new features in the updated version: (i) it allows exploring proteins' upstream E3 ligases and deubiquitinases simultaneously; (ii) it has significantly increased species coverage; (iii) it presents a uniform confidence scoring system to rank predicted ESIs/DSIs. To facilitate the usage of UbiBrowser 2.0, we also redesigned the web interface for exploring these known and predicted ESIs/DSIs, and added functions of 'Browse', 'Download' and 'Application Programming Interface'. We believe that UbiBrowser 2.0, as a discovery tool, will contribute to the study of protein ubiquitination and the development of drug targets for complex diseases.

An evolutionary approach to systematic discovery of novel deubiquitinases, applied to Legionella.

Deubiquitinating enzymes (DUBs) are important regulators of the posttranslational protein ubiquitination system. Mammalian genomes encode about 100 different DUBs, which can be grouped into seven different classes. Members of other DUB classes are found in pathogenic bacteria, which use them to target the host defense. By combining bioinformatical and experimental approaches, we address the question if the known DUB families have a common evolutionary ancestry and share conserved features that set them apart from other proteases. By systematically comparing family-specific hidden Markov models, we uncovered distant relationships between established DUBs and other cysteine protease families. Most DUB families share a conserved aromatic residue linked to the active site, which restricts the cleavage of substrates with side chains at the S2 position, corresponding to Gly-75 in ubiquitin. By applying these criteria to Legionella pneumophila ORFs, we identified lpg1621 and lpg1148 as deubiquitinases, characterized their cleavage specificities, and confirmed the importance of the aromatic gatekeeper motif for substrate selection.

Bacterial DUBs: deubiquitination beyond the seven classes.

Protein ubiquitination is a posttranslational modification that regulates many aspects of cellular life, including proteostasis, vesicular trafficking, DNA repair and NF-κB activation. By directly targeting intracellular bacteria or bacteria-containing vacuoles to the lysosome, ubiquitination is also an important component of cell-autonomous immunity. Not surprisingly, several pathogenic bacteria encode deubiquitinases (DUBs) and use them as secreted effectors that prevent ubiquitination of bacterial components. A systematic overview of known bacterial DUBs, including their cleavage specificities and biological roles, suggests multiple independent acquisition events from host-encoded DUBs and other proteases. The widely used classification of DUBs into seven well-defined families should only be applied to eukaryotic DUBs, since several bacterial DUBs do not follow this classification.

Reactive-site-centric chemoproteomics identifies a distinct class of deubiquitinase enzymes.

Activity-based probes (ABPs) are widely used to monitor the activity of enzyme families in biological systems. Inferring enzyme activity from probe reactivity requires that the probe reacts with the enzyme at its active site; however, probe-labeling sites are rarely verified. Here we present an enhanced chemoproteomic approach to evaluate the activity and probe reactivity of deubiquitinase enzymes, using bioorthogonally tagged ABPs and a sequential on-bead digestion protocol to enhance the identification of probe-labeling sites. We confirm probe labeling of deubiquitinase catalytic Cys residues and reveal unexpected labeling of deubiquitinases on non-catalytic Cys residues and of non-deubiquitinase proteins. In doing so, we identify ZUFSP (ZUP1) as a previously unannotated deubiquitinase with high selectivity toward cleaving K63-linked chains. ZUFSP interacts with and modulates ubiquitination of the replication protein A (RPA) complex. Our reactive-site-centric chemoproteomics method is broadly applicable for identifying the reaction sites of covalent molecules, which may expand our understanding of enzymatic mechanisms.

iUUCD 2.0: an update with rich annotations for ubiquitin and ubiquitin-like conjugations.

Here, we described the updated database iUUCD 2.0 (http://iuucd.biocuckoo.org/) for ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), ubiquitin-protein ligases (E3s), deubiquitinating enzymes (DUBs), ubiquitin/ubiquitin-like binding domains (UBDs) and ubiquitin-like domains (ULDs), which act as key regulators in modulating ubiquitin and ubiquitin-like (UB/UBL) conjugations. In total, iUUCD 2.0 contained 136 512 UB/UBL regulators, including 1230 E1s, 5636 E2s, 93 343 E3s, 9548 DUBs, 30 173 UBDs and 11 099 ULDs in 148 eukaryotic species. In particular, we provided rich annotations for regulators of eight model organisms, especially in humans, by compiling and integrating the knowledge from nearly 70 widely used public databases that cover cancer mutations, single nucleotide polymorphisms (SNPs), mRNA expression, DNA and RNA elements, protein-protein interactions, protein 3D structures, disease-associated information, drug-target relations, post-translational modifications, DNA methylation and protein expression/proteomics. Compared with our previously developed UUCD 1.0 (∼0.41 GB), iUUCD 2.0 has a size of ∼32.1 GB of data with a >75-fold increase in data volume. We anticipate that iUUCD 2.0 can be a more useful resource for further study of UB/UBL conjugations.