Background: Human AlkB homolog 5 (Alkbh5) is an RNA demethylase that erases m 6 A modification. Results: Crystal structures of an enzymatically active Alkbh5 construct in complex with cofactors or small molecules were determined. Conclusion: Structure and activity analyses showed that Alkbh5 strongly prefers single-stranded oligos and small molecule inhibitors. Significance: The Alkbh5 structure reveals potential for structure-based design of selective inhibitors.

Polycomb repressive complex 2 (PRC2) is a histone methyltransferase that maintains cell identity during development in multicellular organisms by marking repressed genes and chromatin domains. In addition to four core subunits, PRC2 comprises multiple accessory subunits that vary in their composition during cellular differentiation and define two major holo-PRC2 complexes: PRC2.1 and PRC2.2. PRC2 binds to RNA, which inhibits its enzymatic activity, but the mechanism of RNA-mediated inhibition of holo-PRC2 is poorly understood. Here we present in vivo and in vitro protein–RNA interaction maps and identify an RNA-binding patch within the allosteric regulatory site of human and mouse PRC2, adjacent to the methyltransferase centre. RNA-mediated inhibition of holo-PRC2 is relieved by allosteric activation of PRC2 by H3K27me3 and JARID2-K116me3 peptides. Both holo-PRC2.1 and -PRC2.2 bind RNA, providing a unified model to explain how RNA and allosteric stimuli antagonistically regulate the enzymatic activity of PRC2.

Covalent modifications of histone N-terminal tails play a critical role in regulating chromatin structure and controlling gene expression. These modifications are controlled by histone-modifying enzymes and read out by histone-binding proteins. Numerous proteins have been identified as histone modification readers. Here we report the family-wide characterization of histone binding abilities of human CW domain-containing proteins. We demonstrate that the CW domains in ZCWPW2 and MORC3/4 selectively recognize histone H3 trimethylated at Lys-4, similar to ZCWPW1 reported previously, while the MORC1/2 and LSD2 lack histone H3 Lys-4 binding ability. Our crystal structures of the CW domains of ZCWPW2 and MORC3 in complex with the histone H3 trimethylated at Lys-4 peptide reveal the molecular basis of this interaction. In each complex, two tryptophan residues in the CW domain form the "floor" and "right wall," respectively, of the methyllysine recognition cage. Our mutation results based on ZCWPW2 reveal that the right wall tryptophan residue is essential for binding, and the floor tryptophan residue enhances binding affinity. Our structural and mutational analysis highlights the conserved roles of the cage residues of CW domain across the histone methyllysine binders but also suggests why some CW domains lack histone binding ability.Chromatin structure is dynamically regulated by histone post-translational modifications, such as methylation, acetylation, phosphorylation, ubiquitination, and sumoylation (1). These post-translational modifications constitute the "histone code," which is written or erased by histone-modifying enzymes and recognized by histone code "reader" proteins (2-4).Histone methylation, such as lysine methylation at the ⑀-amino group at levels from mono-to trimethylation (me1-me3), has received extensive attention (5). A number of domains bind methylated histone tails. Prominent examples include the chromodomain, Tudor domain, MBT domain, PWWP domain, and PHD domain (4, 6, 7). The CW domain has recently been identified as a new member of the lysine methylation reader family (8 -11).The CW domain is a zinc binding domain, composed of ϳ50 amino acid residues with four conserved cysteine (C) and two conserved tryptophan (W) residues, and its name was derived from these conserved residues. CW domains are found in chromatin-associated proteins in animals and plants and grouped into 12 families based on sequence similarity (12). There are seven CW domain-containing proteins in humans, namely ZCWPW1, ZCWPW2, MORC1, MORC2, MORC3, MORC4, and LSD2 (Fig. 1A). Prior studies have shown that the CW domains of ZCWPW1 (8), MORC3 (10), and MORC4 (9) are readers of H3K4 3 methylated histones with differing preferences for histone H3K4 methylation states (i.e. ZCWPW1 and MORC3 preferentially recognize histone H3K4me3 (8, 10), whereas the CW domain of human MORC4 prefers dimethylated H3K4 (9)). The LSD2 CW domain is required for the demethylation function of LSD2 but does not bind to any H3K4 peptides (13). However, th...

LSD2/AOF1/KDM1b catalyzes demethylation of mono-and di-methylated H3K4 and plays an important role in transcriptional regulation and genomic imprinting. Here, we report the high-resolution crystal structures of apo-LSD2 and LSD2 in complex with a peptide that mimics H3K4me2. Three structural domains of LSD2, namely, the novel N-terminal zinc finger, the centrally located SWIRM domain, and the C-terminal oxidase domain, closely pack together to form a boot-shaped structure. The active site cavity in the oxidase domain is large enough to accommodate several residues of the histone H3 tail and cannot discriminate between the different states of H3K4 methylation. The N-terminal zinc-finger domain, composed of a novel C4H2C2-type zinc finger and a specific CW-type zinc finger, is required for demethylase activity and, surprisingly, the binding of cofactor flavin adenine dinucleotide (FAD). In fact, a relay of extensive interactions through the zinc finger-SWIRM-oxidase domains is required for LSD2 demethylase activity and the binding of FAD. These results reveal a novel mechanism for the zinc finger and SWIRM domains in controlling LSD2 demethylase activity and provide a framework for elucidating the regulation and function of LSD2.

USP9X is a conserved deubiquitinase (DUB) that regulates multiple cellular processes. Dysregulation of USP9X has been linked to cancers and X-linked intellectual disability. Here, we report the crystal structure of the USP9X catalytic domain at 2.5-Å resolution. The structure reveals a canonical USP-fold comprised of fingers, palm, and thumb subdomains, as well as an unusual β-hairpin insertion. The catalytic triad of USP9X is aligned in an active configuration. USP9X is exclusively active against ubiquitin (Ub) but not Ub-like modifiers. Cleavage assays with di-, tri-, and tetraUb chains show that the USP9X catalytic domain has a clear preference for K11-, followed by K63-, K48-, and K6-linked polyUb chains. Using a set of activity-based diUb and triUb probes (ABPs), we demonstrate that the USP9X catalytic domain has an exo-cleavage preference for K48- and endo-cleavage preference for K11-linked polyUb chains. The structure model and biochemical data suggest that the USP9X catalytic domain harbors three Ub binding sites, and a zinc finger in the fingers subdomain and the β-hairpin insertion both play important roles in polyUb chain processing and linkage specificity. Furthermore, unexpected labeling of a secondary, noncatalytic cysteine located on a blocking loop adjacent to the catalytic site by K11-diUb ABP implicates a previously unreported mechanism of polyUb chain recognition. The structural features of USP9X revealed in our study are critical for understanding its DUB activity. The new Ub-based ABPs form a set of valuable tools to understand polyUb chain processing by the cysteine protease class of DUBs.

Ubiquitin-specific proteases (USPs) USP15 and USP4 belong to a subset of USPs featuring an N-terminal tandem domain in USP (DUSP) and ubiquitin-like (UBL) domain. Squamous cell carcinoma antigen recognized by T-cell 3 (SART3), a spliceosome recycling factor, binds to the DUSP-UBL domain of USP15 and USP4, recruiting them to the nucleus from the cytosol to control deubiquitination of histone H2B and spliceosomal proteins, respectively. To provide structural insight, we solved crystal structures of SART3 in the apo-form and in complex with the DUSP-UBL domain of USP15 at 2.0 and 3.0 Å, respectively. Structural analysis reveals SART3 contains 12 half-a-tetratricopeptide (HAT) repeats, organized into two subdomains, HAT-N and HAT-C. SART3 dimerizes through the concave surface of HAT-C, whereas the HAT-C convex surface binds USP15 in a novel bipartite mode. Isothermal titration calorimetry measurements and mutagenesis analysis confirmed key residues of USP15 involved in the interaction and indicated USP15 binds 20-fold stronger than USP4.Ubiquitination plays an important role in almost every biological process, including protein homeostasis, DNA damage response, gene transcription, protein trafficking, and RNA splicing. Deubiquitinases (DUBs) 2 remove covalently conjugated ubiquitin tags from substrates and regulate ubiquitin signaling. Ubiquitin-specific proteases (USPs) comprise the largest family of DUBs. In addition to the catalytic domain, most USPs contain auxiliary domains that are involved in substrate recognition, activity regulation, or recruitment of binding partners (1, 2). USP4, USP11, and USP15 are a small set of closely related USPs, sharing a similar domain architecture as follows: a DUSP (domain in USP), followed by a ubiquitin-like (UBL) and a large catalytic domain, bifurcated by a second UBL domain and disordered region (Fig. 1A). Evolutionary analysis suggests the three USPs arose from gene duplication events (3). USP4 and USP15 are more closely related in both sequence and function compared with USP11. Both USP4 and USP15 are implicated in mRNA processing through their interaction with spliceosome components (1). USP4 is recruited by U4/U6 small nuclear RNA recycling factor SART3 (squamous cell carcinoma antigen recognized by T cells 3) to remove the Lys-63-polyubiquitin chain from pre-mRNA processing factor 3 (Prp3), and it controls the assembly of the spliceosome at distinct stages of the splicing process (4). USP15, but not USP4, is recruited by SART3 to regulate deubiquitination of free ubiquitinated histone H2B that has been evicted from the nucleosome during transcription (5).SART3 is the mammalian homolog of yeast Prp24 protein and is essential for the formation of U4/U6 small nuclear ribonucleoprotein complex (6). SART3 plays multiple roles in mRNA splicing, viral and host gene transcription, as well as stem cell survival, proliferation, and differentiation. It is also a potential antigen for cancer immunotherapy (7-9). The biological functions of SART3 have been summarized in a recent c...

The polycomb repressive complex 2 (PRC2) is a histone methyltransferase that maintains cell identities. JARID2 is the only accessory subunit of PRC2 that known to trigger an allosteric activation of methyltransferase. Yet, this mechanism cannot be generalised to all PRC2 variants as, in vertebrates, JARID2 is mutually exclusive with most of the accessory subunits of PRC2. Here we provide functional and structural evidence that the vertebrate-specific PRC2 accessory subunit PALI1 emerged through a convergent evolution to mimic JARID2 at the molecular level. Mechanistically, PRC2 methylates PALI1 K1241, which then binds to the PRC2-regulatory subunit EED to allosterically activate PRC2. PALI1 K1241 is methylated in mouse and human cell lines and is essential for PALI1-induced allosteric activation of PRC2. High-resolution crystal structures revealed that PALI1 mimics the regulatory interactions formed between JARID2 and EED. Independently, PALI1 also facilitates DNA and nucleosome binding by PRC2. In acute myelogenous leukemia cells, overexpression of PALI1 leads to cell differentiation, with the phenotype altered by a separation-of-function PALI1 mutation, defective in allosteric activation and active in DNA binding. Collectively, we show that PALI1 facilitates catalysis and substrate binding by PRC2 and provide evidence that subunit-induced allosteric activation is a general property of holo-PRC2 complexes.

Roquins are a family of highly conserved RNA-binding proteins that also contain a RING-type E3 ubiquitin ligase domain. They repress constitutive decay elements containing mRNAs and play a critical role in RNA homeostasis and immunological self-tolerance. Here we present the crystal structures of the RNA-binding region of Roquin paralog RC3H2 in both apo- and RNA-bound forms. The RNA-binding region has a bipartite architecture composed of ROQ and HEPN domains, and can bind to stem-loop and double-stranded RNAs simultaneously. The two domains undergo a large orientation change to accommodate RNA duplex binding. We profiled E2 ubiquitin-conjugating enzymes that pair with Roquins and found that RC3H1 and RC3H2 interact with two sets of overlapping but not identical E2 enzymes to drive the assembly of polyubiquitin chains of different linkages. Crystal structures, small-angle X-ray scattering, and E2 profiling revealed that while the two paralogs are highly homologous, RC3H2 and RC3H1 are different in their structures and functions. We also demonstrated that RNA duplex binding to RC3H2 cross-talks with its E3 ubiquitin ligase function using an in vitro auto-ubiquitination assay.