LSD1 demethylase and the methyl-binding protein PHF20L1 prevent SET7 methyltransferase-dependent proteolysis of the stem-cell protein SOX2

Lysine-specific methyltransferase Set7/9 in stemness, differentiation, and development

The enzymes performing protein post-translational modifications (PTMs) form a critical post-translational regulatory circuitry that orchestrates literally all cellular processes in the organism. In particular, the balance between cellular stemness and differentiation is crucial for the development of multicellular organisms. Importantly, the fine-tuning of this balance on the genetic level is largely mediated by specific PTMs of histones including lysine methylation. Lysine methylation is carried out by special enzymes (lysine methyltransferases) that transfer the methyl group from S-adenosyl-L-methionine to the lysine residues of protein substrates. Set7/9 is one of the exemplary protein methyltransferases that however, has not been fully studied yet. It was originally discovered as histone H3 lysine 4-specific methyltransferase, which later was shown to methylate a number of non-histone proteins that are crucial regulators of stemness and differentiation, including p53, pRb, YAP, DNMT1, SOX2, FOXO3, and others. In this review we summarize the information available to date on the role of Set7/9 in cellular differentiation and tissue development during embryogenesis and in adult organisms. Finally, we highlight and discuss the role of Set7/9 in pathological processes associated with aberrant cellular differentiation and self-renewal, including the formation of cancer stem cells.

WSB1/2 target chromatin-bound lysine-methylated RelA for proteasomal degradation and NF-κB termination

Proteasome-mediated degradation of chromatin-bound NF-κB is critical in terminating the transcription of pro-inflammatory genes and can be triggered by Set9-mediated lysine methylation of the RelA subunit. However, the E3 ligase targeting methylated RelA remains unknown. Here, we find that two structurally similar substrate-recognizing components of Cullin-RING E3 ligases, WSB1 and WSB2, can recognize chromatin-bound methylated RelA for polyubiquitination and proteasomal degradation. We showed that WSB1/2 negatively regulated a subset of NF-κB target genes via associating with chromatin where they targeted methylated RelA for ubiquitination, facilitating the termination of NF-κB-dependent transcription. WSB1/2 specifically interacted with methylated lysines (K) 314 and 315 of RelA via their N-terminal WD-40 repeat (WDR) domains, thereby promoting ubiquitination of RelA. Computational modeling further revealed that a conserved aspartic acid (D) at position 158 within the WDR domain of WSB2 coordinates K314/K315 of RelA, with a higher affinity when either of the lysines is methylated. Mutation of D158 abolished WSB2's ability to bind to and promote ubiquitination of methylated RelA. Together, our study identifies a novel function and the underlying mechanism for WSB1/2 in degrading chromatin-bound methylated RelA and preventing sustained NF-κB activation, providing potential new targets for therapeutic intervention of NF-κB-mediated inflammatory diseases.

Lysine Methylation-Dependent Proteolysis by the Malignant Brain Tumor (MBT) Domain Proteins

Lysine methylation is a major post-translational protein modification that occurs in both histones and non-histone proteins. Emerging studies show that the methylated lysine residues in non-histone proteins provide a proteolytic signal for ubiquitin-dependent proteolysis. The SET7 (SETD7) methyltransferase specifically transfers a methyl group from S-Adenosyl methionine to a specific lysine residue located in a methylation degron motif of a protein substrate to mark the methylated protein for ubiquitin-dependent proteolysis. LSD1 (Kdm1a) serves as a demethylase to dynamically remove the methyl group from the modified protein. The methylated lysine residue is specifically recognized by L3MBTL3, a methyl-lysine reader that contains the malignant brain tumor domain, to target the methylated proteins for proteolysis by the CRL4DCAF5 ubiquitin ligase complex. The methylated lysine residues are also recognized by PHF20L1 to protect the methylated proteins from proteolysis. The lysine methylation-mediated proteolysis regulates embryonic development, maintains pluripotency and self-renewal of embryonic stem cells and other stem cells such as neural stem cells and hematopoietic stem cells, and controls other biological processes. Dysregulation of the lysine methylation-dependent proteolysis is associated with various diseases, including cancers. Characterization of lysine methylation should reveal novel insights into how development and related diseases are regulated.

A methylation-phosphorylation switch controls EZH2 stability and hematopoiesis

The Polycomb Repressive Complex 2 (PRC2) methylates H3K27 to regulate development and cell fate by transcriptional silencing. Alteration of PRC2 is associated with various cancers. Here, we show that mouse Kdm1a deletion causes a dramatic reduction of PRC2 proteins, whereas mouse null mutation of L3mbtl3 or Dcaf5 results in PRC2 accumulation and increased H3K27 trimethylation. The catalytic subunit of PRC2, EZH2, is methylated at lysine 20 (K20), promoting EZH2 proteolysis by L3MBTL3 and the CLR4DCAF5 ubiquitin ligase. KDM1A (LSD1) demethylates the methylated K20 to stabilize EZH2. K20 methylation is inhibited by AKT-mediated phosphorylation of serine 21 in EZH2. Mouse Ezh2K20R/K20R mutants develop hepatosplenomegaly associated with high GFI1B expression, and Ezh2K20R/K20R mutant bone marrows expand hematopoietic stem cells and downstream hematopoietic populations. Our studies reveal that EZH2 is regulated by methylation-dependent proteolysis, which is negatively controlled by AKT-mediated S21 phosphorylation to establish a methylation-phosphorylation switch to regulate the PRC2 activity and hematopoiesis.

Neddylation-dependent LSD1 destabilization inhibits the stemness and chemoresistance of gastric cancer

Histone lysine-specific demethylase 1 (LSD1) expression has been evaluated in multiple tumors, including gastric cancer (GC). However, the mechanisms underlying LSD1 dysregulation in GC remain largely unclear. In this study, neural precursor cell-expressed developmentally down-regulated protein 8 (NEDD8) was identified to be conjugated to LSD1 at K63 by ubiquitin-conjugating enzyme E2 M (UBE2M), and this neddylated LSD1 could promote LSD1 ubiquitination and degradation, leading to a decrease of GC cell stemness and chemoresistance. Herein, our findings revealed a novel mechanism of LSD1 neddylation and its contribution to decreasing GC cell stemness and chemoresistance. Taken together, our findings may whistle about the future application of neddylation inhibitors.

RNA in situ conformation sequencing reveals novel long-range RNA structures with impact on splicing

Over recent years, long-range RNA structure has emerged as a factor that is fundamental to alternative splicing regulation. An increasing number of human disorders are now being associated with splicing defects; hence it is essential to develop methods that assess long-range RNA structure experimentally. RNA in situ conformation sequencing (RIC-seq) is a method that recapitulates RNA structure within physiological RNA-protein complexes. In this work, we juxtapose pairs of conserved complementary regions (PCCRs) that were predicted in silico with the results of RIC-seq experiments conducted in seven human cell lines. We show statistically that RIC-seq support of PCCRs correlates with their properties, such as equilibrium free energy, presence of compensatory substitutions, and occurrence of A-to-I RNA editing sites and forked eCLIP peaks. Exons enclosed in PCCRs that are supported by RIC-seq tend to have weaker splice sites and lower inclusion rates, which is indicative of post-transcriptional splicing regulation mediated by RNA structure. Based on these findings, we prioritize PCCRs according to their RIC-seq support and show, using antisense nucleotides and minigene mutagenesis, that PCCRs in two disease-associated human genes, PHF20L1 and CASK, and also PCCRs in their murine orthologs, impact alternative splicing. In sum, we demonstrate how RIC-seq experiments can be used to discover functional long-range RNA structures, and particularly those that regulate alternative splicing.

The roles of the SOX2 protein in the development of esophagus and esophageal squamous cell carcinoma, and pharmacological target for therapy

SOX2 is a transcription factor belonging to the SOX gene family, whose activity has been associated with the maintenance of the stemness and self-renewal of embryonic stem cells (ESCs), as well as the induction of differentiated cells into induced pluripotent stem cells (iPSCs). Moreover, accumulating studies have shown that SOX2 is amplified in various cancers, notably in esophageal squamous cell carcinoma (ESCC). In addition, SOX2 expression is linked to multiple malignant processes, including proliferation, migration, invasion, and drug resistance. Taken together, targeting SOX2 might shed light on novel approaches for cancer therapy. In this review, we aim to summarize the current knowledge regarding SOX2 in the development of esophagus and ESCC. We also highlight several therapeutic strategies for targeting SOX2 in different cancer types, which can provide new tools to treat cancers possessing abnormal levels of SOX2 protein.

Lysine-Specific Demethylase 1 Promises to Be a Novel Target in Cancer Drug Resistance: Therapeutic Implications

Chemotherapy, targeted therapy, and immunotherapy are effective against most tumors, but drug resistance remains a barrier to successful treatment. Lysine-specific demethylase 1 (LSD1), a member of histone demethylation modifications, can regulate invasion, metastasis, apoptosis, and immune escape of tumor cells, which are associated with tumorigenesis and tumor progression. Recent studies suggest that LSD1 ablation regulates resensitivity of tumor cells to anticarcinogens containing immune checkpoint inhibitors (ICIs) via multiple upstream and downstream pathways. In this review, we describe the recent findings about LSD1 biology and its role in the development and progression of cancer drug resistance. Further, we summarize LSD1 inhibitors that have a reversal or resensitive effect on drug resistance and discuss the possibility of targeting LSD1 in combination with other agents to surmount resistance.

Targeting CSC-related transcription factors by E3 ubiquitin ligases for cancer therapy

Evidence has revealed that transcription factors play essential roles in regulation of multiple cellular processes, including cell proliferation, metastasis, EMT, cancer stem cells and chemoresistance. Dysregulated expression levels of transcription factors contribute to tumorigenesis and malignant progression. The expression of transcription factors is tightly governed by several signaling pathways, noncoding RNAs and E3 ubiquitin ligases. Cancer stem cells (CSCs) have been validated in regulation of tumor metastasis, reoccurrence and chemoresistance in human cancer. Transcription factors have been verified to participate in regulation of CSC formation, including Oct4, SOX2, KLF4, c-Myc, Nanog, GATA, SALL4, Bmi-1, OLIG2, POU3F2 and FOX proteins. In this review article, we will describe the critical role of CSC-related transcription factors. We will further discuss which E3 ligases regulate the degradation of these CSC-related transcription factors and their underlying mechanisms. We also mentioned the functions and mechanisms of EMT-associated transcription factors such as ZEB1, ZEB2, Snail, Slug, Twist1 and Twist2. Furthermore, we highlight the therapeutic potential via targeting E3 ubiquitin ligases for modulation of these transcription factors.

The assembly of mammalian SWI/SNF chromatin remodeling complexes is regulated by lysine-methylation dependent proteolysis

The assembly of mammalian SWI/SNF chromatin remodeling complexes is developmentally programed, and loss/mutations of SWI/SNF subunits alter the levels of other components through proteolysis, causing cancers. Here, we show that mouse Lsd1/Kdm1a deletion causes dramatic dissolution of SWI/SNF complexes and that LSD1 demethylates the methylated lysine residues in SMARCC1 and SMARCC2 to preserve the structural integrity of SWI/SNF complexes. The methylated SMARCC1/SMARCC2 are targeted for proteolysis by L3MBTL3 and the CRL4^DCAF5 ubiquitin ligase complex. We identify SMARCC1 as the critical target of LSD1 and L3MBTL3 to maintain the pluripotency and self-renewal of embryonic stem cells. L3MBTL3 also regulates SMARCC1/SMARCC2 proteolysis induced by the loss of SWI/SNF subunits. Consistently, mouse L3mbtl3 deletion causes striking accumulation of SWI/SNF components, associated with embryonic lethality. Our studies reveal that the assembly/disassembly of SWI/SNF complexes is dynamically controlled by a lysine-methylation dependent proteolytic mechanism to maintain the integrity of the SWI/SNF complexes.

The Epigenetic Regulation of Nonhistone Proteins by SETD7: New Targets in Cancer

Epigenetic modifications are essential mechanism by which to ensure cell homeostasis. One such modification is lysine methylation of nonhistone proteins by SETD7, a mono-methyltransferase containing SET domains. SETD7 methylates over 30 proteins and is thus involved in various classical pathways. As such, SETD7 has been implicated in both the basic functions of normal tissues but also in several pathologies, such as cancers. In this review, we summarize the current knowledge of SETD7 substrates, especially transcriptional-related proteins and enzymes, and their putative roles upon SETD7-mediated methylation. We focus on the role of SETD7 in cancers, and speculate on the possible points of intervention and areas for future research.

The Role of LSD1 and LSD2 in Cancers of the Gastrointestinal System: An Update

Epigenetic mechanisms are known to play a key role in cancer progression. Specifically, histone methylation involves reversible post-translational modification of histones that govern chromatin structure remodelling, genomic imprinting, gene expression, DNA damage repair, and meiotic crossover recombination, among other chromatin-based activities. Demethylases are enzymes that catalyse the demethylation of their substrate using a flavin adenine dinucleotide-dependent amine oxidation process. Lysine-specific demethylase 1 (LSD1) and its homolog, lysine-specific demethylase 2 (LSD2), are overexpressed in a variety of human cancer types and, thus, regulate tumour progression. In this review, we focus on the literature from the last 5 years concerning the role of LSD1 and LSD2 in the main gastrointestinal cancers (i.e., gastric cancer, liver cancer, pancreatic cancer, and colorectal cancer).

Methyl-lysine readers PHF20 and PHF20L1 define two distinct gene expression-regulating NSL complexes

The methyl-lysine readers plant homeodomain finger protein 20 (PHF20) and its homolog PHF20-like protein 1 (PHF20L1) are known components of the nonspecific lethal (NSL) complex that regulates gene expression through its histone acetyltransferase activity. In the current model, both PHF homologs coexist in the same NSL complex, although this was not formally tested; nor have the functions of PHF20 and PHF20L1 regarding NSL complex integrity and transcriptional regulation been investigated. Here, we perform an in-depth biochemical and functional characterization of PHF20 and PHF20L1 in the context of the NSL complex. Using mass spectrometry, genome-wide chromatin analysis, and protein-domain mapping, we identify the existence of two distinct NSL complexes that exclusively contain either PHF20 or PHF20L1. We show that the C-terminal domains of PHF20 and PHF20L1 are essential for complex formation with NSL, and the Tudor 2 domains are required for chromatin binding. The genome-wide chromatin landscape of PHF20–PHF20L1 shows that these proteins bind mostly to the same genomic regions, at promoters of highly expressed/housekeeping genes. Yet, deletion of PHF20 and PHF20L1 does not abrogate gene expression or impact the recruitment of the NSL complex to those target gene promoters, suggesting the existence of an alternative mechanism that compensates for the transcription of genes whose sustained expression is important for critical cellular functions. This work shifts the current paradigm and lays the foundation for studies on the differential roles of PHF20 and PHF20L1 in regulating NSL complex activity in physiological and diseases states.

Non-histone methylation of SET7/9 and its biological functions

BACKGROUND

(su(var)-3-9,enhancer-of-zeste,trithorax) domain-containing protein 7/9 (SET7/9) is a member of the protein lysine methyltransferases (PLMTs or PKMTs) family. It contains a SET domain. Recent studies demonstrate that SET7/9 methylates both lysine 4 of histone 3 (H3-K4) and lysine(s) of non-histone proteins, including transcription factors, tumor suppressors, and membrane-associated receptors.

OBJECTIVE

This article mainly reviews the non-histone methylation effects of SET7/9 and its functions in tumorigenesis and development.

METHODS

PubMed was screened for this information.

RESULTS

SET7/9 plays a key regulatory role in various biological processes such as cell proliferation, transcription regulation, cell cycle, protein stability, cardiac morphogenesis, and development. In addition, SET7/9 is involved in the pathogenesis of hair loss, breast cancer progression, human carotid plaque atherosclerosis, chronic kidney disease, diabetes, obesity, ovarian cancer, prostate cancer, hepatocellular carcinoma, and pulmonary fibrosis.

CONCLUSION

SET7/9 is an important methyltransferase, which can catalyze the methylation of a variety of proteins. Its substrates are closely related to the occurrence and development of tumors.

Histone Methylation Related Therapeutic Challenge in Cardiovascular Diseases

The epidemic of cardiovascular diseases (CVDs) is predicted to spread rapidly in advanced countries accompanied by the high prevalence of risk factors. In terms of pathogenesis, the pathophysiology of CVDs is featured by multiple disorders, including vascular inflammation accompanied by simultaneously perturbed pathways, such as cell death and acute/chronic inflammatory reactions. Epigenetic alteration is involved in the regulation of genome stabilization and cellular homeostasis. The association between CVD progression and histone modifications is widely known. Among the histone modifications, histone methylation is a reversible process involved in the development and homeostasis of the cardiovascular system. Abnormal methylation can promote CVD progression. This review discusses histone methylation and the enzymes involved in the cardiovascular system and determine the effects of histone methyltransferases and demethylases on the pathogenesis of CVDs. We will further demonstrate key proteins mediated by histone methylation in blood vessels and review histone methylation-mediated cardiomyocytes and cellular functions and pathways in CVDs. Finally, we will summarize the role of inhibitors of histone methylation and demethylation in CVDs and analyze their therapeutic potential, based on previous studies.

Roles of lysine-specific demethylase 1 (LSD1) in homeostasis and diseases

Lysine-specific demethylase 1 (LSD1) targets mono- or di-methylated histone H3K4 and H3K9 as well as non-histone substrates and functions in the regulation of gene expression as a transcriptional repressor or activator. This enzyme plays a pivotal role in various physiological processes, including development, differentiation, inflammation, thermogenesis, neuronal and cerebral physiology, and the maintenance of stemness in stem cells. LSD1 also participates in pathological processes, including cancer as the most representative disease. It promotes oncogenesis by facilitating the survival of cancer cells and by generating a pro-cancer microenvironment. In this review, we discuss the role of LSD1 in several aspects of cancer, such as hypoxia, epithelial-to-mesenchymal transition, stemness versus differentiation of cancer stem cells, as well as anti-tumor immunity. Additionally, the current understanding of the involvement of LSD1 in various other pathological processes is discussed.

Lysine demethylase LSD1 delivered via small extracellular vesicles promotes gastric cancer cell stemness

Several studies have examined the functions of nucleic acids in small extracellular vesicles (sEVs). However, much less is known about the protein cargos of sEVs and their functions in recipient cells. This study demonstrates the presence of lysine-specific demethylase 1 (LSD1), which is the first identified histone demethylase, in the culture medium of gastric cancer cells. We show that sEVs derived from gastric cancer cells and the plasma of patients with gastric cancer harbor LSD1. The shuttling of LSD1-containing sEVs from donor cells to recipient gastric cancer cells promotes cancer cell stemness by positively regulating the expression of Nanog, OCT4, SOX2, and CD44. Additionally, sEV-delivered LSD1 suppresses oxaliplatin response of recipient cells in vitro and in vivo, whereas LSD1-depleted sEVs do not. Taken together, we demonstrate that LSD1-loaded sEVs can promote stemness and chemoresistance to oxaliplatin. These findings suggest that the LSD1 content of sEV could serve as a biomarker to predict oxaliplatin response in gastric cancer patients.

Non-Histone Protein Methylation: Biological Significance and Bioengineering Potential

Protein methylation is a key post-translational modification whose effects on gene expression have been intensively studied over the last two decades. Recently, renewed interest in non-histone protein methylation has gained momentum for its role in regulating important cellular processes and the activity of many proteins, including transcription factors, enzymes, and structural complexes. The extensive and dynamic role that protein methylation plays within the cell also highlights its potential for bioengineering applications. Indeed, while synthetic histone protein methylation has been extensively used to engineer gene expression, engineering of non-histone protein methylation has not been fully explored yet. Here, we report the latest findings, highlighting how non-histone protein methylation is fundamental for certain cellular functions and is implicated in disease, and review recent efforts in the engineering of protein methylation.

Conformational Selection in Ligand Recognition by the First Tudor Domain of PHF20L1

The first Tudor domain (Tudor1) of PHF20L1 recognizes (non)histone methylation to play versatile roles. However, the underlying ligand-recognition mechanism remains unknown as a closed state revealed in the free-form structure. NMR relaxation dispersion and molecular dynamics simulations suggest a pre-existing low-population conformation with a remarkable rearrangement of aromatic cage residues of PHF20L1 Tudor1. Such an open-form conformation is utilized to recognize lysine 142 methylated DNMT1, a cosolvent, and an NMR fragment screening hit, as revealed by the complex crystal structures. Intriguingly, the ligand binding capacity was enhanced by mutation that tunes up the open-state population only. The recognition of DNMT1 by PHF20L1 was further validated in cancer cells. This conformational selection mechanism will enable the discovery of small molecule inhibitors against the seemingly "undruggable" PHF20L1 Tudor1.

Functional characterization of SOX2 as an anticancer target

SOX2 is a well-characterized pluripotent factor that is essential for stem cell self-renewal, reprogramming, and homeostasis. The cellular levels of SOX2 are precisely regulated by a complicated network at the levels of transcription, post-transcription, and post-translation. In many types of human cancer, SOX2 is dysregulated due to gene amplification and protein overexpression. SOX2 overexpression is associated with poor survival of cancer patients. Mechanistically, SOX2 promotes proliferation, survival, invasion/metastasis, cancer stemness, and drug resistance. SOX2 is, therefore, an attractive anticancer target. However, little progress has been made in the efforts to discover SOX2 inhibitors, largely due to undruggable nature of SOX2 as a transcription factor. In this review, we first briefly introduced SOX2 as a transcription factor, its domain structure, normal physiological functions, and its involvement in human cancers. We next discussed its role in embryonic development and stem cell-renewal. We then mainly focused on three aspects of SOX2: (a) the regulatory mechanisms of SOX2, including how SOX2 level is regulated, and how SOX2 cross-talks with multiple signaling pathways to control growth and survival; (b) the role of SOX2 in tumorigenesis and drug resistance; and (c) current drug discovery efforts on targeting SOX2, and the future perspectives to discover specific SOX2 inhibitors for effective cancer therapy.

Inhibition of LSD1 promotes the differentiation of human induced pluripotent stem cells into insulin-producing cells

BACKGROUND

Human induced pluripotent stem cells (hiPSCs) represent a potentially unlimited source of pancreatic endocrine lineage cells. Although insulin-producing β cells derived from hiPSCs have been successfully induced, much work remains to be done to achieve mature β cells. Lysine-specific demethylase 1 (LSD1) plays an important role in the regulation of hiPSC self-renewal and differentiation. We propose a new strategy to acquire insulin-producing cells (IPCs) from hiPSCs by knocking down LSD1.

METHODS

Knockdown of LSD1 in hiPSCs with five shRNA. Assessment of the effects of shRNA on hiPSC proliferation, cell cycle, and apoptosis. Using knockdown hiPSCs with 31.33% LSD1 activity, we achieved a four-step differentiation into IPCs and test its differentiation efficiency, morphology, and marker genes and proteins. We implanted the IPCs into the renal subcapsular of SCID-Beige diabetic mice to evaluate the hypoglycemic effect in vivo. We tested LSD1 and HDAC1 whether they are present in the CoREST complex through IP-WB, and analyzed LSD1, CoREST, HDAC1, H3K4me2/me3, and H3K27me3 protein expression before and after knockdown of LSD1.

RESULTS

Differentiated hiPSCs were 38.32% ± 3.54% insulin-positive cells and released insulin/C-peptide in response to glucose stimulus in a manner comparable to adult human islets. Most of the IPCs co-expressed mature β cell-specific markers. When transplanted under the left renal capsule of SCID-Beige diabetic mice, these IPCs reversed hyperglycemia, leading to a significant increase in the definitive endoderm cells. IP-WB results showed that LSD1, HDAC1, and CoREST formed a complex in hiPSCs. Chip-PCR results showed that LSD1, HDAC1, and CoREST were enriched in the same district during the SOX17 and FOXA2 promoter region. Inhibition of LSD1 would not affect the level of CoREST but decreased the HDAC1 expressions. The H3K4me2/me3 and H3K9act level of SOX17 and FOXA2 promoter region increased after inhibited of LSD1, and promoted transcriptional activation. The H3K4me2/me3 and H3K9act level of OCT4 and SOX2 promoter region decreased with the transcriptional repressed.

CONCLUSIONS

LSD1 regulated histone methylation and acetylation in promoter regions of pluripotent or endodermal genes. Our results suggest a highly efficient approach to producing IPCs from hiPSCs.

Growth Factor Independence 1B-Mediated Transcriptional Repression and Lineage Allocation Require Lysine-Specific Demethylase 1-Dependent Recruitment of the BHC Complex

Growth factor independence 1B (GFI1B) coordinates assembly of transcriptional repressor complexes comprised of corepressors and histone-modifying enzymes to control gene expression programs governing lineage allocation in hematopoiesis. Enforced expression of GFI1B in K562 erythroleukemia cells favors erythroid over megakaryocytic differentiation, providing a platform to define molecular determinants of binary fate decisions triggered by GFI1B. We deployed proteome-wide proximity labeling to identify factors whose inclusion in GFI1B complexes depends upon GFI1B's obligate effector, lysine-specific demethylase 1 (LSD1). We show that GFI1B preferentially recruits core and putative elements of the BRAF-histone deacetylase (HDAC) (BHC) chromatin-remodeling complex (LSD1, RCOR1, HMG20A, HMG20B, HDAC1, HDAC2, PHF21A, GSE1, ZMYM2, and ZNF217) in an LSD1-dependent manner to control acquisition of erythroid traits by K562 cells. Among these elements, depletion of both HMG20A and HMG20B or of GSE1 blocks GFI1B-mediated erythroid differentiation, phenocopying impaired differentiation brought on by LSD1 depletion or disruption of GFI1B-LSD1 binding. These findings demonstrate the central role of the GFI1B-LSD1 interaction as a determinant of BHC complex recruitment to enable cell fate decisions driven by GFI1B.

Proteolysis of methylated SOX2 protein is regulated by L3MBTL3 and CRL4DCAF5 ubiquitin ligase

SOX2 is a dose-dependent master stem cell protein that controls the self-renewal and pluripotency or multipotency of embryonic stem (ES) cells and many adult stem cells. We have previously found that SOX2 protein is monomethylated at lysine residues 42 and 117 by SET7 methyltransferase to promote SOX2 proteolysis, whereas LSD1 and PHF20L1 act on both methylated Lys-42 and Lys-117 to prevent SOX2 proteolysis. However, the mechanism by which the methylated SOX2 protein is degraded remains unclear. Here, we report that L3MBTL3, a protein with the malignant-brain-tumor (MBT) methylation-binding domain, is required for SOX2 proteolysis. Our studies showed that L3MBTL3 preferentially binds to the methylated Lys-42 in SOX2, although mutation of Lys-117 also partially reduces the interaction between SOX2 and L3MBTL3. The direct binding of L3MBTL3 to the methylated SOX2 protein leads to the recruitment of the CRL4^DCAF5 ubiquitin E3 ligase to target SOX2 protein for ubiquitin-dependent proteolysis. Whereas loss of either LSD1 or PHF20L1 destabilizes SOX2 protein and impairs the self-renewal and pluripotency of mouse ES cells, knockdown of L3MBTL3 or DCAF5 is sufficient to restore the protein levels of SOX2 and rescue the defects of mouse ES cells caused by LSD1 or PHF20L1 deficiency. We also found that retinoic acid-induced differentiation of mouse ES cells is accompanied by the enhanced degradation of the methylated SOX2 protein at both Lys-42 and Lys-117. Our studies provide novel insights into the mechanism by which the methylation-dependent degradation of SOX2 protein is controlled by the L3MBTL3-CRL4^DCAF5 ubiquitin ligase complex.

Proliferating cell nuclear antigen interacts with the CRL4 ubiquitin ligase subunit CDT2 in DNA synthesis-induced degradation of CDT1

During DNA replication or repair, the DNA polymerase cofactor, proliferating cell nuclear antigen (PCNA), homotrimerizes and encircles the replicating DNA, thereby acting as a DNA clamp that promotes DNA polymerase processivity. The formation of the PCNA trimer is also essential for targeting the replication-licensing protein, chromatin-licensing, and DNA replication factor 1 (CDT1), for ubiquitin-dependent proteolysis to prevent chromosomal DNA re-replication. CDT1 uses its PCNA-interacting peptide box (PIP box) to interact with PCNA, and the CRL4 E3 ubiquitin ligase subunit CDT2 is recruited through the formation of PCNA-CDT1 complexes. However, it remains unclear how CDT1 and many other PIP box-containing proteins are marked for degradation by the CRL4^CDT2 ubiquitin ligase during DNA replication or damage. Here, using recombinant protein expression coupled with site-directed mutagenesis, we report that CDT2 and PCNA directly interact and this interaction depends on the presence of a highly conserved, C-terminal PIP box-like region in CDT2. Deletion or mutation of this region abolished the CDT2-PCNA interaction between CDT2 and PCNA both in vitro and in vivo Moreover, PCNA-dependent CDT1 degradation in response to DNA damage and replication during the cell cycle requires an intact PIP box in CDT2. The requirement of the PIP boxes in both CDT2 and its substrate CDT1 suggests that the formation of the PCNA trimeric clamp around DNA during DNA replication and repair may bring together CDT1 and CRL4^CDT2 ubiquitin E3 ligase to target CDT1 for proteolysis in a DNA synthesis-dependent manner.