Zhike Lu

Identification of 67 Histone Marks and Histone Lysine Crotonylation as a New Type of Histone Modification

We report the identification of 67 previously undescribed histone modifications, increasing the current number of known histone marks by about 70%. We further investigated one of the marks, lysine crotonylation (Kcr), confirming that it represents an evolutionarily-conserved histone posttranslational modification. The unique structure and genomic localization of histone Kcr suggest that it is mechanistically and functionally different from histone lysine acetylation (Kac). Specifically, in both human somatic and mouse male germ cell genomes, histone Kcr marks either active promoters or potential enhancers. In male germinal cells immediately following meiosis, Kcr is enriched on sex chromosomes and specifically marks testis-specific genes, including a significant proportion of X-linked genes that escape sex chromosome inactivation in haploid cells. These results therefore dramatically extend the repertoire of histone PTM sites and designate Kcr as a specific mark of active sex chromosome-linked genes in postmeiotic male germ cells.

N6-methyladenosine-dependent regulation of messenger RNA stability.

N6-methyladenosine (m6A) is the most prevalent internal (non-cap) modification present in the messenger RNA of all higher eukaryotes. Although essential to cell viability and development, the exact role of m6A modification remains to be determined. The recent discovery of two m6A demethylases in mammalian cells highlighted the importance of m6A in basic biological functions and disease. Here we show that m6A is selectively recognized by the human YTH domain family 2 (YTHDF2) ‘reader’ protein to regulate mRNA degradation. We identified over 3,000 cellular RNA targets of YTHDF2, most of which are mRNAs, but which also include non-coding RNAs, with a conserved core motif of G(m6A)C. We further establish the role of YTHDF2 in RNA metabolism, showing that binding of YTHDF2 results in the localization of bound mRNA from the translatable pool to mRNA decay sites, such as processing bodies. The carboxy-terminal domain of YTHDF2 selectively binds to m6A-containing mRNA, whereas the amino-terminal domain is responsible for the localization of the YTHDF2–mRNA complex to cellular RNA decay sites. Our results indicate that the dynamic m6A modification is recognized by selectively binding proteins to affect the translation status and lifetime of mRNA.

ALKBH5 is a mammalian RNA demethylase that impacts RNA metabolism and mouse fertility.

N(6)-methyladenosine (m(6)A) is the most prevalent internal modification of messenger RNA (mRNA) in higher eukaryotes. Here we report ALKBH5 as another mammalian demethylase that oxidatively reverses m(6)A in mRNA in vitro and in vivo. This demethylation activity of ALKBH5 significantly affects mRNA export and RNA metabolism as well as the assembly of mRNA processing factors in nuclear speckles. Alkbh5-deficient male mice have increased m(6)A in mRNA and are characterized by impaired fertility resulting from apoptosis that affects meiotic metaphase-stage spermatocytes. In accordance with this defect, we have identified in mouse testes 1,551 differentially expressed genes that cover broad functional categories and include spermatogenesis-related mRNAs involved in the p53 functional interaction network. The discovery of this RNA demethylase strongly suggests that the reversible m(6)A modification has fundamental and broad functions in mammalian cells.

A METTL3-METTL14 complex mediates mammalian nuclear RNA N6-adenosine methylation

N6-methyladenosine (m6A) is the most prevalent and reversible internal modification in mammalian messenger and non-coding RNAs. We report here that human METTL14 catalyzes m6A RNA methylation. Together with METTL3, the only previously known m6A methyltransferase, these two proteins form a stable heterodimer core complex of METTL3-14 that functions in cellular m6A deposition on mammalian nuclear RNAs. WTAP, a mammalian splicing factor, can interact with this complex and affect this methylation.

N6-methyladenosine Modulates Messenger RNA Translation Efficiency

N(6)-methyladenosine (m(6)A) is the most abundant internal modification in mammalian mRNA. This modification is reversible and non-stoichiometric and adds another layer to the dynamic control of mRNA metabolism. The stability of m(6)A-modified mRNA is regulated by an m(6)A reader protein, human YTHDF2, which recognizes m(6)A and reduces the stability of target transcripts. Looking at additional functional roles for the modification, we find that another m(6)A reader protein, human YTHDF1, actively promotes protein synthesis by interacting with translation machinery. In a unified mechanism of m(6)A-based regulation in the cytoplasm, YTHDF2-mediated degradation controls the lifetime of target transcripts, whereas YTHDF1-mediated translation promotion increases translation efficiency, ensuring effective protein production from dynamic transcripts that are marked by m(6)A. Therefore, the m(6)A modification in mRNA endows gene expression with fast responses and controllable protein production through these mechanisms.

The first identification of lysine malonylation substrates and its regulatory enzyme

Protein post-translational modifications (PTMs) at the lysine residue, such as lysine methylation, acetylation, and ubiquitination, are diverse, abundant, and dynamic. They play a key role in the regulation of diverse cellular physiology. Here we report discovery of a new type of lysine PTM, lysine malonylation (Kmal). Kmal was initially detected by mass spectrometry and protein sequence-database searching. The modification was comprehensively validated by Western blot, tandem MS, and high-performance liquid chromatography of synthetic peptides, isotopic labeling, and identification of multiple Kmal substrate proteins. Kmal is a dynamic and evolutionarily conserved PTM observed in mammalian cells and bacterial cells. In addition, we demonstrate that Sirt5, a member of the class III lysine deacetylases, can catalyze lysine demalonylation and lysine desuccinylation reactions both in vitro and in vivo. This result suggests the possibility of nondeacetylation activity of other class III lysine deacetylases, especially those without obvious acetylation protein substrates. Our results therefore reveal a new type of PTM pathway and identify the first enzyme that can regulate lysine malonylation and lysine succinylation status.

Structural basis for selective binding of m6A RNA by the YTHDC1 YTH domain

N(6)-methyladenosine (m(6)A) is the most abundant internal modification of nearly all eukaryotic mRNAs and has recently been reported to be recognized by the YTH domain family proteins. Here we present the crystal structures of the YTH domain of YTHDC1, a member of the YTH domain family, and its complex with an m(6)A-containing RNA. Our structural studies, together with transcriptome-wide identification of YTHDC1-binding sites and biochemical experiments, not only reveal the specific mode of m(6)A-YTH binding but also explain the preferential recognition of the GG(m(6)A)C sequences by YTHDC1.

Lysine 2-hydroxyisobutyrylation is a widely distributed active histone mark

We report the identification of a new type of histone mark, lysine 2-hydroxyisobutyrylation (Khib), and identify the mark at 63 human and mouse histone Khib sites, including 27 unique lysine sites that are not known to be modified by lysine acetylation (Kac) and lysine crotonylation (Kcr). This histone mark was initially identified by MS and then validated by chemical and biochemical methods. Histone Khib shows distinct genomic distributions from histone Kac or histone Kcr during male germ cell differentiation. Using chromatin immunoprecipitation sequencing, gene expression analysis and immunodetection, we show that in male germ cells, H4K8hib is associated with active gene transcription in meiotic and post-meiotic cells. In addition, H4K8ac-associated genes are included in and constitute only a subfraction of H4K8hib-labeled genes. The histone Khib mark is conserved and widely distributed, has high stoichiometry and induces a large structural change. These findings suggest its critical role on the regulation of chromatin functions.

Quantitative Acetylome Analysis Reveals the Roles of SIRT1 in Regulating Diverse Substrates and Cellular Pathways

Despite of the progress in identifying many Lys acetylation (Kac) proteins, Kac substrates for Kac-regulatory enzymes remain largely unknown, presenting a major knowledge gap in Kac biology. Here we identified and quantified 4623 Kac sites in 1800 Kac proteins in SIRT1+/+ and SIRT1−/− MEF cells, representing the first study to reveal an enzyme-regulated Kac subproteome and the largest Lys acetylome reported to date from a single study. Four hundred eighty-five Kac sites were enhanced by more than 100% after SIRT1 knockout. Our results indicate that SIRT1 regulates the Kac states of diverse cellular pathways. Interestingly, we found that a number of acetyltransferases and major acetyltransferase complexes are targeted by SIRT1. Moreover, we showed that the activities of the acetyltransferases are regulated by SIRT1-mediated deacetylation. Taken together, our results reveal the Lys acetylome in response to SIRT1, provide new insights into mechanisms of SIRT1 function, and offer biomarker candidates for the clinical evaluation of SIRT1-activator compounds.

Identification of Lysine Succinylation Substrates and the Succinylation Regulatory Enzyme CobB in Escherichia coli

Lysine succinylation is a newly identified protein post-translational modification pathway present in both prokaryotic and eukaryotic cells. However, succinylation substrates and regulatory enzyme(s) remain largely unknown, hindering the biological study of this modification. Here we report the identification of 2,580 bacterial lysine succinylation sites in 670 proteins and 2,803 lysine acetylation (Kac) sites in 782 proteins, representing the first lysine succinylation dataset and the largest Kac dataset in wild-type E. coli. We quantified dynamic changes of the lysine succinylation and Kac substrates in response to high glucose. Our data showed that high-glucose conditions led to more lysine-succinylated proteins and enhanced the abundance of succinyllysine peptides more significantly than Kac peptides, suggesting that glucose has a more profound effect on succinylation than on acetylation. We further identified CobB, a known Sir2-like bacterial lysine deacetylase, as the first prokaryotic desuccinylation enzyme. The identification of bacterial CobB as a bifunctional enzyme with lysine desuccinylation and deacetylation activities suggests that the eukaryotic Kac-regulatory enzymes may have enzymatic activities on various lysine acylations with very different structures. In addition, it is highly likely that lysine succinylation could have unique and more profound regulatory roles in cellular metabolism relative to lysine acetylation under some physiological conditions.