Prdm3 and Prdm16 are H3K9me1 methyltransferases required for mammalian heterochromatin integrity.

Heterochromatin serves important functions, protecting genome integrity and stabilizing gene expression programs. Although the Suv39h methyltransferases (KMTs) are known to ensure pericentric H3K9me3 methylation, the mechanisms that initiate and maintain mammalian heterochromatin organization remain elusive. We developed a biochemical assay and used in vivo analyses in mouse embryonic fibroblasts to identify Prdm3 and Prdm16 as redundant H3K9me1-specific KMTs that direct cytoplasmic H3K9me1 methylation. The H3K9me1 is converted in the nucleus to H3K9me3 by the Suv39h enzymes to reinforce heterochromatin. Simultaneous depletion of Prdm3 and Prdm16 abrogates H3K9me1 methylation, prevents Suv39h-dependent H3K9me3 trimethylation, and derepresses major satellite transcription. Most strikingly, DNA-FISH and electron microscopy reveal that combined impairment of Prdm3 and Prdm16 results in disintegration of heterochromatic foci and disruption of the nuclear lamina. Our data identify Prdm3 and Prdm16 as H3K9me1 methyltransferases and expose a functional framework in which anchoring to the nuclear periphery helps maintain the integrity of mammalian heterochromatin.

sST2 translation is regulated by FGF2 via an hnRNP A1-mediated IRES-dependent mechanism.

Translation is an energy-intensive process and tightly regulated. Generally, translation is initiated in a cap-dependent manner. Under stress conditions, typically found within the tumor microenvironment in association with e.g. nutrient deprivation or hypoxia, cap-dependent translation decreases, and alternative modes of translation initiation become more important. Specifically, internal ribosome entry sites (IRES) facilitate translation of specific mRNAs under otherwise translation-inhibitory conditions. This mechanism is controlled by IRES trans-acting factors (ITAF), i.e. by RNA-binding proteins, which interact with and determine the activity of selected IRESs. We aimed at characterizing the translational regulation of the IL-33 decoy receptor sST2, which was enhanced by fibroblast growth factor 2 (FGF2). We identified and verified an IRES within the 5'UTR of sST2. Furthermore, we found that MEK/ERK signaling contributes to FGF2-induced, sST2-IRES activation and translation. Determination of the sST2-5'UTR structure by in-line probing followed by deletion analyses identified 23 nucleotides within the sST2-5'UTR to be required for optimal IRES activity. Finally, we show that the RNA-binding protein heterogeneous ribonucleoprotein A1 (hnRNP A1) binds to the sST2-5'UTR, acts as an ITAF, and thus controls the activity of the sST2-IRES and consequently sST2 translation. Specifically, FGF2 enhances nuclear-cytoplasmic translocation of hnRNP A1, which requires intact MEK/ERK activity. In summary, we provide evidence that the sST2-5'UTR contains an IRES element, which is activated by a MEK/ERK-dependent increase in cytoplasmic localization of hnRNP A1 in response to FGF2, enhancing the translation of sST2.

Dithiothreitol (DTT) Acts as a Specific, UV-inducible Cross-linker in Elucidation of Protein-RNA Interactions.

Protein-RNA cross-linking by UV irradiation at 254 nm wavelength has been established as an unbiased method to identify proteins in direct contact with RNA, and has been successfully applied to investigate the spatial arrangement of protein and RNA in large macromolecular assemblies, e.g. ribonucleoprotein-complex particles (RNPs). The mass spectrometric analysis of such peptide-RNA cross-links provides high resolution structural data to the point of mapping protein-RNA interactions to specific peptides or even amino acids. However, the approach suffers from the low yield of cross-linking products, which can be addressed by improving enrichment and analysis methods. In the present article, we introduce dithiothreitol (DTT) as a potent protein-RNA cross-linker. In order to evaluate the efficiency and specificity of DTT, we used two systems, a small synthetic peptide from smB protein incubated with U1 snRNA oligonucleotide and native ribonucleoprotein complexes from S. cerevisiae. Our results unambiguously show that DTT covalently participates in cysteine-uracil crosslinks, which is observable as a mass increment of 151.9966 Da (C(4)H(8)S(2)O(2)) upon mass spectrometric analysis. DTT presents advantages for cross-linking of cysteine containing regions of proteins. This is evidenced by comparison to experiments where (tris(2-carboxyethyl)phosphine) is used as reducing agent, and significantly less cross-links encompassing cysteine residues are found. We further propose insertion of DTT between the cysteine and uracil reactive sites as the most probable structure of the cross-linking products.

In vivo characterization of the bacterial intramembrane-cleaving protease RseP using the heme binding tag-based assay iCliPSpy.

Regulated intramembrane proteolysis (RIP) describes the protease-dependent cleavage of transmembrane proteins within the hydrophobic core of cellular membranes. Intramembrane-cleaving proteases (I-CliPs) that catalyze these reactions are found in all kingdoms of life and are involved in a wide range of cellular processes, including signaling and protein homeostasis. I-CLiPs are multispanning membrane proteins and represent challenging targets in structural and enzyme biology. Here we introduce iCLiPSpy, a simple assay to study I-CLiPs in vivo. To allow easy detection of enzyme activity, we developed a heme-binding reporter based on TNFα that changes color after I-CLiP-mediated proteolysis. Co-expression of the protease and reporter in Escherichia coli (E. coli) results in white or green colonies, depending on the activity of the protease. As a proof of concept, we use this assay to study the bacterial intramembrane-cleaving zinc metalloprotease RseP in vivo. iCLiPSpy expands the methodological repertoire for identifying residues important for substrate binding or activity of I-CLiPs and can in principle be adapted to a screening assay for the identification of inhibitors or activators of I-CLiPs, which is of great interest for proteases being explored as biomedical targets.

Merging molecular electron microscopy and mass spectrometry by carbon film-assisted endoproteinase digestion.

Many fundamental processes in the cell are performed by complex macromolecular assemblies that comprise a large number of proteins. Numerous macromolecular assemblies are structurally rather fragile and may suffer during purification, resulting in the partial dissociation of the complexes. These limitations can be overcome by chemical fixation of the assemblies, and recently introduced protocols such as gradient fixation during ultracentrifugation (GraFix) offer advantages for the analysis of fragile macromolecular assemblies. The irreversible fixation, however, is thought to render macromolecular samples useless for studying their protein composition. We therefore developed a novel approach that possesses the advantages of fixation for structure determination by single particle electron microscopy while still allowing a correlative compositional analysis by mass spectrometry. In this method, which we call "electron microscopy carbon film-assisted digestion", macromolecular assemblies are chemically fixed and then adsorbed onto electron microscopical carbon films. Parallel, identically prepared specimens are then subjected to structural investigation by electron microscopy and proteomics analysis by mass spectrometry of the digested sample. As identical sample preparation protocols are used for electron microscopy and mass spectrometry, the results of both methods can directly be correlated. In addition, we demonstrate improved sensitivity and reproducibility of electron microscopy carbon film-assisted digestion as compared with standard protocols. We show that sample amounts of as low as 50 fmol are sufficient to obtain a comprehensive protein composition of two model complexes. We suggest our approach to be an optimization technique for the compositional analysis of macromolecules by mass spectrometry in general.

Mapping the binding site of snurportin 1 on native U1 snRNP by cross-linking and mass spectrometry.

Mass spectrometry allows the elucidation of molecular details of the interaction domains of the individual components in macromolecular complexes subsequent to cross-linking of the individual components. Here, we applied chemical and UV cross-linking combined with tandem mass-spectrometric analysis to identify contact sites of the nuclear import adaptor snurportin 1 to the small ribonucleoprotein particle U1 snRNP in addition to the known interaction of m(3)G cap and snurportin 1. We were able to define previously unknown sites of protein-protein and protein-RNA interactions on the molecular level within U1 snRNP. We show that snurportin 1 interacts with its central m(3)G-cap-binding domain with Sm proteins and with its extreme C-terminus with stem-loop III of U1 snRNA. The crosslinking data support the idea of a larger interaction area between snurportin 1 and U snRNPs and the contact sites identified prove useful for modeling the spatial arrangement of snurportin 1 domains when bound to U1 snRNP. Moreover, this suggests a functional nuclear import complex that assembles around the m(3)G cap and the Sm proteins only when the Sm proteins are bound and arranged in the proper orientation to the cognate Sm site in U snRNA.

Hakai is required for stabilization of core components of the m6A mRNA methylation machinery.

N6-methyladenosine (m6A) is the most abundant internal modification on mRNA which influences most steps of mRNA metabolism and is involved in several biological functions. The E3 ubiquitin ligase Hakai was previously found in complex with components of the m6A methylation machinery in plants and mammalian cells but its precise function remained to be investigated. Here we show that Hakai is a conserved component of the methyltransferase complex in Drosophila and human cells. In Drosophila, its depletion results in reduced m6A levels and altered m6A-dependent functions including sex determination. We show that its ubiquitination domain is required for dimerization and interaction with other members of the m6A machinery, while its catalytic activity is dispensable. Finally, we demonstrate that the loss of Hakai destabilizes several subunits of the methyltransferase complex, resulting in impaired m6A deposition. Our work adds functional and molecular insights into the mechanism of the m6A mRNA writer complex.

CAPRI enables comparison of evolutionarily conserved RNA interacting regions.

RNA-protein complexes play essential regulatory roles at nearly all levels of gene expression. Using in vivo crosslinking and RNA capture, we report a comprehensive RNA-protein interactome in a metazoan at four levels of resolution: single amino acids, domains, proteins and multisubunit complexes. We devise CAPRI, a method to map RNA-binding domains (RBDs) by simultaneous identification of RNA interacting crosslinked peptides and peptides adjacent to such crosslinked sites. CAPRI identifies more than 3000 RNA proximal peptides in Drosophila and human proteins with more than 45% of them forming new interaction interfaces. The comparison of orthologous proteins enables the identification of evolutionary conserved RBDs in globular domains and intrinsically disordered regions (IDRs). By comparing the sequences of IDRs through evolution, we classify them based on the type of motif, accumulation of tandem repeats, conservation of amino acid composition and high sequence divergence.

Cellular differentiation state modulates the mRNA export activity of SR proteins.

SR proteins function in nuclear pre-mRNA processing, mRNA export, and translation. To investigate their cellular dynamics, we developed a quantitative assay, which detects differences in nucleocytoplasmic shuttling among seven canonical SR protein family members. As expected, SRSF2 and SRSF5 shuttle poorly in HeLa cells but surprisingly display considerable shuttling in pluripotent murine P19 cells. Combining individual-resolution cross-linking and immunoprecipitation (iCLIP) and mass spectrometry, we show that elevated arginine methylation of SRSF5 and lower phosphorylation levels of cobound SRSF2 enhance shuttling of SRSF5 in P19 cells by modulating protein-protein and protein-RNA interactions. Moreover, SRSF5 is bound to pluripotency-specific transcripts such as Lin28a and Pou5f1/Oct4 in the cytoplasm. SRSF5 depletion reduces and overexpression increases their cytoplasmic mRNA levels, suggesting that enhanced mRNA export by SRSF5 is required for the expression of pluripotency factors. Remarkably, neural differentiation of P19 cells leads to dramatically reduced SRSF5 shuttling. Our findings indicate that posttranslational modification of SR proteins underlies the regulation of their mRNA export activities and distinguishes pluripotent from differentiated cells.