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An organocatalyst-mediated protein aldol ligation (OPAL) affords C–C linked bioconjugates at neutral pH., The bioconjugation of proteins with small molecules has proved an invaluable strategy for probing and perturbing biological mechanisms. The general use of chemical methods for protein functionalisation can be limited however by the requirement for complicated reaction partners to be present in large excess, and harsh conditions which are incompatible with many protein scaffolds. Herein we describe a site-selective organocatalyst-mediated protein aldol ligation (OPAL) that affords stable carbon–carbon linked bioconjugates at neutral pH. OPAL enables rapid modification of proteins using simple aldehyde probes in minimal excess, and is utilised here in the affinity tagging of proteins in cell lysate. Furthermore we demonstrate that the β-hydroxy aldehyde OPAL product can be functionalised again at neutral pH in a tandem organocatalyst-mediated oxime ligation. This tandem strategy is showcased in the ‘chemical mimicry’ of a previously inaccessible natural dual post-translationally modified protein integral to the pathogenesis of the neglected tropical disease Leishmaniasis.

Double-stranded RNA-binding domains (dsRBDs) are commonly found in modular proteins that interact with RNA. Two varieties of dsRBD exist: canonical Type A dsRBDs interact with dsRNA, while non-canonical Type B dsRBDs lack RNA-binding residues and instead interact with other proteins. In higher eukaryotes, the microRNA biogenesis enzyme Dicer forms a 1:1 association with a dsRNA-binding protein (dsRBP). Human Dicer associates with HIV TAR RNA-binding protein (TRBP) or protein activator of PKR (PACT), while Drosophila Dicer-1 associates with Loquacious (Loqs). In each case, the interaction involves a region of the protein that contains a Type B dsRBD. All three dsRBPs are reported to homodimerize, with the Dicer-binding region implicated in self-association. We report that these dsRBD homodimers display structural asymmetry and that this unusual self-association mechanism is conserved from flies to humans. We show that the core dsRBD is sufficient for homodimerization and that mutation of a conserved leucine residue abolishes self-association. We attribute differences in the self-association properties of Loqs, TRBP and PACT to divergence of the composition of the homodimerization interface. Modifications that make TRBP more like PACT enhance self-association. These data are examined in the context of miRNA biogenesis and the protein/protein interaction properties of Type B dsRBDs.

RNA binding proteins (RBPs) are the primary gene regulators in kinetoplastids as transcriptional control is nearly absent, makingLeishmaniaan exceptional model for investigating methylation of non-histone substrates. Arginine methylation is an evolutionarily conserved protein modification catalyzed by Protein aRginine MethylTransferases (PRMTs). The chromatin modifier PRMT7 is the only Type III PRMT found in higher eukaryotes and a restricted number of unicellular eukaryotes. InLeishmania major, PRMT7 is a cytoplasmic protein implicit in pathogenesis with unknown substrates. Using comparative methyl-SILAC proteomics for the first time in protozoa, we identified 40 putative targets, including 17 RBPs hypomethylated upon PRMT7 knockout. PRMT7 can modify Alba3 and RBP16trans-regulators (mammalian RPP25 and YBX2 homologs, respectively) as direct substratesin vitro. The absence of PRMT7 levelsin vivoselectively reduces Alba3 mRNA-binding capacity to specific target transcripts and can impact the relative stability of RBP16 in the cytoplasm. RNA immunoprecipitation analyses demonstrate PRMT7-dependent methylation promotes Alba3 association with select target transcripts and stability ofδ-amastinsurface antigen. These results highlight a novel role for PRMT7-mediated arginine methylation upon RBP substrates, suggesting a regulatory pathway controlling gene expression and virulence inLeishmania. This work introducesLeishmaniaPRMTs as epigenetic regulators of mRNA metabolism with mechanistic insight into the functional manipulation of RBPs by methylation.

MicroRNAs (miRNAs) are short non-coding RNAs that silence mRNAs. They are generated following transcription and cleavage by the DROSHA/DGCR8 and DICER/TRBP/PACT complexes. Although it is known that components of the miRNA biogenesis machinery can be phosphorylated, it remains poorly understood how these events become engaged during physiological cellular activation. We demonstrate that S6 kinases can phosphorylate the extended C-terminal domain of TRBP and interact with TRBP in situ in primary cells. TRBP serines 283/286 are essential for S6K-mediated TRBP phosphorylation, optimal expression of TRBP, and the S6K-TRBP interaction in human primary cells. We demonstrate the functional relevance of this interaction in primary human dermal lymphatic endothelial cells (HDLECs). Angiopoietin-1 (ANG1) can augment miRNA biogenesis in HDLECs through enhancing TRBP phosphorylation and expression in an S6K2-dependent manner. We propose that the S6K2/TRBP node controls miRNA biogenesis in HDLECs and provides a molecular link between the mTOR pathway and the miRNA biogenesis machinery.

HIV TAR RNA‐binding protein (TRBP) and Protein Activator of PKR (PACT) are double‐stranded (ds) RNA‐binding proteins that participate in both small regulatory RNA biogenesis and the response to viral dsRNA. Despite considerable progress toward understanding the structure–function relationship of TRBP and PACT, their specific roles in these seemingly distinct cellular pathways remain unclear. Both proteins are composed of three copies of the double‐stranded RNA‐binding domain, two of which interact with dsRNA, while the C‐terminal copy mediates protein–protein interactions. PACT and TRBP are found in a complex with the endonuclease Dicer and facilitate processing of immature microRNAs. Their precise contribution to the Dicing step has not yet been defined: possibilities include precursor recruitment, rearrangement of dsRNA within the complex, loading the processed microRNA into the RNA‐induced silencing complex, and distinguishing different classes of small dsRNA. TRBP and PACT also interact with the viral dsRNA sensors retinoic acid‐inducible gene I (RIG‐I) and double‐stranded RNA‐activated protein kinase (PKR). Current models suggest that PACT enables RIG‐I to detect a wider range of viral dsRNAs, while TRBP and PACT exert opposing regulatory effects on PKR. Here, the evidence that implicates TRBP and PACT in regulatory RNA processing and viral dsRNA sensing is reviewed and discussed in the context of their molecular structure. The broader implications of a link between microRNA biogenesis and the innate antiviral response pathway are also considered. WIREs RNA 2015, 6:271–289. doi: 10.1002/wrna.1272 This article is categorized under: 1RNA Interactions with Proteins and Other Molecules > Protein–RNA Interactions: Functional Implications2RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms3Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs

Made available in DSpace on 2016-10-10T03:52:59Z (GMT). No. of bitstreams: 5 Characterization and Manipulation of the.PDF: 747017 bytes, checksum: 6435e6ddbf6e53c28b9c9fa94f1381dc (MD5) license_url: 52 bytes, checksum: 3d480ae6c91e310daba2020f8787d6f9 (MD5) license_text: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) license_rdf: 23892 bytes, checksum: afd5dad10b1d1e6dc10c8c5d25222c7a (MD5) license.txt: 1887 bytes, checksum: 445d1980f282ec865917de35a4c622f6 (MD5) Previous issue date: 2004 In mammals, the enzyme dimethylarginine dimethylaminohydrolase (DDAH) is implicated in the regulation of the cellular levels of asymmetric methylarginines, small molecule metabolites that themselves represent a family of endogenous inhibitors of nitric oxide synthase (NOS). The involvement of DDAH function in the regulation of NOS makes this enzyme a potentially attractive therapeutic target. DDAH from the bacterium Pseudomonas aeruginosa (PaDDAH) is so far the only structurally tractable homologue of mammalian DDAH isoforms. To complement the recent crystal structure of this protein, we show by hydrodynamic measurements that PaDDAH exists in dynamic equilibrium between monomer (ca 29 kDa) and symmetric homodimer (ca 58 kDa) states with a dimer dissociation constant, Kd , 500 nM. For the purposes of NMR-based approaches to the study of this enzyme’s interactions with substrate and inhibitor ligands, it would be useful to obtain the protein in monomeric form. Through detailed analysis of the homodimer PaDDAH crystal structure we identified key residues involved in the protomer–protomer interface and targeted these for mutation. The hydrodynamic and self-associative properties of a series of PaDDAH interface mutants were analyzed by concentration-dependent analytical size-exclusion chromatography and sedimentation equilibrium analytical ultracentrifugation. The individual substitution of several of the interface residues shifts the equilibrium position towards the monomer, which allowed the design of a double mutant variant (Arg40 ! Glu, Arg98 ! His) that behaves exclusively as a stable monomer, yet retains greater than 95% catalytic activity compared to wild-type. Comparative twodimensional 1H, 15N heteronuclear NMR spectra indicate that the double mutant remains a monomer even at ,1 mM concentration. Accordingly, the double mutant PaDDAH is an attractive template for further NMR-based investigations of the enzyme mechanism and characterization of ligand-binding and inhibitor-binding profiles. These results indicate that dimerization of PaDDAH is not critical for the maintenance of the biological function of the protein. These results are discussed in the context of known modes of self-association between structurally related, but functionally distinct, members of the b/a-propeller fold class. Publicado