Conserved Pseudoknots in lncRNA MEG3 Are Essential for Stimulation of the p53 Pathway.

Long non-coding RNAs (lncRNAs) are key regulatory molecules, but unlike with other RNAs, the direct link between their tertiary structure motifs and their function has proven elusive. Here we report structural and functional studies of human maternally expressed gene 3 (MEG3), a tumor suppressor lncRNA that modulates the p53 response. We found that, in an evolutionary conserved region of MEG3, two distal motifs interact by base complementarity to form alternative, mutually exclusive pseudoknot structures ("kissing loops"). Mutations that disrupt these interactions impair MEG3-dependent p53 stimulation in vivo and disrupt MEG3 folding in vitro. These findings provide mechanistic insights into regulation of the p53 pathway by MEG3 and reveal how conserved motifs of tertiary structure can regulate lncRNA biological function.

Scrambling free combinatorial labeling of alanine-ß, isoleucine-δ1, leucine-proS and valine-proS methyl groups for the detection of long range NOEs.

Specific isotopic labeling of methyl groups in proteins has greatly extended the applicability of solution NMR spectroscopy. Simultaneous labeling of the methyl groups of several different amino acid types can offer a larger number of useful probes that can be used for structural characterisations of challenging proteins. Herein, we propose an improved AILV methyl-labeling protocol in which L and V are stereo-specifically labeled. We show that 2-ketobutyrate cannot be combined with Ala and 2-acetolactate (for the stereo-specific labeling of L and V) as this results in co-incorporation incompatibility and isotopic scrambling. Thus, we developed a robust and cost-effective enzymatic synthesis of the isoleucine precursor, 2-hydroxy-2-(1'-[(2)H2], 2'-[(13)C])ethyl-3-keto-4-[(2)H3]butanoic acid, as well as an incorporation protocol that eliminates metabolic leakage. We show that application of this labeling scheme to a large 82 kDa protein permits the detection of long-range (1)H-(1)H NOE cross-peaks between methyl probes separated by up to 10 Å.

Visualizing the functional 3D shape and topography of long noncoding RNAs by single-particle atomic force microscopy and in-solution hydrodynamic techniques.

Long noncoding RNAs (lncRNAs) are recently discovered transcripts that regulate vital cellular processes, such as cellular differentiation and DNA replication, and are crucially connected to diseases. Although the 3D structures of lncRNAs are key determinants of their function, the unprecedented molecular complexity of lncRNAs has so far precluded their 3D structural characterization at high resolution. It is thus paramount to develop novel approaches for biochemical and biophysical characterization of these challenging targets. Here, we present a protocol that integrates non-denaturing lncRNA purification with in-solution hydrodynamic analysis and single-particle atomic force microscopy (AFM) imaging to produce highly homogeneous lncRNA preparations and visualize their 3D topology at ~15-Å resolution. Our protocol is suitable for imaging lncRNAs in biologically active conformations and for measuring structural defects of functionally inactive mutants that have been identified by cell-based functional assays. Once optimized for the specific target lncRNA of choice, our protocol leads from cloning to AFM imaging within 3-4 weeks and can be implemented using state-of-the-art biochemical and biophysical instrumentation by trained researchers familiar with RNA handling and supported by AFM and small-angle X-ray scattering (SAXS) experts.

Topology and enzymatic properties of a canonical Polycomb repressive complex 1 isoform.

Polycomb repressive complex 1 (PRC1) catalyses monoubiquitination of histone H2A on Lys119, promoting gene silencing. Cells at different developmental stages and in different tissues express different PRC1 isoforms. The topology, subunit composition, structural architecture and molecular mechanism of most of these isoforms are still poorly characterized. Here, we have purified a PRC1 isoform comprising subunits RING1B, PCGF2, CBX2 and PHC2, two stable subcomplexes (RING1B-PCGF2 and RING1B-PHC2) and the catalytic subunit RING1B in isolation. By crosslinking mass spectrometry, we identified novel interactions between RING1B and the three non-catalytic subunits. Biochemical, biophysical, and enzymatic data suggest that CBX2 and PHC2 play a structural role, whereas PCGF2 also modulates catalysis. Our data offer insights into the molecular architecture of PRC1 and its histone ubiquitination activity.

An optimized isotopic labelling strategy of isoleucine-γ2 methyl groups for solution NMR studies of high molecular weight proteins.

An efficient synthetic route is proposed to produce 2-hydroxy-2-ethyl-3-oxobutanoate for the specific labelling of Ile methyl-γ(2) groups in proteins. The (2)H, (13)C-pattern of the biosynthetic precursor has been designed to optimize magnetization transfer, in large proteins, between these important structural probes and their corresponding backbone nuclei.

Dynamics induced by ß-lactam antibiotics in the active site of Bacillus subtilis L,D-transpeptidase.

ß-lactams inhibit peptidoglycan polymerization by acting as suicide substrates of essential d,d-transpeptidases. Bypass of these enzymes by unrelated l,d-transpeptidases results in ß-lactam resistance, although carbapenems remain unexpectedly active. To gain insight into carbapenem specificity of l,d-transpeptidases (Ldts), we solved the nuclear magnetic resonance (NMR) structures of apo and imipenem-acylated Bacillus subtilis Ldt and show that the cysteine nucleophile is present as a neutral imidazole-sulfhydryl pair in the substrate-free enzyme. NMR relaxation dispersion does not reveal any preexisting conformational exchange in the apoenzyme, and change in flexibility is not observed upon noncovalent binding of ß-lactams (K(D) > 37.5 mM). In contrast, covalent modification of active cysteine by both carbapenems and 2-nitro-5-thiobenzoate induces backbone flexibility that does not result from disruption of the imidazole-sulfhydryl proton interaction or steric hindrance. The chemical step of the reaction determines enzyme specificity since no differences in drug affinity were observed.