Conformational adaptation of UNCG loops upon crowding

International audienceIf the A-form helix is the major structural motif found in RNA, the loops that cap them constitute the second most important family of motifs. Among those, two are over-represented, the GNRA and the UNCG tetraloops. Although one might think that these consensus sequences imply distinct and specific architectures, such is not the case. Recent surveys of RNA structures deposited in the PDB show that GNRA and UNCG tetraloops can adopt tertiary folds that are very different from their canonical conformations, characterized by the presence of a U-turn of a Z-turn, respectively. In this study, crystallographic data derived from both a Lariat-Capping (LC) ribozyme and a group II intron ribozyme reveal that a given UUCG tetraloop can adopt a distinct fold depending on its direct structural environment. Specifically, when the crystal packing applies relaxed constraints on the loop, the canonical Z-turn conformation is observed. In contrast, a highly-packed environment induces "squashing" of the tetraloop by distorting its sugar-phosphate backbone in a specific way that expels the first and fourth nucleobases out of the loop, and falls in van der Waals distance of the last base pair of the helix, taking the place of the pair formed between the first and fourth residues in Z-turn loops. Importantly, the biological relevance of our observations is supported by the presence of similarly deformed loops in the highly-packed environment of the ribosome and in a complex between a dsRNA and a yeast RNase III. The finding that Z-turn loops can change conformation under higher molecular packing suggests that, in addition to their early demonstrated role in stabilizing RNA folding, they may also contribute to the three-dimensional structure of RNA by mediating tertiary interactions with distal residues

Crystal structure of the aminoglycosides N -acetyltransferase Eis2 from Mycobacterium abscessus

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Differential modes of peptide binding onto replicative sliding clamps from various bacterial origins

Bacterial sliding clamps are molecular hubs that interact with many proteins involved in DNA metabolism through their binding, via a conserved peptidic sequence, into a universally conserved pocket. This interacting pocket is acknowledged as a potential molecular target for the development of new antibiotics. We previously designed short peptides with an improved affinity for the Escherichia coli binding pocket. Here we show that these peptides differentially interact with other bacterial clamps, despite the fact that all pockets are structurally similar. Thermodynamic and modeling analyses of the interactions differentiate between two categories of clamps: group I clamps interact efficiently with our designed peptides and assemble the Escherichia coli and related orthologs clamps, whereas group II clamps poorly interact with the same peptides and include Bacillus subtilis and other Gram-positive clamps. These studies also suggest that the peptide binding process could occur via different mechanisms, which depend on the type of clamp.</p

SECIS-binding protein 2, a key player in selenoprotein synthesis, is an intrinsically disordered protein.

International audienceSelenocysteine (Sec) is co-translationally incorporated into selenoproteins at a reprogrammed UGA codon. In mammals, this requires a dedicated machinery comprising a stem-loop structure in the 3' UTR RNA (the SECIS element) and the specific SECIS Binding Protein 2. In this report, disorder-prediction methods and several biophysical techniques showed that ca. 70% of the SBP2 sequence is disordered, whereas the RNA binding domain appears to be folded and functional. These results are consistent with a recent report on the role of the Hsp90 chaperone for the folding of SBP2 and other functionally unrelated proteins bearing an RNA binding domain homologous to SBP2