Base-type-selective high-resolution 13C edited NOESY for sequential assignment of large RNAs.

Extensive spectral overlap presents a major problem for the NMR study of large RNAs. Here we present NMR techniques for resolution enhancement and spectral simplification of fully 13C labelled RNA. High-resolution 1H-13C correlation spectra are obtained by combining TROSY-type experiments with multiple-band-selective homonuclear 13C decoupling. An additional C-C filter sequence performs base-type-selective spectral editing. Signal loss during the filter is significantly reduced because of TROSY-type spin evolution. These tools can be inserted in any 13C-edited multidimensional NMR experiment. As an example we have chosen the 13C-edited NOESY which is a crucial experiment for sequential resonance assignment of RNA. Application to a 33-nucleotide RNA aptamer and a 76-nucleotide tRNA illustrates the potential of this new methodology.

Simultaneous determination of disulphide bridge topology and three-dimensional structure using ambiguous intersulphur distance restraints: possibilities and limitations.

Knowledge of the native disulphide bridge topology allows the introduction of conformational restraints between remote parts of the peptide chain. This information is therefore of great importance for the successful determination of the three-dimensional structure of cysteine-rich proteins by NMR spectroscopy. In this paper we investigate the limitations of using ambiguous intersulphur restraints [Nilges, M. (1995) J. Mol. Biol., 245, 645-660] associated with NMR experimental information to determine the native disulphide bridge pattern. Using these restraints in a simulated annealing protocol we have determined the correct topology of numerous examples, including a protein with seven disulphide bridges (phospholipase A2) and a protein in which 25% of the total number of residues are cysteines (mu-conotoxin GIIIB). We have also characterised the behaviour of the method when only limited experimental data is available, and find that the proposed protocol permits disulphide bridge determination even with a small number of restraints (around 5 NOEs--including a long-range restraint--per residue). In addition, we have shown that under these conditions the use of a reduced penalty function allows the identification of misassigned NOE restraints. These results indicate that the use of ambiguous intersulphur distances with the proposed simulated annealing protocol is a general method for the determination of disulphide bridge topology, particularly interesting in the first steps of NMR study of cysteine-rich proteins. Comparison with previously proposed protocols indicates that the presented method is more reliable and the interpretation of results is straightforward.

Pi7, an orphan peptide from the scorpion Pandinus imperator: a 1H-NMR analysis using a nano-NMR Probe.

The three-dimensional solution structure of a novel peptide, Pi7, purified from the venom of the scorpion Pandinus imperator, and for which no specific receptor has been found yet, was determined by two-dimensional homonuclear proton NMR methods from a nanomole amount of compound using a nano-nmr probe. Pandinus imperator peptide 7 does not block voltage-dependent K(+)-channels and does not displace labeled noxiustoxin from rat brain synaptosomal membranes. The toxin has 38 amino acid residues and, similarly to Pi1, is stabilized by four disulfide bridges (Cys6-Cys27, Cys12-Cys32, Cys16-Cys34, and Cys22-Cys37). In addition, the lysine at position 26 crucial for potassium-channel blocking is replaced in Pi7 by an arginine. Tyrosine 34, equivalent to Tyr36 of ChTX is present, but the N-terminal positions 1 and 2 are occupied by two acidic residues Asp and Glu, respectively. The dihedral angles and distance restraints obtained from measured NMR parameters were used in structural calculations in order to determine the conformation of the peptide. The disulfide-bridge topology was established using distance restraints allowing ambiguous partners between S atoms combined with NMR-derived structural information. The structure is organized around a short alpha-helix spanning residues Thr9 to Thr20/Gly21 and a beta-sheet. These two elements of secondary structure are stabilized by two disulfide bridges, Cys12-Cys32 and Cys16-Cys34. The antiparallel beta-sheet is composed of two strands extending from Asn22 to Cys34 with a tight turn at Ile28-Asn29 in contact with the N-terminal fragment Ile4 to Cys6.

Structural information on a cecropin-like synthetic peptide, Shiva-3 toxic to the sporogonic development of Plasmodium berghei.

This study is a contribution towards the understanding of the mode of action of Shiva-3 and more generally that of cecropin-like peptides. Structural information on Shiva-3 (a cecropin-like synthetic peptide) in water and in a membrane-mimicking environment (trifluoroethanol alcohol, SDS) were obtained using analytical centrifugation, CD and NMR spectroscopies. A total of 20 converged structures were retained on the basis of 197 non-redundant experimental constraints, including 166 distance constraints from the nuclear Overhauser effect measurements and 31 dihedral angle restraints derived from the purged COSY experiments. Some results obtained in presence of SDS are also presented. The toxic effects of the peptides obtained by cleavage (trypsin and lysine-C hydrolysis) of Shiva-3 on Escherichia coli and on Plasmodium berghei sporogonic stages are reported. Biological effects are discussed in relation to the calculated structure. The antiparasite activity and the low mosquito toxicity of Shiva-3 make this peptide a good candidate for genetic transformation of mosquito vectors which warrants further studies aimed at the improvement of the molecule.

A structural homologue of colipase in black mamba venom revealed by NMR floating disulphide bridge analysis.

The solution structure of mamba intestinal toxin 1 (MIT1), isolated from Dendroaspis polylepis polylepis venom, has been determined. This molecule is a cysteine-rich polypeptide exhibiting no recognised family membership. Resistance to MIT1 to classical specific endoproteases produced contradictory NMR and biochemical information concerning disulphide-bridge topology. We have used distance restraints allowing ambiguous partners between S atoms in combination with NMR-derived structural information, to correctly determine the disulphide-bridge topology. The resultant solution structure of MIT1, determined to a resolution of 0.5 A, reveals an unexpectedly similar global fold with respect to colipase, a protein involved in fatty acid digestion. Colipase exhibits an analogous resistance to endoprotease activity, indicating for the first time the possible topological origins of this biochemical property. The biochemical and structural homology permitted us to propose a mechanically related digestive function for MIT1 and provides novel information concerning snake venom protein evolution.