ELISE NMR: experimental liquid sealing of NMR samples

Formation of β-Lactoglobulin Aggregates from Quite, Unfolded Conformations upon Heat Activation

In presence of calcium ions, β-lactoglobulin (BLG) unfolds and subsequently aggregates after heating. This process has important pharmaceutical and agroalimentary applications. Nowadays, the molecular mechanism of unfolding and BLG aggregation, and the role of calcium in the mechanism, is poorly understood. Actually, in most studies, data have been acquired at room temperature, after heating and after aggregation, which makes it difficult to establish a clear causal-temporal relation between calcium binding, heat, and aggregation. Thus, the goal of the present study is to get accurate, nanoscale data about the molecular events leading to BLG unfolding and calcium-dependent aggregation. The molecular transformation of BLG during heating has been investigated, using the NMR pulse field gradient technique, operating in a high field (900 MHz). Thanks to this technique, the molecular conformation of newly formed unfolded BLG molecules can be distinguished in a large pool of native ones. The present work shows that BLG at neutral pH at 65 °C displays fast, cooperative-like unfolding, in which no long-lived intermediary state (as a molten globule one) is detected, before aggregation. These data also indicate that calcium ions bind unfolded BLG in specific sites which might be a necessary feature to form the aggregate. Finally, these data also provide an NMR-based methodology to monitor the rate of protein unfolding using NMR.

Proline Conformation in a Functional Tau Fragment

The conformational state of distinct prolines can determine the folding of a protein but equally other biological processes when coupled to a conformation-sensitive secondary reaction. For the neuronal tau protein, the importance of proline conformation is underscored by its interaction with different prolyl cis/trans isomerases. The proline conformation would gain even further importance after phosphorylation of the preceding residue by various proline-directed kinases. A number of molecular diseases including Alzheimer's disease and traumatic brain injury were thereby recently qualified as "cistauosis", as they would imply a cis conformation for the pThr231-Pro232 prolyl bond. We here investigate by NMR spectroscopy the conformation of all prolines in a functional Tau fragment, Tau[208-324]. Although we can detect and identify some minor conformers in the cis form, we show that all prolines are for over 90% in the trans conformation. Phosphorylation by CDK2/CycA3, which notably leads to complete modification of the Thr231 residue, does not change this conclusion. Our data hence disagree with the notion that specific prolyl bonds in tau would adopt preferentially the cis conformation.

Impact of a stereocentre inversion in cyclic lipodepsipeptides from the viscosin group: a comparative study of the viscosinamide and pseudodesmin conformation and self-assembly

The viscosin group covers a series of cyclic lipodepsipeptides (CLPs) produced by Pseudomonas bacteria, with a range of biological functions and antimicrobial activities. Their oligopeptide moieties are composed of both L- and D-amino acids. Remarkably, the Leu5 amino acid-centrally located in the nonapeptide sequence-is the sole residue found to possess either an L or D configuration, depending on the producing strain. The impact of this D/L switch on the solution conformation was investigated by NMR-restrained molecular modelling of the epimers pseudodesmin A and viscosinamide A. Although the backbone fold remained unaffected, the D/L switch adjusted the segregation between hydrophobic and hydrophilic residues, and thus the amphipathicity. It also influenced the self-assembly capacity in organic solvents. Additionally, several new minor variants of viscosinamide A from Pseudomonas fluorescens DR54 were identified, and an NMR assay is proposed to assess the presence of either an L- or D-Leu5.

The solution structure and self-association properties of the cyclic lipodepsipeptide pseudodesmin A support its pore-forming potential

Pseudodesmin A is a cyclic lipodepsipeptide (CLP) of the viscosin group with a moderate in vitro biological activity. For several CLPs, including members of this group, this activity has been related to the ability to form ion pores in cellular membranes. As their size does not allow individual CLPs to span the membrane bilayer, individual monomers must somehow assemble into a larger structure. NMR spectroscopy has been used to demonstrate that in chloroform and other apolar organic solvents, pseudodesmin A monomers assemble into a supramolecular structure. These self-assembled structures can become sufficiently large to span the membrane bilayer as demonstrated with translational diffusion NMR spectroscopic measurements. With the aim to obtain more insight into the structural nature of this assembly, the solution conformation of pseudodesmin A was first determined by using ROESY (rOe) restraints measured in acetonitrile, in which no self-association occurs. The structure, which is found to be mostly similar to the previously described crystal structure, is shown to be retained within the supramolecular complex. Intermolecular rOe contacts obtained in chloroform together with chemical shift perturbation data provides structural insight into the organization of the self-associated complex. Based upon this analysis, a model for the organization of pseudodesmin A monomers in the supramolecular assembly is proposed, which is in agreement with the formation of bilayer spanning hydrophilic pores and provides the basis for a structure-function relationship for this type of CLPs. Finally, it is demonstrated that the differences previously reported between the crystal and solution conformation of the white line inducing principle (WLIP), a close analogue of pseudodesmin A, are the result of the use of dimethyl sulfoxide as solvent, whose strong hydrogen-bonding capacity induces conformational exchange.

Antimalarial activities of ferroquine conjugates with either glutathione reductase inhibitors or glutathione depletors via a hydrolyzable amide linker

Based on the prodrug concept as well as the combination of two different classes of antimalarial agents, we designed and synthesized two series of ferrocenic antimalarial dual molecules consisting of a ferroquine analogue conjugated with a glutathione reductase inhibitor (or a glutathione depletor) through a cleavable amide bond in order to target two essential pathways in the malarial parasites. The results showed no enhancement of the antimalarial activity of the dual molecules but evidenced a unique mode of action of ferroquine and ferrocenyl analogues distinct of those of chloroquine and nonferrocenic 4-aminoquinoline analogues.

Reducing the NMR sample volume using a single organic liquid: increased sensitivity for mass-limited samples with standard NMR probes

A simple inexpensive protocol for confining an aqueous sample to the active region of a standard NMR probe is examined for high-resolution NMR. The aqueous sample is sandwiched between an inert perfluorinated organic liquid that has been exploited in the design of micro-coil NMR probes. The procedure is demonstrated with 3mm NMR tubes at ambient and elevated temperatures but should be equally applicable to smaller diameter tubes. It is shown that confinement has minimal effects on line shape and provides at least a two fold increase in sensitivity over a conventional sample, for the same mass of solute.

Sandwich-ELISE NMR: reducing the sample volume of NMR samples

We present Sandwich-ELISE, a concatenated version of our previously proposed Experimental LIquid SEaling (ELISE) protocol, in which an aqueous sample is effectively sealed by the addition of a small layer of mineral oil, or, alternatively, a chloroform sample was sealed by a water layer. With Sandwich-ELISE, a triple layered geometry composed of deuterated chloroform/aqueous buffer/mineral oil can be used to limit the sample to the active coil volume, effectively replacing the popular Shigemi tubes. Importantly, this procedure is readily applicable to smaller diameter tubes, for which no Shigemi tubes are available. We further present spectra of a 1 microl protein sample sandwiched between the chloroform and Nujol phases in a 1mm tube, demonstrating thereby that the volume of the aqueous phase of interest can be reduced even further.

Growth of Mycobacterium tuberculosis biofilms containing free mycolic acids and harbouring drug-tolerant bacteria

Successful treatment of human tuberculosis requires 6-9 months' therapy with multiple antibiotics. Incomplete clearance of tubercle bacilli frequently results in disease relapse, presumably as a result of reactivation of persistent drug-tolerant Mycobacterium tuberculosis cells, although the nature and location of these persisters are not known. In other pathogens, antibiotic tolerance is often associated with the formation of biofilms--organized communities of surface-attached cells--but physiologically and genetically defined M. tuberculosis biofilms have not been described. Here, we show that M. tuberculosis forms biofilms with specific environmental and genetic requirements distinct from those for planktonic growth, which contain an extracellular matrix rich in free mycolic acids, and harbour an important drug-tolerant population that persist despite exposure to high levels of antibiotics.

Thiacetazone, an antitubercular drug that inhibits cyclopropanation of cell wall mycolic acids in mycobacteria

Background

Mycolic acids are a complex mixture of branched, long-chain fatty acids, representing key components of the highly hydrophobic mycobacterial cell wall. Pathogenic mycobacteria carry mycolic acid sub-types that contain cyclopropane rings. Double bonds at specific sites on mycolic acid precursors are modified by the action of cyclopropane mycolic acid synthases (CMASs). The latter belong to a family of S-adenosyl-methionine-dependent methyl transferases, of which several have been well studied in Mycobacterium tuberculosis, namely, MmaA1 through A4, PcaA and CmaA2. Cyclopropanated mycolic acids are key factors participating in cell envelope permeability, host immunomodulation and persistence of M. tuberculosis. While several antitubercular agents inhibit mycolic acid synthesis, to date, the CMASs have not been shown to be drug targets.

Methodology/Principle Findings

We have employed various complementary approaches to show that the antitubercular drug, thiacetazone (TAC), and its chemical analogues, inhibit mycolic acid cyclopropanation. Dramatic changes in the content and ratio of mycolic acids in the vaccine strain Mycobacterium bovis BCG, as well as in the related pathogenic species Mycobacterium marinum were observed after treatment with the drugs. Combination of thin layer chromatography, mass spectrometry and Nuclear Magnetic Resonance (NMR) analyses of mycolic acids purified from drug-treated mycobacteria showed a significant loss of cyclopropanation in both the α- and oxygenated mycolate sub-types. Additionally, High-Resolution Magic Angle Spinning (HR-MAS) NMR analyses on whole cells was used to detect cell wall-associated mycolates and to quantify the cyclopropanation status of the cell envelope. Further, overexpression of cmaA2, mmaA2 or pcaA in mycobacteria partially reversed the effects of TAC and its analogue on mycolic acid cyclopropanation, suggesting that the drugs act directly on CMASs.

Conclusions/Significance

This is a first report on the mechanism of action of TAC, demonstrating the CMASs as its cellular targets in mycobacteria. The implications of this study may be important for the design of alternative strategies for tuberculosis treatment.