André Fournel

Lid opening and unfolding in human pancreatic lipase at low pH revealed by site-directed spin labeling EPR and FTIR spectroscopy.

The structural changes induced in human pancreatic lipase (HPL) by lowering the pH were investigated using a combined approach involving the use of site-directed spin labeling coupled to electron paramagnetic resonance (SDSL-EPR) and Fourier transform infrared (ATR-FTIR) spectroscopy. The secondary structure of HPL observed with ATR-FTIR spectroscopy was found to be stable in the pH range of 3.0-6.5, where HPL remained active. Using a spin-label introduced into the lid of HPL at position 249, a reversible opening of the lid controlling the access to the active site was observed by EPR spectroscopy in the pH range of 3.0-5.0. In the same pH range, some structural changes were also found to occur outside the lid in a peptide stretch located near catalytic aspartate 176, using a spin-label introduced at position 181. Below pH 3.0, ATR-FTIR measurements indicated that HPL had lost most of its secondary structure. At these pH levels, the loss of enzyme activity was irreversible and the ability of HPL to bind to lipid emulsions was abolished. The EPR spectrum of the spin-label introduced at position 181, which was typical of a spin-label having a high mobility, confirmed the drastic structural change undergone by HPL in this particular region. The EPR spectrum of the spin-label at position 249 indicated, however, that the environment of this residue within the lid was not affected at pH 3.0 in comparison with that observed in the pH range of 3.0-5.0. This finding suggests that the disulfide bridge between the hinges of the lid kept the secondary structure of the lid intact, whereas the HPL was completely unfolded.

The interaction between the measles virus nucleoprotein and the Interferon Regulator Factor 3 relies on a specific cellular environment

BackgroundThe genome of measles virus consists of a non-segmented single-stranded RNA molecule of negative polarity, which is encapsidated by the viral nucleoprotein (N) within a helical nucleocapsid. The N protein possesses an intrinsically disordered C-terminal domain (aa 401–525, NTAIL) that is exposed at the surface of the viral nucleopcapsid. Thanks to its flexible nature, NTAIL interacts with several viral and cellular partners. Among these latter, the Interferon Regulator Factor 3 (IRF-3) has been reported to interact with N, with the interaction having been mapped to the regulatory domain of IRF-3 and to NTAIL. This interaction was described to lead to the phosphorylation-dependent activation of IRF-3, and to the ensuing activation of the pro-immune cytokine RANTES gene.ResultsAfter confirming the reciprocal ability of IRF-3 and N to be co-immunoprecipitated in 293T cells, we thoroughly investigated the NTAIL-IRF-3 interaction using a recombinant, monomeric form of the regulatory domain of IRF-3. Using a large panel of spectroscopic approaches, including circular dichroism, fluorescence spectroscopy, nuclear magnetic resonance and electron paramagnetic resonance spectroscopy, we failed to detect any direct interaction between IRF-3 and either full-length N or NTAIL under conditions where these latter interact with the C-terminal X domain of the viral phosphoprotein. Furthermore, such interaction was neither detected in E. coli nor in a yeast two hybrid assay.ConclusionAltogether, these data support the requirement for a specific cellular environment, such as that provided by 293T human cells, for the NTAIL-IRF-3 interaction to occur. This dependence from a specific cellular context likely reflects the requirement for a human or mammalian cellular co-factor.

EPR spectroscopy: A powerful technique for the structural and functional investigation of metalloproteins

Numerous metal centers in proteins can be prepared in a redox state in which their ground state is paramagnetic. Complementary data provided by EPR, Mössbauer, electron nuclear double resonance, magnetic circular dichroism, and NMR spectroscopies have therefore played a major role in the elucidation of the structure and function of these centers. Among those techniques the most commonly used is certainly EPR spectroscopy. In this article various aspects of the current applications of EPR to the structural and functional study of metalloproteins are presented. They are illustrated by recent studies carried out in our laboratory in the field of metalloenzymes and electron transfer systems. The power of numerical simulation techniques is emphasized throughout this work.

Amplitude of Pancreatic Lipase Lid Opening in Solution and Identification of Spin Label Conformational Subensembles by Combining Continuous Wave and Pulsed EPR Spectroscopy and Molecular Dynamics

The opening of the lid that controls the access to the active site of human pancreatic lipase (HPL) was measured from the magnetic interaction between two spin labels grafted on this enzyme. One spin label was introduced at a rigid position in HPL where an accessible cysteine residue (C181) naturally occurs. A second spin label was covalently bound to the mobile lid after introducing a cysteine residue at position 249 by site-directed mutagenesis. Double electron-electron resonance (DEER) experiments allowed the estimation of a distance of 19 +/- 2 A between the spin labels when bilabeled HPL was alone in a frozen solution, i.e., with the lid in the closed conformation. A magnetic interaction was however detected by continuous wave EPR experiments, suggesting that a fraction of bilabeled HPL contained spin labels separated by a shorter distance. These results could be interpreted by the presence of two conformational subensembles for the spin label lateral chain at position 249 when the lid was closed. The existence of these conformational subensembles was revealed by molecular dynamics experiments and confirmed by the simulation of the EPR spectrum. When the lid opening was induced by the addition of bile salts and colipase, a larger distance of 43 +/- 2 A between the two spin labels was estimated from DEER experiments. The distances measured between the spin labels grafted at positions 181 and 249 were in good agreement with those estimated from the known X-ray structures of HPL in the closed and open conformations, but for the first time, the amplitude of the lid opening was measured in solution or in a frozen solution in the presence of amphiphiles.

Magnetic interactions between a [4Fe–4S]1+ cluster and a flavin mononucleotide radical in the enzyme trimethylamine dehydrogenase: A high-field electron paramagnetic resonance study

Trimethylamine dehydrogenase is a bacterial enzyme which contains two redox centers: a flavin mononucleotide (FMN) group which constitutes the active site and a [4Fe–4S]1+,2+ cluster which transfers the electrons provided by the FMN to an electron-transferring flavoprotein. According to the x-ray crystal structure, the center-to-center distance is equal to 12 A and the nearest atoms of the two centers are separated by a 4 A gap. Although this arrangement does not appear especially favorable for mediating strong magnetic interactions, a triplet state electron paramagnetic resonance (EPR) spectrum arising from the intercenter magnetic coupling is observed at X band (9 GHz) when the enzyme is reduced by its substrate. In earlier work, the temperature dependence of this spectrum and its analysis based on a triplet state spin Hamiltonian were used to propose the range (0.8–100 cm−1) for the parameter J0 of the isotropic interaction J0SA.SB, but neither the magnitude of J0 nor its sign could be further specifie...

A new approach for the structural study of metalloproteins: the quantitative analysis of intercenter magnetic interactions

Abstract The quantitative analysis of intercenter magnetic interactions, based on the simulation of EPR spectra recorded at different microwave frequencies, is a powerful technique to determine the relative arrangement of paramagnetic centers in metalloproteins. Such simulations generally rely on a model Hamiltonian in which the interacting centers are approximated by point dipoles. This approximation is often sufficient when these centers are mononuclear metal complexes or organic radicals in which the spin density is not too delocalized and keeps a constant sign. It is used in the present paper to study the magnetic interactions among several hemes and between a heme and a FMN radical in cytochromes. In the case of metalloproteins containing polynuclear metal clusters, the point dipole approximation is no longer valid and must be replaced by a local spin model in which the magnetic interactions among all the paramagnetic sites of the system are explicitly considered. Numerical simulations based on this model provide a description of the relative arrangement of the interacting centers at atomic resolution and can be used to assign a valence state to the different metal ions of the clusters. This is illustrated by recent studies carried out on metalloproteins containing [2Fe-2S]1+ and [4Fe-4S]1+ clusters.