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International audience; The gymnodimines and spirolides are phycotoxins classified into a heterogeneous group of marine biocompounds called cyclic imines. Although there is no clear evidence of their toxicity to humans, gymnodimines and spirolides are highly toxic to rodents and constitute a source of false positives in lipophilic toxin detection by the mouse bioassay. Using nicotinic acetylcholine receptor-enriched membranes of Torpedo, and fluorescent alpha-bungarotoxin, we developed a fluorescence polarization assay to detect and quantify gymnodimine-A and 13-desmethyl C spirolide. The presence of these cyclic imines in solution inhibited the interaction of fluorescent-labeled alpha-bungarotoxin with nicotinic acetylcholine receptors in a concentration-dependent manner. The sensitivity of the assay is in the order of nanomolar concentrations of gymnodimine and 13-desmethyl C spirolide. Okadaic acid, yessotoxin, and brevetoxin-2, three lipophilic marine toxins, did not interfere with this assay. A suitable extraction method in shellfish was also developed. The gymnodimine-A and 13-desmethyl C spirolide recovery rates of mussel matrix extraction with acetone/chloroform were 63.6% +/- 3.5% and 87.4% +/- 5.3%, respectively. In summary, this inhibition assay is capable of gymnodimine-A and 13-desmethyl C spirolide detection in mussel extracts with enough sensitivity and specificity to quantify these toxins in the range of 50-2000 microg/kg and 70-700 microg/kg of shellfish meat, respectively.

CggR belongs to the SorC family of bacterial transcriptional regulators which control the expression of genes and operons involved in carbohydrate catabolism. CggR was first identified in Bacillus subtilis where it represses the gapA operon encoding the five enzymes that catalyze the central part of glycolysis. Here we present a structure/function study demonstrating that the C-terminal region of CggR regulates the DNA binding activity of this repressor in response to binding of a phosphorylated sugar. Molecular modeling of CggR revealed a winged-helix DNA-binding motif followed by a C-terminal domain presenting weak but significant homology with glucosamine-6-phosphate deaminases from the NagB family. In silico ligand screening suggested that the CggR C-terminal domain would bind preferentially bi-phosphorylated compounds, in agreement with previous studies that proposed fructuose-1,6-biphosphate (FBP) as the inducer metabolite. In vitro, FBP was the only sugar compound capable of interfering with CggR cooperative binding to DNA. FBP was also found to protect CggR against trypsin degradation at two arginine residues predicted to reside in a mobile loop forming the active site lid of the NagB enzymes. Replacement of residues predicted to interact with FBP led to mutant CggR with altered repressor activity in vivo but retaining their structural integrity and DNA binding activity in vitro. Interestingly, some of the mutant repressors responded with different specificity towards mono- and di-phospho-fructosides. Based on these results, we propose that the activity of the CggR-like repressors is controlled by a phospho-sugar binding (PSB) domain presenting structural and functional homology with NagB enzymes.

Anatoxin-a (ATX) is a potent neurotoxin produced by several species of Anabaena spp. Cyanobacteria blooms around the world have been increasing in recent years; therefore, it is urgent to develop sensitive techniques that unequivocally confirm the presence of these toxins in fresh water and cyanobacterial samples. In addition, the identification of different ATX analogues is essential to later determine its toxicity. In this paper we designed a fluorescent polarization (FP) method to detect ATXs in water samples. A nicotinic acetylcholine receptor (nAChR) labeled with a fluorescein derivative was used to develop this assay. Data showed a direct relationship between the amount of toxin in a sample and the changes in the polarization degree of the emitted light by the labeled nAChR, indicating an interaction between the two molecules. This method was used to measure the amount of ATX in three Anabaena spp. cultures. Results indicate that it is a good method to show ATXs presence in algal samples. In order to check the toxin profile of Anabaena cultures a LC-MS/MS method was also developed. Within this new method, ATX-a, retention time (RT) 5 min, and three other molecules with a mass m/z 180.1 eluting at 4.14 min, 5.90 min and 7.14 min with MS/MS spectra characteristic of ATX toxin group not previously identified were detected in the Anabaena spp. cultures. These ATX analogues may have an important role in the toxicity of the sample The research leading to these results has received funding from the following FEDER cofunded-grants: From Ministerio de Ciencia y Tecnología, Spain: AGL2009-13581-CO2-01, AGL2012-40485-CO2-01. From Xunta de Galicia, Spain: 10PXIB261254 PR. From the European Union’s Seventh Framework Programme managed by REA—Research Executive Agency http://ec.europa.eu/research/rea (FP7/2007-2013) under grant agreement Nos. 211326—CP (CONffIDENCE), 265896 BAMMBO, 265409 µAQUA, and 262649 BEADS, 315285 Ciguatools and 312184 PharmaSea. From the Atlantic Area Programme (Interreg IVB Trans-national): 2008-1/003 (Atlantox) and 2009-1/117 (Pharmatlantic) SI

International audience; Fatty acid-binding proteins (FABPs) are small cytosolic proteins, largely distributed in invertebrates and vertebrates, which accomplish uptake and intracellular transport of hydrophobic ligands such as fatty acids. Although long chain fatty acids play multiple crucial roles in cellular functions (structural, energy metabolism, regulation of gene expression), the precise functions of FABPs, especially those of invertebrate species, remain elusive. Here, we have identified and characterized a novel FABP family member, Cq-FABP, from the hepatopancreas of red claw crayfish Cherax quadricarinatus. We report the characterization of fatty acid-binding affinity of Cq-FABP by four different competitive fluorescence-based assays. In the two first approaches, the fluorescent probe 8-Anilino-1-naphthalenesulfonate (ANS), a binder of internal cavities of protein, was used either by directly monitoring its fluorescence emission or by monitoring the fluorescence resonance energy transfer occurring between the single tryptophan residue of Cq-FABP and ANS. The third and the fourth approaches were based on the measurement of the fluorescence emission intensity of the naturally fluorescent cis-parinaric acid probe or the steady-state fluorescence anisotropy measurements of a fluorescently labeled fatty acid (BODIPY-C16), respectively. The four methodologies displayed consistent equilibrium constants for a given fatty acid but were not equivalent in terms of analysis. Indeed, the two first methods were complicated by the existence of non specific binding modes of ANS while BODIPY-C16 and cis-parinaric acid specifically targeted the fatty acid binding site. We found a relationship between the affinity and the length of the carbon chain, with the highest affinity obtained for the shortest fatty acid, suggesting that steric effects primarily influence the interaction of fatty acids in the binding cavity of Cq-FABP. Moreover, our results show that the binding affinities of several fatty acids closely parallel their prevalences in the hepatopancreas of C. quadricarinatus as measured under specific diet conditions.

International audience; The tendency of GFP-like fluorescent proteins to dimerize in vitro is a permanent concern as it may lead to artifacts in FRET imaging applications. However, we have found recently that CFP and YFP (the couple of GFP variants mostly used in FRET studies) show no trace of association in the cytosol of living cells up to millimolar concentrations. In this study, we investigated the oligomerization properties of purified CFP, by fluorescence anisotropy and sedimentation velocity. Surprisingly, we found that CFP has a much weaker homoaffinity than other fluorescent proteins (K(d) ≥ 3 × 10(-3) M), and that this is due to the constitutive N146I mutation, originally introduced into CFP to improve its brightness.

International audience; BACKGROUND INFORMATION: The idea that GPCRs (G-protein-coupled receptors) may exist as homo- or hetero-oligomers, although still controversial, is now widely accepted. Nevertheless, the functional roles of oligomerization are still unclear and gaining greater insight into the mechanisms underlying the dynamics of GPCR assembly and, in particular, assessing the effect of ligands on this process seems important. We chose to focus our present study on the effect of MT7 (muscarinic toxin 7), a highly selective allosteric peptide ligand, on the oligomerization state of the hM1 (human M1 muscarinic acetylcholine receptor subtype). RESULTS: We analysed the hM1 oligomerization state in membrane preparations or in live cells and observed the effect of MT7 via four complementary techniques: native-PAGE electrophoresis analysed by both Western blotting and autoradiography on solubilized membrane preparations of CHO-M1 cells (Chinese-hamster ovary cells expressing muscarinic M1 receptors); FRET (fluorescence resonance energy transfer) experiments on cells expressing differently tagged M1 receptors using either an acceptor photobleaching approach or a novel fluorescence emission anisotropy technique; and, finally, by BRET (bioluminescence resonance energy transfer) assays. Our results reveal that MT7 seems to protect the M1 receptor from the dissociating effect of the detergent and induces an increase in the FRET and BRET signals, highlighting its ability to affect the dimeric form of the receptor. CONCLUSIONS: Our results suggest that MT7 binds to a dimeric form of hM1 receptor, favouring the stability of this receptor state at the cellular level, probably by inducing some conformational rearrangements of the pre-existing muscarinic receptor homodimers.

International audience; Recent efforts have underlined the role of serine/threonine protein kinases in growth, pathogenesis, and cell wall metabolism in Mycobacterium tuberculosis. Although most kinases have been investigated for their physiological roles, little information is available regarding how serine/threonine protein kinase-dependent phosphorylation regulates the activity of kinase substrates. Herein, we focused on M. tuberculosis Rv2175c, a protein of unknown function, conserved in actinomycetes, and recently identified as a substrate of the PknL kinase. We solved the solution structure of Rv2175c by multidimensional NMR and demonstrated that it possesses an original winged helix-turn-helix motif, indicative of a DNA-binding protein. The DNA-binding activity of Rv2175c was subsequently confirmed by fluorescence anisotropy, as well as in electrophoretic mobility shift assays. Mass spectrometry analyses using a combination of MALDI-TOF and LC-ESI/MS/MS identified Thr(9) as the unique phosphoacceptor. This was further supported by complete loss of PknL-dependent phosphorylation of an Rv2175c_T9A mutant. Importantly, the DNA-binding activity was completely abrogated in a Rv2175c_T9D mutant, designed to mimic constitutive phosphorylation, but not in a mutant lacking the first 13 residues. This implies that the function of the N-terminal extension is to provide a phosphoacceptor (Thr(9)), which, following phosphorylation, negatively regulates the Rv2175c DNA-binding activity. Interestingly, the N-terminal disordered extension, which bears the phosphoacceptor, was found to be restricted to members of the M. tuberculosis complex, thus suggesting the existence of an original mechanism that appears to be unique to the M. tuberculosis complex.

In a previous work, we reported that contrary to native calcium-loaded alpha-lactalbumin (holo alpha-LA), calcium-depleted form (apo alpha-LA) has the ability to self-assemble with lysozyme (LYS) to form different supramolecular structures in temperature-dependent manner. In this study, we examine what happens at molecular scale using fluorescence techniques. Fluorescence anisotropy coupled with fluorescence lifetime measurements provides a means to measure intermolecular interactions. We showed that LYS interacts with both apo alpha-LA and holo alpha-LA to form oligomers, assumed to be heterodimers, at 10 degrees C and 45 degrees C. The dissociation constants for dimerization were found to be in the muM range and increased significantly with increasing ionic strength from 39 to 124 mM. Although the binding constants of holo alpha-LA-LYS and apo alpha-LA-LYS complexes were of the same order of magnitude, the shape or conformation of formed heterodimers differed as assessed by fluorescence parameters in particular correlation time calculations. Such conformation differences could explain why holo alpha-LA-LYS complexes are trapped as heterodimers while the apo alpha-LA-LYS complexes have the ability to further self-assemble into various supramolecular structures.

International audience; Crystallization trials of the human retinoid X receptor alpha ligand-binding domain (RXRalpha LBD) in complex with various ligands have been carried out. Using fluorescence anisotropy, it has been found that when compared with agonists these small-molecule effectors enhance the dynamics of the RXRalpha LBD C-terminal helix H12. In some cases, the mobility of this helix could be dramatically reduced by the addition of a 13-residue co-activator fragment (CoA). In keeping with these observations, crystals have been obtained of the corresponding ternary RXRalpha LBD-ligand-CoA complexes. In contrast, attempts to crystallize complexes with a highly mobile H12 remained unsuccessful. These experimental observations substantiate the previously recognized role of co-regulator fragments in facilitating the crystallization of nuclear receptor LBDs.

International audience; Serum response factor (SRF) is a MADS transcription factor that binds to the CArG box sequence of the serum response element (SRE). Through its binding to CArG sequences, SRF activates several muscle-specific genes as well as genes that respond to mitogens. The thermodynamic parameters of the interaction of core-SRF (the 124-245 fragment of serum response factor) with specific oligonucleotides from c-fos and desmin promoters, were determined by spectroscopy. The rotational correlation time of core-SRF labeled with bis-ANS showed that the protein is monomeric at low concentration (10(-7) m). The titration curves for the fluorescence anisotropy of fluorescein-labeled oligonucleotide revealed that under equilibrium conditions, the core-SRF monomers were bound sequentially to SRE at very low concentration (10(-9) m). Curve-fitting data showed also major differences between the wild-type sequence and the oligonucleotide sequences mutated within the CArG box. The fluorescence of the core-SRF tyrosines was quenched by the SRE oligonucleotide. This quenching indicated that under stoichiometric conditions, core-SRF was bound as a dimer to the wild-type oligonucleotide, and as a monomer or a tetramer to the mutant oligonucleotides. Far-UV CD spectra indicated that the flexibility of core-SRF changed profoundly upon its binding to its specific target SRE. Lastly, the rotational correlation time of fluorescein-labeled SRE revealed that formation of the specific complex was accompanied by a change in the SRE internal dynamics. These results indicated that the flexibility of the two partners is crucial for the DNA-protein interaction.

International audience; Retinoid receptors (RARs and RXRs) are ligand-activated transcription factors that regulate the transcription of target genes by recruiting coregulator complexes at cognate promoters. To understand the effects of heterodimerization and ligand binding on coactivator recruitment, we solved the crystal structure of the complex between the RARbeta/RXRalpha ligand-binding domain heterodimer, its 9-cis retinoic acid ligand, and an LXXLL-containing peptide (termed NR box 2) derived from the nuclear receptor interaction domain (NID) of the TRAP220 coactivator. In parallel, we measured the binding affinities of the isolated NR box 2 peptide or the full-length NID of the coactivator SRC-1 for retinoid receptors in the presence of various types of ligands. Our correlative analysis of three-dimensional structures and fluorescence data reveals that heterodimerization does not significantly alter the structure of individual subunits or their intrinsic capacity to interact with NR box 2. Similarly, we show that the ability of a protomer to recruit NR box 2 does not vary as a function of the ligand binding status of the partner receptor. In contrast, the strength of the overall association between the heterodimer and the full-length SRC-1 NID is dictated by the combinatorial action of RAR and RXR ligands, the simultaneous presence of the two receptor agonists being required for highest binding affinity. We identified an LXXLL peptide-driven mechanism by which the concerted reorientation of three phenylalanine side chains generates an "aromatic clamp" that locks the RXR activation helix H12 in the transcriptionally active conformation. Finally, we show how variations of helix H11-ligand interactions can alter the communication pathway linking helices H11, H12, and the connecting loop L11-12 to the coactivator-binding site. Together, our results reveal molecular and structural features that impact on the ligand-dependent interaction of the RAR/RXR heterodimer with nuclear receptor coactivators.

Human immunodeficiency virus type 1 integrase is one of three viral enzymes, and it realizes a key process of the viral replication cycle, i.e. viral DNA integration into infected cell genome. Integrase recognizes nucleotide sequences located at the ends of the viral DNA U3 and U5 LTRs and catalyzes 3'-processing and strand transfer reactions. To study the interactions between integrase and viral DNA at present work, we used modified integrase substrates mimicking the terminal U5 LTR sequence and containing non-nucleoside insertions in one or/and both strands. It is shown that the substrate modifications have no influence on the integrase binding rate, while the heterocyclic bases removal in the 5th and 6th substrate positions and in the 3rd position of the substrate processed strand distinctly inhibits the integrase catalytic activity. This fact demonstrates these bases significance for the active enzyme/substrate complex formation. On the contrary, modification of the 3rd position within substrate non-processed strand stimulates 3'-processing. Since heterocyclic base elimination results in disruption of the DNA complementary and staking interactions, this result shows that DNA double helix destabilization close to the cleaved bond promotes the 3'-processing.

International audience; The last decade has contributed to our understanding of the three-dimensional structure of the human immunodeficiency virus, type 1 (HIV-1) integrase (IN) and to the description of how the enzyme catalyzes the viral DNA integration into the host DNA. Recognition of the viral DNA termini by IN is sequence-specific, and that of the host DNA does not require particular sequence, although in physicochemical studies IN fails to discriminate between the two interactions. Here, such discrimination was allowed thanks to a model system using designed oligonucleotides and peptides as binding structures. Spectroscopic (circular dichroism, NMR, and fluorescence anisotropy) techniques and biochemical (enzymatic and filter binding) assays clearly indicated that the amphipathic helix alpha4, located at the catalytic domain surface, is responsible for the specific high affinity binding of the enzyme to viral DNA. Analogues of the alpha4 peptide having increased helicity and still bearing the biologically relevant lysines 156 and 159 on the DNA binding face, and oligonucleotides conserving an intact attachment site, are required to achieve high affinity complexes (Kd of 1.5 nm). Data corroborate previous in vivo results obtained with mutated viruses.

International audience; H-NS, a protein found in Gram-negative bacteria, is involved in structuring the bacterial chromosome and acts as a global regulator for the expression of a wide variety of genes. These functions are correlated with both its DNA-binding and oligomerization properties. We have identified the minimal dimerization domain of H-NS, a 46 amino acid-long N-terminal fragment, and determined its structure using heteronuclear NMR spectroscopy. The highly intertwined structure of the dimer, reminiscent of a handshake, defines a new structural fold, which may offer a possibility for discriminating prokaryotic from eukaryotic proteins in drug design. Using mutational analysis, we also show that this N-terminal domain actively contributes to DNA binding, conversely to the current paradigm. Together, our data allows us to propose a model for the action of full length H-NS.

Viral integrase (IN) and Vpr are both components of the human immunodeficiency virus type 1 (HIV-1) pre-integration complex. To investigate whether these proteins interact within this complex, we investigated the effects of Vpr and its subdomains on IN activity in vitro. When a 21mer oligonucleotide was used as a donor and acceptor, both Vpr and its C-terminal DNA-binding domain [(52-96)Vpr] inhibited the integration reaction, whereas the (1-51)Vpr domain did not affect IN activity. Steady-state fluorescence anisotropy showed that both full-length and (52-96)Vpr bind to the short oligonucleotide, thereby extending previous observations with long DNA. The concentrations of the two proteins required to inhibit IN activity were consistent with their affinities for the oligonucleotide. The use of a 492 bp mini-viral substrate confirmed that Vpr can inhibit the IN-mediated reaction. However, the activity of (52-96)Vpr differed notably since it stimulated specifically integration events involving two homologous mini-viral DNAs. Order of addition experiments indicated that the stimulation was maximal when IN, (50-96)Vpr and the mini-viral DNA were allowed to form a complex. Furthermore, in the presence of (50-96)Vpr, the binding of IN to the mini-viral DNA was dramatically enhanced. Taken together, these data suggest that (52-96)Vpr stimulates the formation of a specific complex between IN and the mini-viral DNA.

International audience; Information on the structure and dynamics of condensed forms of DNA is important in understanding both natural situations such as DNA packaging and artificial systems such as gene delivery complexes. We have established the fluorescence of bisintercalator 1,1'-(4,4,8,8-tetramethyl-4,8-diazaundecamethylene)bis[4-[[3-methylbenz-1,3-oxazol-2-yl]methylidine]-1,4-dihydroquinolinium] tetraiodide (YOYO-1) as a novel probe for DNA condensation. When the level of DNA-bound YOYO-1 is sufficiently large, condensation by either polyethylenimine (PEI) or the cationic detergent cetyltrimethylammonium bromide (CTAB) leads to electronic interaction among YOYO-1 molecules bound on the same DNA molecule. This interaction results in an excitonic blue shift of the absorption spectra of YOYO-1 and dramatic decrease in the fluorescence quantum yield. These observations constitute a signature of the condensation of DNA. We further examined the comparative properties of DNA condensed by PEI, CTAB, or Co(NH(3))(6)(3+) through the steady-state and dynamic fluorescence of YOYO-1. Condensation by either PEI or CTAB was associated with a blue shift in the absorption spectra of YOYO-1, although the magnitude of the shift was larger in the case of PEI when compared to that of CTAB. In contrast, condensation by Co(NH(3))(6)(3+) was not associated with a measurable shift in the absorption spectra. These results were interpreted as signifying the varying level of compactness of the DNA condensates. Quenching of fluorescence by acrylamide showed that condensation by all three agents led to an increase in the level of solvent exposure of the base pairs. Observation of the decay of fluorescence intensity and anisotropy of DNA-bound YOYO-1 showed that while condensation by either PEI or CTAB froze the segmental mobility of the helix, condensation by Co(NH(3))(6)(3+) enhanced the flexibility of DNA. The relevance of our findings to functions such as efficiency of gene delivery is discussed.

In iron limitation conditions, Pseudomonas aeruginosa secretes a major fluorescent siderophore named pyoverdin (PaA). PaA has an extremely high affinity for Fe(3+) but also chelates other ions such as Al(3+) and Ga(3+) with a lower affinity. The transfer of PaA-Fe(3+) across the outer membrane of the bacteria is mediated by the receptor FpvA, a TonB-dependent outer membrane transport protein. FpvA binds the iron-free and iron-loaded forms of pyoverdin with similar affinities, but only PaA-Fe(3+) is taken up by the cell, suggesting that FpvA adopts different conformations depending on its loading status. We used time-resolved fluorescence spectroscopy to characterize the different forms of FpvA-PaA in vitro. We showed that the FpvA-PaA complex adopts two different conformations depending on how it was prepared (formed in vitro or in vivo prior to purification). The dihydroquinoline moiety of both conformers is fully protonated, or coordinated by protein charged groups, but the polarity of its environment, its solvent accessibility, and its rotational dynamics are much slower when the FpvA-PaA complex is formed in vivo than in vitro. In the presence of Ga(3+) or Al(3+) ions, the solvent accessibility and mobility of the dihydroquinoline moiety in the two FpvA-PaA complexes are intermediate between those observed for the metal-free ones. In addition, the Förster resonance energy transfer kinetics from FpvA tryptophan residues to the PaA chromophore differs from one complex to the other, revealing differences in one or more of the donor-acceptor topologies.

International audience; Reaction of the melanotropin hormone analogs [Nle(4),D-Phe(7)]-alpha-MSH and [Nle(4),D-Phe(7)]-alpha-MSH(4-10), which were extended at their N-terminus by a thiol-functionalized spacer arm, with preformed liposomes containing thiol-reactive (phospho)lipid derivatives resulted in the aggregation of the vesicles and in a partial leakage of their inner contents. This aggregation/leakage effect, which was only observed when the peptides were covalently conjugated to the surface of the liposomes, was correlated with the fusion of the vesicles as demonstrated by the observed decrease in resonance energy transfer between probes in a membrane lipid mixing assay. A limited fusion was confirmed by monitoring the mixing of the liposome inner contents (formation of 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylene bis(pyridinium bromide) complex). The membrane-active properties of the peptides could be correlated with changes in the fluorescence emission spectra of their tryptophan residue, which suggested that after their covalent binding to the outer surface of the liposomes they can partition within the core of the bilayers. A blue shift of 10 nm was observed for [Nle(4),D-Phe(7)]-alpha-MSH which was correlated with an increase in fluorescence anisotropy and with changes in the accessibility of the coupled peptide as assessed by the quenching of fluorescence of its tryptophan residue by iodide (Stern-Volmer plots). These results should be related to the previously described capacity of alpha-MSH, and analogs, to interact with membranes and with the favored conformation of these peptides which, via a beta-turn, segregate their central hydrophobic residues into a domain that could insert into membranes and, as shown here, trigger their destabilization.

Self-assembly of HIV-1 integrase (IN) in solution has been studied previously by time-resolved fluorescence, using tryptophan anisotropy decay. This approach provides information on the size of macromolecules via the determination of rotational correlation times (θ). We have shown that, at submicromolar concentration, IN is characterized by a long rotational correlation time (θ 20°C = 90–100 ns) corresponding to a high-order oligomeric form, likely a tetramer. In the present work, we investigated the self-assembly properties of the DNA-bound IN by using three independent fluorophores. Under enzymatic assay conditions (10 −7 M IN, 2 × 10 −8 M DNA), using either fluorescein-labeled or fluorescent guanosine analog-containing oligonucleotides that mimic a viral end long terminal repeat sequence, we found that the DNA–IN complex was characterized by shorter θ 20°C values of 15.5–19.5 and 23–27 ns, calculated from experiments performed at 25°C and 37°C, respectively. These results were confirmed by monitoring the Trp anisotropy decay as a function of the DNA substrate concentration: the θ of IN shifted from 90–100 ns to lower values (20°C values were significantly higher when monitored at 37°C as compared with 25°C. These results indicate that upon binding the viral DNA end, the multimeric enzyme undergoes a dissociation, most likely into a homogenous monomeric form at 25°C and into a monomer–dimer equilibrium at 37°C.

Two molecular dynamics simulations have been carried out on the HIV-1 integrase catalytic core starting from fully determined crystal structures. During the first one, performed in the absence of divalent cation (6-ns long), the catalytic core took on two main conformations. The conformational transition occurs at approximately 3.4 ns. In contrast, during the second one, in the presence of Mg(2+) (4-ns long), there were no such changes. The molecular dynamics simulations were used to compute the fluorescence intensity decays emitted by the four tryptophan residues considered as the only chromophores. The decay was computed by following, frame by frame, the amount of chromophores that remained excited at a certain time after light absorption. The simulation took into account the quenching through electron transfer to the peptide bond and the fluorescence resonance energy transfer between the chromophores. The fit to the experimental intensity decays obtained at 5 degrees C and at 30 degrees C is very good. The fluorescence anisotropy decays were also simulated. Interestingly, the fit to the experimental anisotropy decay was excellent at 5 degrees C and rather poor at 30 degrees C. Various hypotheses such as dimerization and abnormal increase of uncorrelated internal motions are discussed.

International audience; The aim of this work was to examine the possible influence of the integrity of the microtubule network on the plasma membrane fluidity of L929 mouse fibroblasts. The L929 cell line was selected for the ease of culture and the stability of its characteristics. The cells were treated with colchicine, nocodazole and vinblastine, three microtubule-depolymerizing drugs, at various concentrations and for various times. Membrane fluidity was assessed from fluorescence depolarization measurements with the plasma membrane probe TMA-DPH. Each of the drugs induced a significant, dose-dependent decrease in fluorescence anisotropy. The effect levelled off (5-7% decrease) after approximately 90 min of treatment, and could be unambiguously interpreted as resulting from an increase in membrane fluidity. The cumulative action of the drugs did not significantly increase the effect. The effects of colchicine and nocodazole could be reversed by incubation in drug-free medium, but not that of vinblastine. The results are discussed in correlation with the kinetics of the three drugs interaction with tubulin or microtubules. It is concluded that the microtubule integrity contributed to the high plasma membrane lipidic order, but less than other factors, like the lipid composition and the cholesterol content.

Self-assembly properties of HIV-1 integrase were investigated by time-resolved fluorescence anisotropy using tryptophanyl residues as a probe. From simulation analyses, we show that suitable photon counting leads to an accurate determination of long rotational correlation times in the range of 20-80 ns, permitting the distinction of the monomer, dimer, and tetramer from higher oligomeric forms of integrase. The accuracy of correlation times higher than 100 ns is too low to distinguish the octamer from other larger species. The oligomeric states of the widely used detergent-solubilized integrase were then studied in solution under varying parameters known to influence the activity. In the micromolar range, integrase exists as high-order multimers such as an octamer and/or aggregates and a well-defined tetramer, at 25 and 35 degrees C, respectively. However, integrase is monomeric at catalytically active concentrations (in the sub-micromolar range). Detergents (NP-40 and CHAPS) and divalent cation cofactors (Mg(2+) and Mn(2+)) have a clear dissociative effect on the high multimeric forms of integrase. In addition, we observed that Mg(2+) and Mn(2+) have different effects on both the oligomeric state and the conformation of the monomer. This could explain in part why these two metal cations are not equivalent in terms of catalytic activity in vitro. In contrast, addition of Zn(2+) stimulates dimerization. Interestingly, this role of Zn(2+) in the multimerization process was evident only in the presence of Mg(2+) which by itself does not induce oligomerization. Finally, it is highly suggested that the presence of detergent during the purification procedure plays a negative role in the proper self-assembly of integrase. Accordingly, the accompanying paper [Leh, H., et al. (2000) Biochemistry 39, 9285-9294] shows that a detergent-free integrase preparation has self-assembly and catalytic properties different from those of the detergent-solubilized enzyme.