Amphipols from A to Z.

Amphipols (APols) are short amphipathic polymers that can substitute for detergents to keep integral membrane proteins (MPs) water soluble. In this review, we discuss their structure and solution behavior; the way they associate with MPs; and the structure, dynamics, and solution properties of the resulting complexes. All MPs tested to date form water-soluble complexes with APols, and their biochemical stability is in general greatly improved compared with MPs in detergent solutions. The functionality and ligand-binding properties of APol-trapped MPs are reviewed, and the mechanisms by which APols stabilize MPs are discussed. Applications of APols include MP folding and cell-free synthesis, structural studies by NMR, electron microscopy and X-ray diffraction, APol-mediated immobilization of MPs onto solid supports, proteomics, delivery of MPs to preexisting membranes, and vaccine formulation.

15N NMR relaxation as a probe for helical intrinsic propensity: the case of the unfolded D2 domain of annexin I.

The isolated D2 domain of annexin I is unable to adopt a tertiary fold but exhibits both native and non-native residual structures. It thus constitutes an attractive model for the investigation of dynamics of partially folded states in the context of protein folding and stability. 15N relaxation parameters of the D2 domain have been acquired at three different magnetic fields, 500, 600 and 800 MHz. This enables the estimation of the contribution of conformational exchange to the relaxation parameters on the micro- to millisecond time scale, thus providing a suitable data set for the description of motions on the pico- and nanosecond time scale. The analysis of the seven spectral densities obtained (J(0), J(50 MHz), J(60 MHz), J(80 MHz), , , ) provides complementary and meaningful results on the conformational features of the D2 domain structure previously depicted by chemical shift and NOE data. Especially, residual helix segments exhibit distinct dynamical behaviors that are related to their intrinsic helical propensity. Beside the spectral density analysis, a series of models derived from the Lipari and Szabo model-free approach are investigated. Two models containing three parameters are able to reproduce equally well the experimental data within experimental errors but provide different values of order parameters and correlation times. The inability to find a unique model to describe the data emphasizes the difficulty to use and interpret the model-free parameters in the case of partially or fully unfolded proteins consisting of a wide range of interconverting conformers.

NMR characterization of a kissing complex formed between the TAR RNA element of HIV-1 and a DNA aptamer.

This work presents the first structural analysis of an RNA-DNA complex consisting of an 18 nt RNA hairpin and a 20 nt DNA aptamer. The DNA molecule was previously selected, from a randomly synthesized library, against the transactivation response element (TAR) involved in transcriptional regulation of the HIV genome. The DNA aptamer used in the present study is an imperfect stem-loop with the sequence 5'-ACTCCCAT-3', characteristic of the selected candidates, in the apical loop. This octameric motif contains five bases complementary to the TAR loop sequence 5'-CUGGGA-3'. The use of homo- and heteronuclear NMR spectroscopy allowed assignment of the complex resonances and resolution of its secondary structure. Evidence is given for a kissing complex fold, which consists of a quasi-continuous helix formed by one stem of DNA, one stem of RNA and a central hybrid helix comprising 5 bp. Two out of helices residues of DNA and one of RNA connect the DNA-RNA loop-loop helix to the stem of either partner in the complex. In addition, two thymines of the DNA stem are engaged in a non-canonical T.T base pair.

New (15)N NMR exchange experiments for the unambiguous assignment of (1)H(N)/(15)N resonances of proteins in complexes in slow chemical exchange with free form.

The potentialities of a 2D proton-detected heteronuclear exchange experiment to assign the nitrogen and amide proton resonances in a uniformly (15)N-enriched macromolecule involved in a complex, starting from the free form assignments, are demonstrated on a protein-DNA complex. This 2D experiment is further extended to a 3D experiment in the case of severe superpositions.

Dynamics of the DNA binding domain of the fructose repressor from the analysis of linear correlations between the 15N-1H bond spectral densities obtained by nuclear magnetic resonance spectroscopy.

The spectral densities of the backbone and arginine side chain NH bonds of the DNA binding domain of the fructose repressor (FruR) were extensively analyzed in order to extract reliable motions parameters. An accurate measurement of 15N NMR relaxation rates allowed their calculation at three frequencies, zero, omegaN, and omegaH + omegaN, using a reduced matrix approach. Linear correlations were found between J(omegaN) and J(0) and between and J(0). The analysis of the compatibility between the motions parameters obtained independently from the two correlation lines allowed further development of the linear correlation approach proposed recently [Lefèvre, J. F., Dayie, K. T., Peng, J. W., and Wagner, G. (1996) Biochemistry 35, 2674-2686]. The results demonstrate (i) the existence of a concerted motion along the whole backbone with a global correlation time equal to 5.95 ns.rad-1, and (ii) the presence of complex internal movements at an intermediate time scale around 1 ns. The extracted motion parameters have been related to those obtained with the extended Lipari and Szabo approach but are incompatible with those obtained using the usual simple Lipari and Szabo approach. They were correlated to the features of the NMR structure of FruR(1-57)*. Some residues in the turns and in the third helix experience slow motions in the micro- to millisecond time scale. Side-chain motions are not correlated to the backbone dynamics. A direct examination of spectral densities reveals a higher flexibility for the side chains of arginines that are not involved in ionic bridges.

1H and 15N resonance assignment and secondary structure of capsicein, an alpha-elicitin, determined by three-dimensional heteronuclear NMR.

The backbone 1H and 15N resonance assignments and solution secondary structure determination of capsicein, a protein of 98 residues with a molecular mass of 10161 Da, are presented. Capsicein belongs to the elicitin family, elicitor molecules having toxic and signaling properties that are secreted by Phytophthora fungi, responsible for the incompatible hypersensitive reaction of diverse plant species leading to resistance against fungal or bacterial plant pathogens. The protein was uniformly labeled with 15N to overcome spectral overlap of the proton resonances. A combination of 3D HOHAHA-HMQC and 3D NOESY-HMOC experiments allowed the identification of spin systems with through-bond correlations, which were in turn correlated by through-space connections. The sequential assignment was obtained for main- and side-chain resonances and led to the identification of all secondary structures. A 3D HMQC-NOESY-HMQC experiment was performed which characterized the NH(i)-NH(i+1) connections specific to alpha-helical structures. This proved particularly useful for the assignment of degenerate amide protons of successive residues in alpha-helical structures. The data show five alpha-helical regions comprising residues 5-18, 26-33, 44-58, 59-67, and 86-98 and a two-stranded antiparallel beta-sheet involving residues 70-75 and 80-85, packed around a hydrophobic core grouping all of the aromatic residues. The C-terminal secondary structure motifs of capsicein evoke phospholipase structural features, which suggests that elicitins might interact with the lipidic molecules of the plasma membrane.

Resonance assignment, cysteine-pairing elucidation and secondary-structure determination of capsicein, an alpha-elicitin, by three-dimensional 1H NMR.

Difficulties encountered in the interpretation of two-dimensional NMR spectra of proteins exceeding roughly 100 amino acids, including resonance overlap and line broadening due to longer correlation times and/or aggregation phenomena, can be overcome by using three-dimensional 1H-NMR experiments. The improvement of spectral resolution using these experiments allows the size of molecules amenable to structure determination by NMR spectroscopy to be extended. A three-dimensional non-selective homonuclear Hartmann-Hahn/nuclear Overhauser effect spectroscopy experiment was performed on capsicein, a 10161-Da elicitin secreted by the Phytophthora capsici fungus. Sequential assignment and secondary structure determination is illustrated for beta-sheet, alpha-helix and loop structures by analysis of planar cross sections perpendicular to the omega 2 or omega 3 axis at the amide proton resonance frequencies. Cysteine pairing was established in the course of the investigation. The secondary structure topology of the molecule is composed of five helices and an antiparallel beta-sheet. Four of the helices compose two pairs running antiparallel while the last one is parallel to the beta-sheet.

Reassessment of structural characteristics of the d(CGCG)2:actinomycin D complex from complete 1H and 31P NMR.

Complexes formed between Actinomycin D (ActD) and the tetranucleotides d(AGCT)2 and d(CGCG)2 were studied in detail by one and two-dimensional 1H and 31P NMR. The 31P two dimensional chemical exchange experiment, at room temperature on saturated complexes (1:1), showed unambiguously that the asymmetrical phenoxazone ring binds to the unique GC site under the two possible orientations in the d(AGCT)2 tetranucleotide but adopts a single orientation in the d(CGCG)2 tetranucleotide. For the d(CGCG)2:Act D saturated complex, complete assignments of all protons and phosphorus signals of the two-nucleotide strands, as well as of the two cyclic pentapeptide chains has allowed us to study in details the conformational features of the complex from NOE and coupling constants analysis. The tetranucleotide remains in a right-handed duplex, but the sugar puckers are modified for residues at the intercalation site. A uniform C2' endo pucker is observed for residues on the strand facing the quinoid side of the phenoxazone ring while a C2' endo-C3-endo equilibrium about 60% of C2' endo is proposed for the two residues on the strand facing the benzenoid side of the phenoxazone ring. In contrast to previous studies on ActD-DNA interactions, we have been able to measure the 3J phosphorus-proton coupling constants at the intercalation site but also adjacent to it, showing that 31P chemical shifts are not simply related to the backbone conformation. Molecular mechanics calculations, using empirical distances deduced from NOE effects as restrained distances during minimizations, led to a model differing mainly from those previously published by orientation of the N methyl groups of both N-Methyl-Valines.