HIV-1 nucleocapsid protein activates transient melting of least stable parts of the secondary structure of TAR and its complementary sequence

The nucleocapsid protein NCp7 of HIV-1 possesses a nucleic acid chaperone activity that is critical in minus and plus strand transfer during reverse transcription. The minus strand transfer notably relies on the ability of NCp7 to destabilize the stable stem with five contiguous, double-stranded segments of both the TAR sequence at the 3' end of the viral genome and the complementary sequence, cTAR, in minus strong-stop DNA. In order to examine the nature and the extent of NCp7 destabilizing activity, we investigated, by absorbance and fluorescence spectroscopy, the interaction of TAR and cTAR with a (12-55)NCp7 peptide containing the zinc-finger motifs but lacking the ability to aggregate the oligonucleotides. The absorbance changes in the UV band of cTAR show that seven to eight base-pairs, on average, are melted per oligonucleotide at a ratio of one peptide to 7.5 nucleotides. In contrast, the melting of TAR does not exceed an average of one base-pair per oligonucleotide. This may be linked to the greater stability of TAR, since a strong correlation between NCp7 destabilizing effect and oligonucleotide stability was observed. The effect of (12-55)NCp7 on the stem terminus was investigated by using a cTAR molecule doubly labeled at the 3' and 5' ends by a donor/acceptor couple. In the absence of the peptide, about 80 % of the oligonucleotides are in a dark non-fluorescent state, having a close proximity of the two dyes. The remaining 20 % are distributed between three fluorescent species, having either the terminal segment, the two terminal segments or all segments of the stem melted. This is in line with a fraying mechanism wherein the stem terminus fluctuates rapidly between open and closed states. Addition of (12-55)NCp7 shifts the equilibrium toward the open species, suggesting that NC enhances fraying of the stem terminus. Taken together, our data suggest that NCp7 activates the transient opening of base-pairs in the least stable parts of the stem. Also, this activity of NCp7 was found to be dependent on the zinc-finger motifs, since no melting was observed with a fingerless NCp7 peptide.

Effects of temperature on the dynamic behaviour of the HIV-1 nucleocapsid NCp7 and its DNA complex

The nucleocapsid protein NCp7 of human immunodeficiency virus type 1 (HIV-1) contains two highly conserved CCHC zinc fingers and is involved in many crucial steps of the virus life-cycle. A large number of physiological rôles of NCp7 involve its binding to single-stranded nucleic acid chains. Several solution structures of NCp7 and its complex with single-stranded RNA or DNA have been reported. We have investigated the changes in the dynamic behaviour experienced by the (12-53)NCp7 peptide upon DNA binding using (15)N heteronuclear relaxation measurements at 293 K and 308 K, and fluorescence spectroscopy. The relaxation data were interpreted using the reduced spectral density approach, which allowed the high-frequency motion, overall tumbling rates and the conformational exchange contributions to be characterized for various states of the peptide without using a specific motional model. Analysis of the temperature-dependent correlation times derived from both NMR and fluorescence data indicated a co-operative change of the molecular shape of apo (12-53)NCp7 around 303 K, leading to an increased hydrodynamic radius at higher temperatures. The binding of (12-53)NCp7 to a single-stranded d(ACGCC) pentanucleotide DNA led to a reduction of the conformational flexibility that characterized the apo peptide. Translational diffusion experiments as well as rotational correlation times indicated that the (12-53)NCp7/d(ACGCC) complex tumbles as a rigid object. The amplitudes of high-frequency motions were restrained in the complex and the occurrence of conformational exchange was displaced from the second zinc finger to the linker residue Ala30.

Rapid internalization and recycling of the human neuropeptide Y Y(1) receptor

Desensitization of G protein-coupled receptors (GPCRs) involves receptor phosphorylation and reduction in the number of receptors at the cell surface. The neuropeptide Y (NPY) Y(1) receptor undergoes fast desensitization. We examined agonist-induced signaling and internalization using NPY Y(1) receptors fused to green fluorescent protein (EGFP). When expressed in HEK293 cells, EGFP-hNPY Y(1) receptors were localized at the plasma membrane, desensitized rapidly as assessed using calcium responses, and had similar properties compared to hNPY Y(1) receptors. Upon agonist challenge, the EGFP signal decreased rapidly (t(1/2) = 107 +/- 3 s) followed by a slow recovery. This decrease was blocked by BIBP3226, a Y(1) receptor antagonist, or by pertussis toxin, in agreement with Y(1) receptor activation. Internalization of EGFP-hNPY Y(1) receptors to acidic endosomal compartments likely accounts for the decrease in the EGFP signal, being absent after pretreatment with monensin. Concanavalin A and hypertonic sucrose, which inhibit clathrin-mediated endocytosis, blocked the decrease in fluorescence. After agonist, intracellular EGFP signals were punctate and co-localized with transferrin-Texas Red, a marker of clathrin-associated internalization and recycling, but not with LysoTracker Red, a lysosomal pathway marker, supporting receptor trafficking to recycling endosomes rather than the late endosomal/lysosomal pathway. Pulse-chase experiments revealed no receptor degradation after internalization. The slow recovery of fluorescence was unaffected by cycloheximide or actinomycin D, indicating that de novo synthesis of receptors was not limiting. Use of a multicompartment model to fit our fluorescence data allows simultaneous determination of internalization and recycling rate constants. We propose that rapid internalization of receptors via the clathrin-coated pits recycling pathway may largely account for the rapid desensitization of NPY Y(1) receptors.

Role of endocytosis in the transfection of L929 fibroblasts by polyethylenimine/DNA complexes

Polyethylenimine (PEI) is one of the most efficient nonviral vectors for gene therapy. The aim of this study was to investigate the role of endocytosis in the transfection of synchronized L929 fibroblasts by PEI/DNA complexes. This was performed by confocal microscopy and flow cytometry, using the endocytosis marker FM4-64 and PEI/DNA complexes labeled either with the DNA intercalator YOYO-1, or with fluorescein covalently linked to PEI. Endocytosis appeared as the major if not the sole mode of entry of the PEI/DNA complexes into the L929 cells. The complexes followed a typical fluid phase endocytosis pathway and were efficiently taken up in less than 10 min in endosomes that did not exceed 200 nm in diameter. Later, the localization of the complexes became perinuclear and fusion between late endosomes was shown to occur. Comparison with the intracellular trafficking of the same complexes in EA.hy 926 cells (W.T. Godbey, K. Wu, A.G. Mikos, Proc. Natl. Acad. Sci. USA 96 (1999)) revealed that endocytosis of PEI/DNA complexes is strongly cell-dependent. In L929 cells, escape of the complexes from the endosomes is a major barrier for transfection. This limited the number of transfected cells to a few percent, even though an internalization of PEI/DNA complexes was observed in most cells. In addition, the entry of the complexes into the nucleus apparently required a mitosis and did not involve the lipids of the endosome membrane. This entry seems to be a short-lived event that involves only a few complexes.

Determination of the pK(a) of the four Zn2+-coordinating residues of the distal finger motif of the HIV-1 nucleocapsid protein: consequences on the binding of Zn2+

The nucleocapsid protein NCp7 of human immunodeficiency virus type 1 is characterized by two highly conserved CCHC motifs that bind Zn2+ strongly. To elucidate the striking pH-dependence of the apparent Zn2+-binding constants of these motifs further, we investigated, using 1H NMR, potentiometry and fluorescence spectroscopy, the acid-base properties of the four Zn2+-coordinating residues of (35-50)NCp7, a peptide corresponding to the distal finger motif of NCp7. With the exception of the H(beta2) proton of Cys39, the pH-dependence of the H(beta) proton resonances of the three Cys residues and, the H(delta) and H(epsilon) resonances of His44 in the apopeptide could be fitted adequately with a single pK(a). This suggests that the protonating groups are non-interacting, a feature that was confirmed by a potentiometric titration. The pK(a) of His44, Cys36, Cys39, and Cys49 in the apopeptide were found to be 6.4, 8.0, 8.8 and 9.3, respectively. Accordingly, the deprotonation is almost sequential and may thus induce a sequential binding of Zn2+ to the four coordinating residues. The high pK(a) of Cys49 is probably related to the negative charge of the neighboring Asp48. Such a high pK(a) may be a general feature in nucleocapsid proteins (NCs), since an acidic residue generally occupies the (i-1) position of the C-terminal Cys residue of single-finger NCs and distal finger motifs in two-finger NCs. Molecular dynamics simulation suggested the formation of a hydrogen bonded network that weakly structured the Cys36-Cys39 segment in the apopeptide. This network depends on the protonation state of Cys36 and may thus explain the biphasic behavior of the pH-dependence of the Cys39 H(beta2) resonance. Finally, the pK(a) values were used to build up a model describing the coordination of Zn2+ to (35-50)NCp7 at equilibrium. It appears that each protonation step of the coordination complex decreases the Zn2+-binding constant by about four orders of magnitude and that a significant dissociation of Zn2+ from the holopeptide can be achieved in acidic cell compartments.

Exciton interaction in molecular beacons: a sensitive sensor for short range modifications of the nucleic acid structure

Molecular beacons are hairpin-shaped, single-stranded oligonucleotides constituting sensitive fluorescent DNA probes widely used to report the presence of specific nucleic acids. In its closed form the stem of the hairpin holds the fluorophore covalently attached to one end, close to the quencher, which is covalently attached to the other end. Here we report that in the closed form the fluorophore and the quencher form a ground state intramolecular heterodimer whose spectral properties can be described by exciton theory. Formation of the heterodimers was found to be poorly sensitive to the stem sequence, the respective positions of the dyes and the nature of the nucleic acid (DNA or RNA). The heterodimer allows strong coupling between the transition dipoles of the two chromophores, leading to dramatic changes in the absorption spectrum that are not compatible with a Förster-type fluorescence resonance energy transfer (FRET) mechanism. The excitonic heterodimer and its associated absorption spectrum are extremely sensitive to the orientation of and distance between the dyes. Accordingly, the application of molecular beacons can be extended to monitoring short range modifications of the stem structure. Moreover, the excitonic interaction was also found to operate for doubly end-labeled duplexes.

DNA condensation by an oxidizable cationic detergent. Interactions with lipid vesicles

Cationic amphiphile-mediated delivery of plasmid DNA is the non-viral gene transfer method most often used. In the present work, we considered a new cysteine-detergent, ornithinyl-cysteinyl-tetradecylamide (C(14)-CO), able to convert itself, via oxidative dimerization, into a cationic cystine-lipid. By using fluorescence techniques, we first characterized the structure of complexes of plasmid DNA with C(14)-CO molecules either kept as monomers, or oxidized into dimers. Both forms are able to condense DNA, with the formation of hydrophobic micelle-like domains along the DNA chain. Domains with a larger molecular order were obtained with dimeric C(14)-CO/DNA complexes. In a second step, the interactions of these complexes with lipid vesicles considered as membrane models were investigated. In the presence of vesicles, we observed a decondensation of the DNA involved in complexes obtained with C(14)-CO monomers. With anionic vesicles, the DNA is released into the bulk solution, while with neutral vesicles, it remains bound to the vesicles via electrostatic interactions with inserted C(14)-CO molecules. In sharp contrast, the complexes with C(14)-CO dimers are unaffected by the addition of either neutral or anionic vesicles and show no interaction with them. These results may partly explain the low transfection efficiency of these complexes at the +/-charge ratios used in this study.

Gene transfer by cationic surfactants is essentially limited by the trapping of the surfactant/DNA complexes onto the cell membrane: a fluorescence investigation

The interaction between complexes of plasmid DNA with cetyltrimethylammonium bromide (CTAB) and L929 fibroblasts was first examined using confocal microscopy. The complexes labeled with the DNA intercalator, YOYO-1, were found to be trapped onto the external face of the plasma membrane; a feature that may constitute a major limiting step in transfection. Moreover, since no cytotoxic effect appeared in these conditions, we further inferred that the CTAB molecules remained bound to the DNA. The interaction of the complexes with the membranes was best modeled with neutral vesicles. From anisotropy thermotropic curves of DPHpPC-labeled vesicles and fluorescence resonance energy transfer measurements between these vesicles and YOYO-labeled complexes, we evidenced that the binding of the complexes to the vesicle surface opened the micelle-like domains and unwound DNA. However, DNA was not released but remained stably bound via electrostatic interactions to the CTAB molecules incorporated in the external liposome leaflet. Consequently, the large diameter of the unwound plasmid DNA is likely the major factor that precludes its internalization into the cells by endocytosis. In contrast, anionic vesicles that mimic the cytoplasmic facing monolayer of the plasma membrane rapidly released DNA from the complex. This may explain the previously reported high transfection efficiency of DNA complexed with liposomes composed of neutral lipids and cationic surfactants, since the latter may destabilize the endosomal membrane and induce the release of DNA in the cytoplasm.

Circular dichroic investigation of the native and non-native conformational states of the growth factor receptor-binding protein 2 N-terminal src homology domain 3: effect of binding to a proline-rich peptide from guanine nucleotide exchange factor

SH3 (src homology domain 3) domains are small protein modules that interact with proline-rich peptides. The structure of the N-terminal SH3 domain from growth factor receptor-binding protein 2 (Grb2), an adapter protein in the intracellular signaling pathway to Ras, was investigated by circular dichroic (CD) spectroscopy. The compact native beta-barrel conformation, previously elucidated by NMR spectroscopy, was largely predominant at pH = 4.8, in the absence of salt. From the structural changes induced by varying pH, ionic strength, temperature, or hydrophobicity of the environment, evidence for the existence of distinct nonnative conformations was obtained in the far- and near-UV domains. Along the free energy scale, these appear to distribute into two conformational ensembles, depending on the extent of structural and thermodynamic differences compared to the native conformation. The first ensemble consists of non-native conformations with a nativelike secondary structure, and the second is composed of partially unfolded conformations having short alpha-helical fragments or turnlike motifs in their nonnative secondary structure. Most of the observed nonnative conformations exist in mild or nondenaturing conditions. They probably have distinct compactness of their inner structure, depending on the strength of nonlocal interactions, but only the native all-beta conformation possesses a condensed protein exterior, appropriate for the binding to the VPPPVPPRRR decapeptide from Sos. Upon binding, the native conformation undergoes a local tertiary structure change in a hydrophobic pocket at the binding site. This is accompanied by the PP-II helix folding of the proline-rich peptide. Interestingly, in the near-UV domain, a significant change in the spectral contribution of an aromatic exciton was observed, thus allowing quantitative tracking of the binding process.

Nucleic acid sequence discrimination by the HIV-1 nucleocapsid protein NCp7: a fluorescence study

The critical functions of the HIV-1 nucleocapsid protein NCp7 in genomic RNA packaging and reverse transcription, essentially rely on interactions with nucleic acids. A significant progress in the knowledge of these interactions has been recently achieved with the NMR-derived structures of NCp7 derivatives in complex with two short sequences of the HIV-1 psi packaging signal, namely ACGCC and the stem-loop 3 (SL3) motif. To further identify the key nucleotides in the formation of both NCp7-d(ACGCC) and NCp7-SL3 complexes, we quantitatively analyzed by steady-state and time-resolved fluorescence, the interaction of NCp7 with d(ACGCC) and SL3 mutants where each nucleotide in interaction with the protein has been systematically substituted. Moreover, by using several NCp7 derivatives, we investigated the contributions of Phe16, Trp37, and Trp61, and the various NCp7 domains, in the binding process. The binding of NCp7 appeared essentially driven by the interaction of the zinc finger domain and notably Trp37 with a G residue, irrespective of its location in the oligonucleotide. The involvement of Trp37 in the binding process depended on its location in the C-terminal finger motif and the proper folding of this motif. Phe16 in the N-terminal finger motif also strongly contributed to the binding energy, while in contrast, Trp61 in the C-terminal domain only marginally interacted with the oligonucleotides. The stem-loop structure of SL3 stabilized the binding of NCp7 by about -7 kJ/mol (at 0.1 M NaCl) by favoring the electrostatic binding of both N- and C-terminal domains. Finally, we found that NCp7 bound to nucleic acid single-stranded regions with the following preference: X(i)()TGX(j)() > X(i)()GXGX(j)() approximately X(i)()TXGX(j)() > X(i)()GX(j)() >> X(i)()X(j)(), where X corresponds to either A or C. This implies that recognition of nucleic acids by NCp7 may be achieved by a limited number of sites, and hence, no strong affinities are required in order to get a selective binding.

Time-resolved fluorescence investigation of the human immunodeficiency virus type 1 nucleocapsid protein: influence of the binding of nucleic acids

Depending on the HIV-1 isolate, MN or BH10, the nucleocapsid protein, NCp7, corresponds to a 55- or 71-amino acid length product, respectively. The MN NCp7 contains a single Trp residue at position 37 in the distal zinc finger motif, and the BH10 NCp7 contains an additional Trp, at position 61 in the C-terminal chain. The time-resolved intensity decay parameters of the zinc-saturated BH10 NCp7 were determined and compared to those of single-Trp-containing derivatives. The fluorescence decay of BH10 NCp7 could be clearly represented as a linear combination (with respect to both lifetimes and fractional intensities) of the individual emitting Trp residues. This suggested the absence of interactions between the two Trp residues, a feature that was confirmed by molecular modeling and fluorescence energy transfer studies. In the presence of tRNAPhe, taken as a RNA model, the same conclusions hold true despite the large fluorescence decrease induced by the binding of tRNAPhe. Indeed, the fluorescence of Trp37 appears almost fully quenched, in keeping with a stacking of this residue with the bases of tRNAPhe. Despite the multiple binding sites in tRNAPhe, the large prevalence of ultrashort lifetimes, associated with the stacking of Trp37, suggests that this stacking constitutes a major feature in the binding process of NCp7 to nucleic acids. In contrast, Trp61 only stacked to a small extent with tRNAPhe. The behavior of this residue in the tRNAPhe-NCp7 complexes appeared to be rather heterogeneous, suggesting that it does not constitute a major determinant in the binding process. Finally, our data suggested that the binding of NCp7 proteins from the two HIV-1 strains to nonspecific nucleic acid sequences was largely similar.

Properties and growth mechanism of the ordered aggregation of a model RNA by the HIV-1 nucleocapsid protein: an electron microscopy investigation

NCp7, the nucleocapsid protein of the human immunodeficiency virus type 1, induces an ordered aggregation of RNAs, a mechanism that is thought to be involved in the NCp7-induced promotion of nucleic acid annealing. To further investigate this aggregation the morphology and the properties of the NCp7-induced aggregates of the model RNA homoribopolymer, polyA, were investigated by electron microscopy in various conditions. In almost all the tested conditions, the aggregates were spherical and consisted of a central dense core surrounded by a less dense halo made of NCp7-covered polyA molecules. The formation of these aggregates with a narrow distribution of sizes constitutes a distinctive feature of NCp7 over other single-stranded nucleic acid binding proteins. In most conditions, at the shortest times that can be reached experimentally, all the polyA molecules were already incorporated in small aggregates, suggesting that the nucleation step and the first aggregation events took place rapidly. The aggregates then orderly grew with time by fusion of the smaller aggregates to give larger ones. The aggregate halo was important in the fusion process by initiating the bridging between the colliding aggregates. In the presence of an excess of protein, the aggregates grew rapidly but were loosely packed and dissociated easily, suggesting adverse protein-protein interactions in the aggregates obtained in these conditions. In the presence of an excess of nucleotides, the presence of both amorphous nonspherical and slowly growing spherical aggregates suggested some changes in the mechanism of aggregate growth due to an incomplete covering of polyA molecules by NCp7. Finally, we showed that in the absence of added salt, the aggregate fusions were unfavored but not the initial events giving the first aggregates, the reverse being true in the presence of high salt concentrations (> or = 300 mM).

Cysteine oxidation in the mitogenic S100B protein leads to changes in phosphorylation by catalytic CKII-alpha subunit

The glial-derived calcium-binding protein S100B can be secreted to act as a neurotrophic factor or a mitogen, stimulating proliferation of glial cells. The extracellular S100B activities rely on the oxidation of the protein cysteine residues (Kligman, D., and Marshak, D. R. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 7136-7139; Winningham-Major, F., Staecker, J. L., Barger, S. W., Coats, S., and Van Eldik, L. J. (1989) J. Cell Biol. 109, 3063-3071). Here we show that oxidation of the S100B cysteine residues, Cys-68 and Cys-84, induces a conformational change in the protein structure, unmasking a canonical CKII phosphorylation site located within the typical EF-hand calcium-binding site IIbeta. Intrasubunit disulfide-bridged S100B monomer and disulfide-bonded S100B dimer are phosphorylated by the catalytic CKII-alpha subunit on Ser-62 with a Km of 0.5 microM and a Vmax of 10 pmol/min/100 pmol of S100B. Oxidized S100B is the best in vitro CKII-alpha substrate identified so far. Next we show that intrasubunit disulfide-bridged S100B monomer is the most potent S100B species to stimulate [3H]thymidine uptake by C6 glial cells in culture. In addition, the phosphorylated intrasubunit disulfide-bridged S100B monomer retains apparent mitogenic activity toward C6 glial cells, and hence, 32P-labeled S100B should be a useful probe for characterizing the mechanisms by which extracellular oxidized S100B functions. Finally, we show that formation of intrasubunit disulfide-bridged S100B monomer is stimulated by peroxynitrite anion, suggesting that production of mitogenic S100B species could be enhanced in neuropathology associated with peroxynitrite anion production.

Mapping the suramin-binding sites of human neutrophil elastase: investigation by fluorescence resonance energy transfer and molecular modeling

Neutrophil elastase (NE), a mediator of inflammation, binds with high affinity numerous anionic molecules including suramin, a polysulfated naphthylurea, which inhibits it with a Ki of 0.2 microM and a 4:1 suramin:NE stoichiometry and thus constitutes a potential therapeutic agent. In an attempt to locate the suramin molecules on NE, we investigated the NE-suramin interaction using steady-state and time-resolved fluorescence spectroscopy. The time-resolved intensity decay of NE, a protein with three Trp residues, in positions 27, 141, and 237 (chymotrypsin numbering system) was best described by a three-exponential function with lifetimes ranging from 0.22 to 2.28 ns. Comparison of the accessibility of the three lifetime classes to the fluorescence quenchers acrylamide and iodide with the computed solvent accessibility of the three Trp residues in the crystal structure of NE indicates that the main, if not the sole, contribution to the 2.28 ns lifetime class is brought about by the fully buried Trp 141 residue. The addition of suramin to NE induces a sharp decrease in NE fluorescence and a corresponding increase in suramin fluorescence due to an efficient fluorescence resonance energy transfer (FRET) between the Trp residues of NE, acting as donors, and the naphthalene rings of suramin, behaving as acceptors. From the fate of the longest lifetime class in the presence of variable suramin concentrations, we deduce that two suramins are bound at less than 17 A from Trp 141, whereas the two others are located at least 29 A from Trp 141. Moreover, neither the binding of suramin to NE nor the FRET process was modified when NE was complexed with a peptide chloromethylketone inhibitor, suggesting that suramin does not directly interfere with the substrate binding site of NE. These data were used as constraints to model the NE-suramin complex.

Ordered aggregation of ribonucleic acids by the human immunodeficiency virus type 1 nucleocapsid protein

The nucleocapsid protein NCp7, which is the major genomic RNA binding protein of human immunodeficiency virus type 1, plays an important role in several key steps of the viral life cycle. Many of the NCp7 activities, notably the nucleic acid annealing and the genomic RNA wrapping ones, are thought to be linked to a nonspecific binding of NCp7 to its nucleic acid targets. The mechanism of these activities is still debated but several clues are in favor of an intermediate aggregation of nucleic acids by NCp7. To check and characterize the nucleic acid aggregating properties of NCp7, we investigated the interaction of NCp7 with the model RNA homopolymer, polyA, by quasielastic light scattering and optical density measurements. The ordered growth of monodisperse large particles independently of the nucleic acid size and the almost complete covering of polyA by NCp7 strongly suggested an ordered aggregation mechanism. The aggregate kinetics of growth in the optimum protein concentration range (> or = 2 microM) were governed by a so-called Ostwald ripening mechanism limited by transfer of NCp7-covered polyA complexes from small to large aggregates. The aggregation process was strongly dependent on both Na+ and Mg2+ concentrations, the optimum concentrations being in the physiological range. Similar conclusions held true when polyA was replaced by 16S + 23S ribosomal RNA, suggesting that the NCp7 aggregating properties were only poorly dependent on the nucleic acid sequence and structure. Finally, as in the NCp7 annealing activities, the basic regions of NCp7, but not the zinc fingers, were found critical in nucleic acid aggregation. Taken together, our data indicate that NCp7 is a highly efficient nucleic acid aggregating agent and strengthen the hypothesis that aggregation may constitute a transient step in various NCp7 functions.

Zinc binding to the HIV-1 nucleocapsid protein: a thermodynamic investigation by fluorescence spectroscopy

The HIV-1 nucleocapsid protein, NCp7, is characterized by two CCHC zinc finger motifs which have been shown to stoichiometrically bind zinc in mature virions. Moreover, this binding of zinc proves to be critical in various NCp7 functions, especially in the encapsidation process. To further understand the central role of zinc binding to NCp7, we closely investigated the zinc binding properties of NCp7 and various deleted or substituted derivatives. To this end, the fluorescence of wither the naturally occurring Trp37 or the conservatively substituted Trp16 was used to monitor the binding of zinc to the N- and C-terminal finger motifs, respectively. At pH 7.5, the NCp7 proximal motif was found to bind zinc strongly with 2.8 x 10(14) M-1 binding constant about five times higher than the NCp7 distal motif. Moreover, the binding of zinc to one finger motif decreased the affinity of the second one, and this negative cooperativity was shown to be related to the spatial proximity of the zinc-saturated finger motifs. The binding seemed to be almost equally driven by entropy and enthalpy, and the binding information was essentially encoded by the finger motifs themselves whereas the other parts of the protein only played a marginal stabilization role. As expected, the Cys and His residues of the CCHC motifs were critical and competition between protons and zinc ions to these residues induced a steep pH-dependence of the zinc binding constants to both sites. Taken together, our data provide further evidence for the nonequivalence of the two NCp7 finger motifs.

Binding of the HIV-1 nucleocapsid protein to the primer tRNA(3Lys), in vitro, is essentially not specific

The nucleocapsid protein NCp7 of human immunodeficiency virus, type 1, is a key component in the viral life cycle. Since, the first common step of all reported NCp7 activities corresponds to a nucleic acid-binding step, the NCp7 binding parameters to the natural primer tRNA(3Lys) were investigated. Using NCp7 intrinsic fluorescence, we found that (i) in 0.1 M NaCl, NCp7 bound noncooperatively to tRNA(3Lys) with a Kobs = 3.2 x 10(6) M-1 association constant and a n = 6 binding site size, (ii) four ionic interactions were formed in the NCp7.tRNA(3Lys) complex, and (iii) nonelectrostatic factors provided about 60% of the binding energy. These binding parameters were not significantly altered when the natural tRNA(3Lys) was replaced by either an in vitro synthetic tRNA(3Lys) transcript, the heterologous yeast tRNA(Phe) or the structurally unrelated 5 S RNA from Escherichia coli. Moreover, the environment of the intrinsic fluorescent reporters (Trp37 and Trp61) was similar in the various complexes. Finally, experiments performed at low protein concentration provide no evidence of high affinity binding sites. Taken together, our data strongly suggested an essentially nonspecific binding of NCp7 to tRNA(3Lys) and thus did not seem to support a direct role of NCp7, per se, in the selection of tRNA(3Lys) from the pool of cellular tRNAs.

Spatial proximity of the HIV-1 nucleocapsid protein zinc fingers investigated by time-resolved fluorescence and fluorescence resonance energy transfer

The three-dimensional structure of peptides encompassing the two zinc-saturated finger motifs of the nucleocapsid protein NCp7 of HIV-1 has been reported by several groups. Whereas the folded structures of the finger motifs were in good agreement, discrepancies existed concerning their spatial relationship since the fingers were found either close to each other [Morellet, N., Jullian, N., De Rocquigny, H., Maigret, B., Darlix, J. L., & Roques, B. P. (1992) Embo J. 11, 3059-3065] or independently folded [Omichinski, J. G., Clore, G. M., Sakaguchi, K., Appella, E., & Gronenborn, A. M. (1991) FEBS Lett. 292, 25-30, Summers, M. F., Henderson, L. E., Chance, M. R., Bess, J. W., Jr., South, T. L., Blake, P. R., Sagi, I., Perez-Alvarado, G., Sowder, R.C., III, Hare, D.R., & Arthur, L. O. (1992) Protein Sci. 1, 563-574]. As in the interacting finger model, Phe16 in the NH2-terminal finger and Trp37 in the COOH-terminal finger were found to be spatially close, the fluorescence properties of the aromatic residues at positions 16 and 37 in the wild-type and two conservatively substituted (12-53) NCp7 peptides were investigated and compared with those of three negative control derivatives where the finger motifs were not in close contact. Direct distance measurements by Tyr-Trp fluorescence resonance energy transfer of the former derivatives yielded a 7-12 A interchromophore distance range which is clearly inconsistent with the 12.5-18 A range measured for the negative controls and thus a random orientation of the zinc finger motifs.(ABSTRACT TRUNCATED AT 250 WORDS)

Conformational behaviour of the active and inactive forms of the nucleocapsid NCp7 of HIV-1 studied by 1H NMR

The nucleocapsid protein NCp7 of the human immunodeficiency virus type I (HIV-1) is a 72 amino acid peptide containing two zinc fingers of the type CX2CX4HX4C linked by a short basic sequence 29RAPRKKG35. NCp7 was shown to activate in vitro both viral RNA dimerization and replication primer tRNA(Lys,3) annealing to the initiation site of reverse transcription. In order to clarify the possible structural role of the zinc fingers in the various functions of NCp7, complete sequence specific 1H NMR assignment of the entire protein was achieved by two-dimensional NMR experiments. Moreover, to characterize the role of the peptide linker in NCp7 folding, a synthetic analogue with an inversion of Pro31 configuration was studied by NMR and fluorescence techniques. Several long range NOEs implying amino acid protons from the folded zinc fingers and the spacer, such as Ala25 and Trp37, Phe16 and Trp37, Arg32 and Trp37, Lys33 and Trp37, Cys18 and Lys33 disappeared in the D-Pro31 (12-53)NCp7, confirming the spatial proximity of the two CCHC boxes observed in the (13-51)NCp7. This was also confirmed by iodide fluorescence quenching experiments. The N and C-terminal parts of NCp7 displayed a large flexibility except for two short sequences Tyr56 to Gly58 and Tyr64 to Gly66, which seemed to oscillate between random-coil and helical conformations. The biological relevance of the structural characteristics of NCp7 was studied in vitro and in vivo. Substitution of Pro31 by D-Pro31 in the active (13-64)NCp7 peptide led to a severe reduction of dimerization in vitro. Moreover, site-directed mutagenesis substituting Leu for Pro31 resulted in the formation of non-infectious and immature viral particles. These results suggest that the spatial proximity of the zinc fingers induced by the peptide linker, plays a critical role in encapsidation of genomic RNA and morphogenesis of HIV-1 infectious particles.

Structural and dynamic characterization of the aromatic amino acids of the human immunodeficiency virus type I nucleocapsid protein zinc fingers and their involvement in heterologous tRNA(Phe) binding: a steady-state and time-resolved fluorescence study

The steady-state and time-resolved fluorescence properties of two zinc-saturated 18-residue synthetic peptides with the amino acid sequence of the NH2-terminal (NCp7 13-30 F16W, where the naturally occurring Phe was replaced by a Trp residue) and the COOH-terminal (NCp7 34-51) zinc finger domains of human immunodeficiency virus type I nucleocapsid protein were investigated. Fluorescence intensity decay of both Trp 16 and Trp 37 residues suggested the existence of two fully solvent-exposed ground-state classes governed by a C = 2.2 equilibrium constant. The lifetimes of Trp 16 classes differed from those of Trp 37 essentially because of differences in nonradiative rate constants. Arrhenius plots of the temperature-dependent nonradiative rate constants suggested that the fluorescence quenchers involved in both classes and in both peptides were different and the collisional rate of these quenchers with the indole ring was very low, probably because of the highly constrained peptide chain conformation. The nature of the ground-state classes was discussed in relation to 1H nuclear magnetic resonance data. Using Trp fluorescence to monitor the interaction of both peptides with tRNA(Phe) we found that a stacking between the indole ring of both Trp residues and the bases of tRNA(Phe) occurred. This stacking constituted the main driving force of the interaction and modified the tRNA(Phe) conformation. Moreover, the binding of both fingers to tRNA(Phe) was noncooperative with similar site size (3 nucleotide residues/peptide), but the affinity of the NH2-terminal finger domain (K = 1.3 (+/- 0.2) 10(5) M-1) in low ionic strength buffer was one order of magnitude larger than the COOH-terminal one due to additional electrostatic interactions involving Lys 14 and/or Arg 29 residues.