Conformational states of the nuclear GTP-binding protein Ran and its complexes with the exchange factor RCC1 and the effector protein RanBP1

It has been shown before by (31)P NMR that Ras bound to the nonhydrolyzable GTP analogue guanosine 5'-O-(beta, gamma-imidotriphosphate) (GppNHp) exists in two conformations which are rapidly interconverting with a rate constant of 3200 s-1 at 30 degrees C [Geyer, M., et al. (1996) Biochemistry 35, 10308-10320]. Here we show that Ran complexed with GTP also exists in two conformational states, 1 and 2, which can be directly inferred from the occurrence of two (31)P NMR resonance lines for the gamma-phosphate group of bound GTP. The exchange between the two states is slow on the NMR time scale with a value of <200 s-1 at 5 degrees C for the corresponding first-order rate constants. In wild-type Ran, the equilibrium constant K' between the two states is 0.7 at 278 K, is different for various mutants, and is strongly dependent on the temperature. The standard enthalpy DeltaH degrees and the standard entropy DeltaS degrees for the conformational transitions determined from the NMR spectra are as follows: DeltaH degrees = 37 kJ mol-1 and DeltaS degrees = 130 J mol-1 K-1 for wild-type Ran.GTP. In complex with the Ran-binding protein RanBP1, one of the Ran.GTP conformations (state 2) is stabilized. The interaction of Ran with the guanine nucleotide exchange factor protein RCC1 was also studied by (31)P NMR spectroscopy. In the presence of nucleotide, the ternary complex of Ran.nucleotide.RCC1, an intermediate in the guanine nucleotide exchange reaction, could be observed. A model for the conformational transition of Ran.GTP is proposed where the two states observed are caused by the structural flexibility of the effector loop of Ran; in solution, state 2 resembles the GTP-bound form found in the crystal structure of the Ran-RanBP complex.

Structure of the anchor-domain of myristoylated and non-myristoylated HIV-1 Nef protein

Negative factor (Nef) is a regulatory myristoylated protein of human immunodeficiency virus (HIV) that has a two-domain structure consisting of an anchor domain and a core domain separated by a specific cleavage site of the HIV proteases. For structural analysis, the HIV-1 Nef anchor domain (residues 2-57) was synthesized with a myristoylated and non-myristoylated N terminus. The structures of the two peptides were studied by1H NMR spectroscopy and a structural model was obtained by restrained molecular dynamic simulations. The non-myristoylated peptide does not have a unique, compactly folded structure but occurs in a relatively extended conformation. The only rather well-defined canonical secondary structure element is a short two-turn alpha-helix (H2) between Arg35 and Gly41. A tendency for another helical secondary structure element (H1) can be observed for the arginine-rich region (Arg17 to Arg22). Myristoylation of the N-terminal glycine residue leads to stabilization of both helices, H1 and H2. The first helix in the arginine-rich region is stabilized by the myristoylation and now contains residues Pro14 to Arg22. The second helix appears to be better defined and to contain more residues (Ala33 to Gly41) than in the absence of myristoylation. In addition, the hydrophobic N-terminal myristic acid residue interacts closely with the side-chain of Trp5 and thereby forms a loop with Gly2, Gly3 and Lys4 in the kink region. This interaction could possibly be disturbed by phosphorylation of a nearby serine residue, and modifiy the characteristic membrane interactions of the HIV-1 Nef anchor domain.

Structural and biochemical analysis of Ras-effector signaling via RalGDS

The structure of the complex of Ras with the Ras-binding domain of its effector RalGDS (RGS-RBD), the first genuine Ras-effector complex, has been solved by X-ray crystallography. As with the Rap-RafRBD complex (Nasser et al., 1995), the interaction is via an inter-protein beta-sheet between the switch I region of Ras and the second strand of the RGS-RBD sheet, but the details of the interactions in the interface are remarkably different. Mutational studies were performed to investigate the contribution of selected interface residues to the binding affinity. Gel filtration experiments show that the Ras x RGS-RBD complex is a monomer. The results are compared to a recently determined structure of a similar complex using a Ras mutant (Huang et al., 1998) and are discussed in relation to partial loss-of-function mutations and the specificity of Ras versus Rap binding.

Thermodynamic and kinetic characterization of the interaction between the Ras binding domain of AF6 and members of the Ras subfamily

Cellular signaling downstream of Ras is highly diversified and may involve many different effector molecules. A potential candidate is AF6 which was originally identified as a fusion to ALL-1 in acute myeloid leukemia. In the present work the interaction between Ras and AF6 is characterized and compared with other effectors. The binding characteristics are quite similar to Raf and RalGEF, i.e. nucleotide dissociation as well as GTPase-activating protein activity are inhibited, whereas the intrinsic GTPase activity of Ras is unperturbed by AF6 binding. Particularly, the dynamics of interaction are similar to Raf and RalGEF with a lifetime of the Ras. AF6 complex in the millisecond range. As probed by 31P NMR spectroscopy one of two major conformational states of Ras is stabilized by the interaction with AF6. Looking at the affinities of AF6 to a number of Ras mutants in the effector region, a specificity profile emerges distinct from that of other effector molecules. This finding may be useful in defining the biological function of AF6 by selectively switching off other pathways downstream of Ras using the appropriate effector mutant. Notably, among the Ras-related proteins AF6 binds most tightly to Rap1A which could imply a role of Rap1A in AF6 regulation.

Determination of mean and standard deviation of dihedral angles

Backbone torsional angles are a characteristic and useful parameter for the description and characterisation of protein structures determined by x-ray crystallography or NMR spectroscopy. For the comparison of an ensemble of three-dimensional structures the calculation of the statistical parameters mean and standard deviation would be very useful. However, they are not defined unambiguously for periodic quantities such as the dihedral angles. In this paper a plausible and unique definition of these parameters is introduced and a straightforward method for their calculation is given.

Computer assisted assignment of 13C or 15N edited 3D-NOESY-HSQC spectra using back calculated and experimental spectra

A new tool, for the simulation of 15N or 13C edited 3D-NOESY-HSQC spectra using the complete relaxation matrix approach, has been developed and integrated in the program AURELIA. This tool should be particularly useful for the fast and reliable computer assisted assignment of 3D-NOESY-HSQC spectra by comparing back-calculated and experimental spectra in an iterative process. Folded spectra are sometimes used to enhance the digital resolution in the indirect dimensions of multidimensional spectra. However, these spectra are usually difficult to analyze. To simplify this assignment process we have incorporated the simulation and automated annotation of folded peaks into the program. It is hereby possible to simulate multiple folding in all three dimensions of 3D 15N- or 13C-NOESY-HSQC spectra. By comparing experimental 3D-NOESY-HSQC spectra with spectra back calculated from a single trial structure or a set of trial structures, a user can easily check if the final structures explain all experimental NOEs. The new feature has been successfully tested with the histidine-containing phosphocarrier protein HPr from Staphylococcus carnosus.

Control of K+ channel gating by protein phosphorylation: structural switches of the inactivation gate

Fast N-type inactivation of voltage-dependent potassium (Kv) channels controls membrane excitability and signal propagation in central neurons and occurs by a 'ball-and-chain'-type mechanism. In this mechanism an N-terminal protein domain (inactivation gate) occludes the pore from the cytoplasmic side. In Kv3.4 channels, inactivation is not fixed but is dynamically regulated by protein phosphorylation. Phosphorylation of several identified serine residues on the inactivation gate leads to reduction or removal of fast inactivation. Here, we investigate the structure-function basis of this phospho-regulation with nuclear magnetic resonance (NMR) spectroscopy and patch-clamp recordings using synthetic inactivation domains (ID). The dephosphorylated ID exhibited compact structure and displayed high-affinity binding to its receptor. Phosphorylation of serine residues in the N- or C-terminal half of the ID resulted in a loss of overall structural stability. However, depending on the residue(s) phosphorylated, distinct structural elements remained stable. These structural changes correlate with the distinct changes in binding and unbinding kinetics underlying the reduced inactivation potency of phosphorylated IDs.

Structure of the inactivating gate from the Shaker voltage gated K+ channel analyzed by NMR spectroscopy

Rapid inactivation of voltage-gated K+ (Kv) channels is mediated by an N-terminal domain (inactivating ball domain) which blocks the open channel from the cytoplasmic side. Inactivating ball domains of various Kv channels are also biologically active when synthesized separately and added as a peptide to the solution. Synthetic inactivating ball domains from different Kv channels with hardly any sequence homology mediate quite similar effects even on unrelated Kv channel subtypes whose inactivation domain has been deleted. The solution structure of the inactivating ball peptide from Shaker (Sh-P22) was analyzed with NMR spectroscopy. The NMR data indicate a non-random structure in an aqueous environment. However, while other inactivating ball peptides showed well-defined three-dimensional structures under these conditions, Sh-P22 does not have a unique, compactly folded structure in solution.

Structural studies of histidine-containing phosphocarrier protein from Enterococcus faecalis

Based on the complete sequential assignment of the 1H-NMR spectrum by multidimensional NMR techniques the secondary structure and the local geometry of the active site of histidine-containing phosphocarrier protein (HPr) from Enterococcus faecalis were elucidated. We present a comparative analysis of the active site in the seven known structures of HPr from different organisms determined by NMR or X-ray crystallography. In catalysis, HPr is phosphorylated at the ring N61 of His15. No general agreement exists in literature regarding the structure of the active-centre loop. In the crystal structure of HPr from E. faecalis, a torsion strain of the backbone at position 16 was observed, which was assumed to be important to the catalytic mechanism. Coupling constants were determined in order to calculate phi angles to establish whether there are strained torsion angles in HPr from E. faecalis in the solution state. The evaluation of data obtained indicate a stable and well-defined structure of HPr from E. faecalis, with an overall fold similar to that found in HPr from other bacteria. We find that in the active-site region there are relatively large variations in local geometry between the evaluated structures. In HPr from E. faecalis, a particularly detailed view of the phosphate-binding His15 and residues in close spatial proximity was obtained by determination of coupling constants obtained from the double-quantum-filtered COSY spectrum. Our data indicate that in aqueous solution, in the dominant conformational state there is no torsion strain of the backbone at position 16, as observed in the crystal state. The maximum population of a strained conformation in solution can be estimated to be smaller than 23%. The analysis of the data suggests that the active-centre loop is able to adopt different conformations in solution. A similar observation was made for HPr from E. faecalis phosphorylated at its regulatory site (Ser46). 31P-NMR shows that phosphorylated HPr exists in two conformational substates with nearly equal populations.

Use of global symmetries in automated signal class recognition by a bayesian method

Automated or semiautomated pattern recognition in multidimensional NMR spectroscopy is strongly hampered by the large number of noise and artifact peaks occurring under practical conditions. A general Bayesian method which is able to assign probabilities that observed peaks are members of given signal classes (e.g., the class of true resonance peaks or the class of noise and artifact peaks) was proposed previously. The discriminative power of this approach is dependent on the choice of the properties characterizing the peaks. The automated class recognition is improved by the addition of a nonlocal feature, the similarities of peak shapes in symmetry-related positions. It turns out that this additional property strongly decreases the overlap of the multivariate probability distributions for true signals and noise and hence largely increases the discrimination of true resonance peaks from noise and artifacts. Copyright 1997 Academic Press. Copyright 1997Academic Press

Structure of the Ras-binding domain of RalGEF and implications for Ras binding and signalling

The solution structure of the Ras-binding domain (RBD) of Ral guanine-nucleotide exchange factor RalGEF was solved by NMR spectroscopy. The overall structure is similar to that of Raf-RBD, another effector of Ras, although the sequence identity is only 13%. 15N chemical shifts changes in the complex of RalGEF-RBD with Ras indicate an interaction similar to the intermolecular beta-sheet observed for the complex between Ras and Raf-RBD.

Denaturation and reactivation of dimeric human glutathione reductase--an assay for folding inhibitors

Human glutathione reductase (GR; which catalyzes the reaction NADPH + GSSG + H+ --> 2 GSH + NADP+) is an obligatory FAD-containing homodimer of known geometry. Native human GR, a potential target of antimalarial and cytostatic agents, cannot be dissociated by dilution or by means of subunit-interface mimetics, similarly to well-studied viral dimeric proteins. However, ab initio folding and/or dimerization of human GR can be inhibited by point mutations or by peptides corresponding to subunit-interface areas, for example synthetic peptide P11, which represents the intersubunit-contact helix H11. The structure of this peptide, which might assist inhibitor design, was solved by high-resolution NMR spectroscopy. Residues 440-453, were found to be alpha helical in the isolated peptide. To quantitate the efficacy of inhibitors such as P11, we developed the following unfolding/reactivation assay. The effects of various guanidine hydrochloride (Gdn/HCl) concentrations were studied by analytical ultracentrifugation. It was shown that human GR denatured by greater than 3 M Gdn/HCl is monomeric and free of FAD. Circular-dichroism experiments at 223 nm indicated a half-life of approximately 20 s at 20 degrees C for the unfolding process. To optimize the reactivation yield, four parameters [protein concentration (x) in the range 0.3-10 microg/ml, cofactor supplementation, temperature (y: 0-32 degrees C), and time (0-72 h)] were varied systematically, and a reactivation score z was given to each constellation of parameters. This type of analysis might be useful to optimize refolding and activation yields for other proteins. For human GR, the highest recovery was found not to occur at one of the corners of the x,y plane, but close to its center. Consequently, the optimal assay conditions for folding and dimerization inhibitors are as follows. The enzyme (at 300 microg/ml) is denatured by 5 M guanidine hydrochloride/5 mM dithiothreitol, then reactivated by dilution to 1 microg/ml at pH 6.9 and 20 degrees C. In the absence of inhibitors, this procedure leads to 70% of the control activity within 8 h. Peptides representing the upper subunit interface (for instance residues 436-478) of human GR were found to inhibit refolding with EC50% values in the micromolar range, whereas fragments from other regions of the protein had no influence on this process. For peptide P11, the EC50% value was 20 microM. In conclusion, hGR, enzyme with a tight intersubunit contact area of 21 nm2, appears to be suitable for studying protein folding, dimerization, and prosthetic-group complexation in the absence and presence of compounds that inhibit these processes. There is a shortage, at least for oligomeric enzymes of eukaryotes, of published systematic studies on protein (re)activation.

Synthesis and NMR spectroscopy of peptides containing either phosphorylated or phosphonylated cis- or trans-4-hydroxy-L-proline

Many proteins are regulated by reversible O-glycosylation and O-phosphorylation. Whereas O-glycosylation of hydroxy-L-proline is common and well investigated, phosphorylation has not been proved so far in vivo, but this post-translational modification is entirely possible. As a first step to identify this phosphoamino acid, we describe both the syntheses of peptides phosphorylated at 4-hydroxy-L-proline and the 1H and 31P NMR parameters of these phosphopeptides. The model peptides were synthesized on solid-phase using Fmoc-strategy. Both natural isomers of 4-hydroxy-L-proline (containing the hydroxyl group in either the cis or trans position) were introduced without side-chain protection. All peptides were globally phosphorylated with O,O'-tert-butyl-N,N-diethylphosphoramidite on the solid phase and cleaved with trifluoroacetic acid. Additionally, we synthesized two classes of phosphonopeptides that mimic phosphopeptides, namely H- and methylphosphonopeptides. The NMR data were based on the model peptide Gly-Gly-Hyp-Ala, which is regarded as a typical random-coil sequence. The NMR parameters showed a significant influence of the phosphate group on the cis-trans isomerization of the Gly-Hyp bond, which may reflect a possible regulation of proteins by changing their local conformations. The 1H and 31P NMR parameters differed for each isomer, and were distinct from the parameters of phosphorylated serine, threonine and tyrosine. These known shifts can be used to identify both cis- and trans-O-phospho-4-hydroxy-L-proline in vivo.

NMR structure of inactivation gates from mammalian voltage-dependent potassium channels

The electrical signalling properties of neurons originate largely from the gating properties of their ion channels. N-type inactivation of voltage-gated potassium (Kv) channels is the best-understood gating transition in ion channels, and occurs by a 'ball-and-chain' type mechanism. In this mechanism an N-terminal domain (inactivation gate), which is tethered to the cytoplasmic side of the channel protein by a protease-cleavable chain, binds to its receptor at the inner vestibule of the channel, thereby physically blocking the pore. Even when synthesized as a peptide, ball domains restore inactivation in Kv channels whose inactivation domains have been deleted. Using high-resolution nuclear magnetic resonance (NMR) spectroscopy, we analysed the three-dimensional structure of the ball peptides from two rapidly inactivating mammalian K. channels (Raw3 (Kv3.4) and RCK4 (Kv1.4)). The inactivation peptide of Raw3 (Raw3-IP) has a compact structure that exposes two phosphorylation sites and allows the formation of an intramolecular disulphide bridge between two spatially close cysteine residues. Raw3-IP exhibits a characteristic surface charge pattern with a positively charged, a hydrophobic, and a negatively charged region. The RCK4 inactivation peptide (RCK4-IP) shows a similar spatial distribution of charged and uncharged regions, but is more flexible and less ordered in its amino-terminal part.

Relax, a flexible program for the back calculation of NOESY spectra based on complete-relaxation-matrix formalism

RELAX is a flexible program for the quantitative analysis of NOESY spectra. It allows the simultaneous application of different models describing the internal and overall motion of the molecule under investigation for individual spin pairs or groups of spins. A correction for anisotropy effects due to the deviation of the molecule from a spherical shape is calculated automatically from the trial structure. The program can deal with completely relaxed spectra as well as spectra recorded with a short relaxation delay. An execution-time-controlled splitting of the relaxation matrix reduces the computation time significantly without any loss of accuracy. This is especially important for large molecules or medium distance cutoffs.

Specific cleavage sites of Nef proteins from human immunodeficiency virus types 1 and 2 for the viral proteases

Human immunodeficiency virus type 2 (HIV-2) Nef is proteolytically cleaved by the HIV-2-encoded protease. The proteolysis is not influenced by the absence or presence of the N-terminal myristoylation. The main cleavage site is located between residues 39 and 40, suggesting a protease recognition sequence, GGEY-SQFQ. As observed previously for Nef protein from HIV-1, a large, stable core domain with an apparent molecular mass of 30 kDa is produced by the proteolytic activity. Cleavage of Nef from HIV-1 in two domains by its own protease or the protease from HIV-2 is also independent of Nef myristoylation. However, processing of HIV-1 Nef by the HIV-2 protease is less selective than that by the HIV-1 protease: the obtained core fragment is heterogeneous at its N terminus and has an additional cleavage site between amino acids 99 and 100. Preliminary experiments suggest that the full-length Nef of HIV-2 and the core domain are part of the HIV-2 particles, analogous to the situation reported recently for HIV-1.

Linear free energy relationships in the intrinsic and GTPase activating protein-stimulated guanosine 5'-triphosphate hydrolysis of p21ras

Controlling the hydrolysis rate of GTP bound to guanine nucleotide binding proteins is crucial for the right timing of many biological processes. Theoretical, structural, and functional studies have demonstrated that in p21ras the substrate of the reaction, GTP itself, plays a central role by acting as the base catalyst. This substrate-assisted reaction mechanism was analyzed with the help of linear free energy relationships (LFERs). Here we present experimental data that further support the proposed mechanism. We extend the LFER analysis to a wide range of oncogenic as well as nontransforming Ras mutants. It is illustrated that almost all Ras variants follow the observed LFER and thus also the same reaction path. Further, the reduced GTPase reaction rate that characterizes the oncogenic effect of many of the p21 mutants found in human tumors seems to be a consequence of a slightly reduced pKa of the gamma-phosphate group of bound GTP. Factors causing a pKa deviation of just 0.5 unit are enough to slow the intrinsic GTPase reaction rate significantly, and the system may exhibit as a consequence of this an oncogenic potential. Interestingly, we also found oncogenic mutations that do not follow the regular LFER. This suggests that the oncogenic effect of distinct Ras mutants has a different physical origin. The results presented might aid in the design of drugs aimed at reactivating the GTPase reaction of many oncogenic p21ras mutants. We also analyzed the stimulated GTPase reaction of p21ras by the GTPase activating protein (GAP) and the GTPase reaction of Rap1A, a Ras-related GTP binding protein, with similar approaches. The corresponding results indicate that the GAP-stimulated GTPase as well as the Rap1A-catalyzed reaction seem to follow the same substrate-assisted reaction mechanism. However, the correlation coefficient for the GAP-catalyzed reaction is different from the corresponding coefficient for the intrinsic reaction. While the intrinsic reaction exhibits a Brønsted slope of beta = 2.1, the corresponding value for the GAP-activated reaction is beta = 4.9.

Mobility of the N-terminal segment of rabbit skeletal muscle F-actin detected by 1H and 19F nuclear magnetic resonance spectroscopy

After polymerization filamentous actin (F-actin) still shows a number of rather narrow 1H NMR signals in its Mg2+ form which are quenched when Mg2+ is replaced by Ca2+. These resonances originate from mobile residues in F-actin. For assignment of these resonances three different strategies were used, the fluorine labeling of Cys-374 by 4-(perfluoro-tert-butyl)phenyliodoacetamide, binding studies with antibodies (Fab) against the seven N-terminal amino acids of actin, and two-dimensional 1H NMR spectroscopy of a highly concentrated F-actin sample. In contrast to the effects detected earlier by 1H NMR spectroscopy, 19F NMR spectroscopy of actin labeled at its C-terminal cysteine shows no significant spectral changes in dependence on the divalent ion present. In its G- (globular) form a strong, narrow 19F resonance can be observed at 15.06 ppm (relative to the external standard trifluoroacetic acid) which is broadened substantially after polymerization of actin. At 283 K the corresponding transverse relaxation time T2 decreases from 16.7 ms to approximately 0.6 ms. These data suggest that the highly mobile residues observed by 1H NMR spectroscopy do not originate from the C-terminus. Binding of Fab directed against the N-terminal amino acids of actin to Mg-F-actin leads to the disappearing of the 1H NMR resonances assigned to a mobile domain in F-actin. This indicates that the mobile region probably comprises the N-terminal amino acids. By homonuclear two-dimensional 1H NMR spectroscopy it was finally possible to sequentially assign the resonances of the mobile domain of F-actin. It turned out that amino acids 1-22 are in a highly mobile state in Mg-F-actin. The nuclear Overhauser effect data indicate that, rather surprisingly, in this high mobility state some of the beta-pleated structure is still conserved. The population of F-actin protomers in the M- (mobile) state can be obtained from the NMR spectra and was determined under different experimental conditions. In the presence of 150 mM KCl approximately half of the protomers in Mg-F-actin are in the M-state. This number is largely independent of the pH in the range studied (pH 7.2-7.8) and of the temperature in range studied (283-310 K). The equilibrium constant KMI for the equilibrium between the I- and M-states is approximately 1.3 under these conditions.

The recombinant dehydrin-like desiccation stress protein from the resurrection plant Craterostigma plantagineum displays no defined three-dimensional structure in its native state

Dehydration stress in the drought-tolerant resurrection plant Craterostigma plantagineum is accompanied by the accumulation of a large number of desiccation stress proteins (Dsp). One abundant class of these is represented by the dehydrin-related Dsp16 protein which contains 15 amino acid conserved lysine-rich repeats and a stretch of eight serine residues providing extremely hydrophilic characteristics. Recombinant Dsp16 from Craterostigma plantagineum has been cloned and expressed in Escherichia coli. The protein was purified and characterized regarding its physicochemical properties. Irrespective of successful crystallization experiments, dilute aqueous buffer solutions do not display a well-defined three-dimensional structure in terms of the canonical secondary structural elements. 1H-NMR (nuclear magnetic resonance) spectra in aqueous solution are characterized by a small chemical shift dispersion typical for an unfolded protein; however, the observed line-widths are not typical for a highly mobile random coil structure. Instead they indicate an equilibrium between conformational states with preferentially extended substructures. As a consequence of its loose structure, Dsp16 is extremely sensitive towards proteolysis unless its structure is stabilized by structure-making additives such as trifluoroethanol. Denaturants such as guanidinium chloride do not induce cooperative structural transitions. pH-dependent fluorescence changes reflect protonation/deprotonation rather than conformational changes. Sedimentation/diffusion experiments confirm the predicted molecular mass of 16 kDa. Due to the high serine/threonine content and its loose structure, Dsp16 is accessible to phosphorylation, supporting the idea that in situ the structurally relatively undefined protein may be involved in both water binding and phosphorylation.

Polyamine spider toxins and mammalian N-methyl-D-aspartate receptors. Structural basis for channel blocking and binding of argiotoxin636

Recombinant N-methyl-D-aspartate receptors composed of NR1/NR2A subunits were expressed in Xenopus oocytes to analyse the voltage-dependent and use-dependent channel blocking activity of argiotoxin636. Functional assays demonstrate that the toxin competes with other open channel blockers such as Mg2+ and MK-801. Direct binding or competition assays using radiolabeled ligands and isolated rat brain membranes, in contrast, reveal no specific binding or yield binding constants which differ by orders of magnitude from the IC50 values of the functional assays. One explanation is that argiotoxin636 does not bind with high affinity to the inhibitory site in the N-methyl-D-aspartate-receptor channel under in vitro conditions when membranes are depolarised. The structure of argiotoxin636 was investigated by NMR spectroscopy. In solution the positively charged argiotoxin636 acquires an extended conformation and its dimensions might allow permeation deep into the channel. In the absence of direct structural information on the channel protein, the detailed analysis of blockade in conjunction with structural information, as provided here, may be of aid in the deduction of structural features of glutamate-receptor channel ion pores.

Conformational transitions in p21ras and in its complexes with the effector protein Raf-RBD and the GTPase activating protein GAP

31P NMR revealed that the complex of p21ras with the GTP analog GppNHp.Mg2+ exists in two conformational states, states 1 and 2. In wild-type p21ras the equilibrium constant K1(12) between the two states is 1.09. The population of these states is different for various mutants but independent of temperature. The activation enthalpy delta H ++ and activation entropy delta S ++ for the conformational transitions were determined by full-exchange matrix analysis for wild-type p21ras and p21ras(S65P). For the wild-type protein one obtains delta H ++ = 89 +/- 2 kJ mol-1 and delta S ++ = 102 +/- 20 J mol-1 K-1 and for the mutant protein delta H ++ = 93 +/- 7 kJ mol-1 and delta S ++ = 138 +/- 30 J mol-1 K-1. The study of various p21ras mutants suggests that the two states correspond to different conformations of loop L2, with Tyr-32 in two different positions relative to the bound nucleotide. High-field EPR at 95 GHz suggest that the observed conformational transition does not directly influence the coordination sphere of the protein-bound metal ion. The influence of this transition on loop L4 was studied by 1H NMR with mutants E62H and E63H. There was no indication that L4 takes part in the transition described in L2, although a reversible conformational change could be induced by decreasing the pH value. The exchange between the two states is slow on the NMR time scale (< 10 s-1): at approximately pH 5 the population of the two states is equal. The interaction of p21ras-triphosphate complexes with the Ras-binding domain (RBD) of the effector protein c-Raf-1, Raf-RBD, and with the GTPase activating protein GAP was studied by 31P NMR spectroscopy. In complex with Raf-RBD the second conformation of p21ras (state 2) is stabilized. In this conformation Tyr-32 is located in close proximity to the phosphate groups of the nucleotide, and the beta-phosphate resonance is shifted upfield by 0.7 ppm. Spectra obtained in the presence of GAP suggest that in the ground state GAP does not interact directly with the nucleotide bound to p21ras and does not induce larger conformational changes in the neighborhood of the nucleotide. The experimental data are consistent with a picture where GAP accelerates the exchange process between the two states and simultaneously increases the population of state 1 at higher temperature.

Human immunodeficiency virus type 1 Nef protein is incorporated into virus particles and specifically cleaved by the viral proteinase

The Nef protein of primate immunodeficiency viruses is essential for establishing a highly productive pathogenic infection in vivo. In tissue culture, Nef is not required for infection but enhances viral infectivity. This effect is most pronounced in unstimulated primary lymphocytes and occurs in the early phase of infection prior to viral gene expression. Since Nef expression does not lead to obvious changes in virus composition, it was of interest to analyze whether Nef is incorporated into virus particles. Here, we show that Nef is specifically immunoprecipitated from radioactively labeled human immunodeficiency virus type 1 (HIV-1)-infected cells and virus particle preparations. Quantitative analysis revealed Nef to be incorporated on the order of 10% of reverse transcriptase incorporation, which corresponds to 5 to 10 molecules of Nef per virion. In infected cells, Nef was detected as a full-length 27-kDa protein. In contrast, approximately 50% of particle-associated Nef corresponded to an 18-kDa species which comigrated with the larger product after in vitro cleavage of purified HIV-1 Nef by the viral proteinase. Nef cleavage in particle preparations was completely abolished by a specific inhibitor of HIV-1 proteinase. Most likely, Nef is cleaved concomitantly with viral structural proteins on maturation of virus particles. This cleavage is likely to be functionally significant because it dissociates the conserved core domain from the N-terminal membrane attachment region. Our results suggest that the profound influence of Nef on establishing infection of unstimulated cells in tissue culture and in vivo is mediated by virion-associated Nef which functions in early infection before viral gene expression.