Backbone and side chain dynamics of uncomplexed human adipocyte and muscle fatty acid-binding proteins.

Adipocyte lipid-binding protein (A-LBP) and muscle fatty acid-binding protein (M-FABP) are members of a family of small ( approximately 15 kDa) cytosolic proteins that are involved in the metabolism of fatty acids and other lipid-soluble molecules. Although highly homologous (65%) and structurally very similar, A-LBP and M-FABP display distinct ligand binding characteristics. Since ligand binding may be influenced by intrinsic protein dynamical properties, we have characterized the backbone and side chain dynamics of uncomplexed (apo) human A-LBP and M-FABP. Backbone dynamics were characterized by measurements of 15N T1 and T2 values and ¿1H¿-15N NOEs. These data were analyzed using model-free spectral density functions and reduced spectral density mapping. The dynamics of methyl-containing side chains were charaterized by measurements of 2H T1 and T1rho relaxation times of 13C1H22H groups. The 2H relaxation data were analyzed using the model-free approach. For A-LBP, 15N relaxation data were obtained for 111 residues and 2H relaxation data were obtained for 42 methyl groups. For M-FABP, 15N relaxation data were obtained for 111 residues and 2H relaxation data were obtained for 53 methyl groups. The intrinsic flexibilities of these two proteins are compared, with particular emphasis placed on binding pocket residues. There are a number of distinct dynamical differences among corresponding residues between the two proteins. In particular, many residues display greater backbone picosecond to nanosecond and/or microsecond to millisecond time scale mobility in A-LBP relative to M-FABP, including F57, K58, and most residues in alpha-helix 2 (residues 28-35). Variations in the dynamics of this region may play a role in ligand selectivity. The side chains lining the fatty acid binding pocket display a wide range of motional restriction in both proteins. Side chains showing distinct dynamical differences between the two proteins include those of residues 20, 29, and 51. This information provides a necessary benchmark for determining dynamical changes induced by ligand binding and may ultimately lead to an enhanced understanding of ligand affinity and selectivity among fatty acid-binding proteins.

Characterization of NADP+ binding to perdeuterated MurB: backbone atom NMR assignments and chemical-shift changes.

Backbone-atom resonances have been assigned for both the substrate-free and the NADP+-complexed forms of UDP-N-acetylenolpyruvylglucosamine reductase (MurB), a monomeric, 347-residue (38.5 kDa) flavoenzyme essential for bacterial cell-wall biosynthesis. NMR studies were performed using perdeuterated, uniformly 13C/15N-labeled samples of MurB. In the case of substrate-free MurB, one or more backbone atoms have been assigned for 334 residues (96%). The assigned backbone atoms include 309 1HN and 15N atoms (94%), 315 13CO atoms (91%), 331 13C(alpha) atoms (95%), and 297 13C(beta) atoms (93%). For NADP+-complexed MurB, one or more backbone atoms have been assigned for 313 residues (90%); these include 283 1HN and 15N atoms (86%), 305 13CO atoms (88%), 310 13C(alpha) atoms (89%), and 269 13C(beta) atoms (84%). The strategies used for obtaining resonance assignments are described in detail. Information on the secondary structure in solution for both the substrate-free and NADP+-complexed forms of the enzyme has been derived both from 13C(alpha) and 13C(beta) chemical-shift deviations from random-coil values and from 1HN-1HN NOEs. These data are compared to X-ray crystallographic structures of substrate-free MurB and MurB complexed with the UDP-N-acetylglucosamine enolpyruvate (UNAGEP) substrate. NADP+ binding induces significant chemical-shift changes in residues both within the known UNAGEP and FAD binding pockets and within regions known to undergo conformational changes upon UNAGEP binding. The NMR data indicate that NADP+ and UNAGEP utilize the same binding pocket and, furthermore, that the binding of NADP+ induces structural changes in MurB. Finally, many of the residues within the UNAGEP/NADP+ binding pocket were difficult to assign due to dynamic processes which weaken and/or broaden the respective resonances. Overall, our results are consistent with MurB having a flexible active site.

Solution structure of the Grb2 N-terminal SH3 domain complexed with a ten-residue peptide derived from SOS: direct refinement against NOEs, J-couplings and 1H and 13C chemical shifts.

Refined ensembles of solution structures have been calculated for the N-terminal SH3 domain of Grb2 (N-SH3) complexed with the ac-VPPPVPPRRR-nh2 peptide derived from residues 1135 to 1144 of the mouse SOS-1 sequence. NMR spectra obtained from different combinations of both 13C-15N-labeled and unlabeled N-SH3 and SOS peptide fragment were used to obtain stereo-assignments for pro-chiral groups of the peptide, angle restraints via heteronuclear coupling constants, and complete 1H, 13C, and 15N resonance assignments for both molecules. One ensemble of structures was calculated using conventional methods while a second ensemble was generated by including additional direct refinements against both 1H and 13C(alpha)/13C(beta) chemical shifts. In both ensembles, the protein:peptide interface is highly resolved, reflecting the inclusion of 110 inter-molecular nuclear Overhauser enhancement (NOE) distance restraints. The first and second peptide-binding sub-sites of N-SH3 interact with structurally well-defined portions of the peptide. These interactions include hydrogen bonds and extensive hydrophobic contacts. In the third highly acidic sub-site, the conformation of the peptide Arg8 side-chain is partially ordered by a set of NOE restraints to the Trp36 ring protons. Overall, several lines of evidence point to dynamical averaging of peptide and N-SH3 side-chain conformations in the third subsite. These conformations are characterized by transient charge stabilized hydrogen bond interactions between the peptide arginine side-chain hydrogen bond donors and either single, or possibly multiple, acceptor(s) in the third peptide-binding sub-site.

Orientation of peptide fragments from Sos proteins bound to the N-terminal SH3 domain of Grb2 determined by NMR spectroscopy.

NMR spectroscopy has been used to characterize the protein-protein interactions between the mouse Grb2 (mGrb2) N-terminal SH3 domain complexed with a 15-residue peptide (SPLLPKLPP-KTYKRE) corresponding to residues 1264-1278 of the mouse Sos-2 (mSos-2) protein. Intermolecular interactions between the peptide and 13C-15N-labeled SH3 domain were identified in half-reverse-filtered 2D and 3D NOESY experiments. Assignments for the protons involved in interactions between the peptide and the SH3 domain were confirmed in a series of NOESY experiments using a set of peptides in which different leucine positions were fully deuterated. The peptide ligand-binding site of the mGrb2 N-terminal SH3 domain is defined by the side chains of specific aromatic residues (Tyr7, Phe9, Trp36, Tyr52) that form two hydrophobic subsites contacting the side chains of the peptide Leu4 and Leu7 residues. An adjacent negatively charged subsite on the SH3 surface is likely to interact with the side chain of a basic residue at peptide position 10 that we show to be involved in binding. The peptide-binding site of the SH3 is characterized by large perturbations of amide chemical shifts when the peptide is added to the SH3 domain. The mGrb2 N-terminal SH3 domain structure in the complex is well-defined (backbone RMSD of 0.56 +/- 0.21 calculated over the backbone N, C alpha, and C atoms of residues 1-54). The structure of the peptide in the complex is less well-defined but displays a distinct orientation.(ABSTRACT TRUNCATED AT 250 WORDS)

Production and characterization of an antibody Fv fragment 15N-labeled in the VL domain only.

Procedures for separating and recombining the light-chain variable (VL) and heavy-chain variable (VH) domains of antibody Fv fragments have been applied to produce a recombinant Fv of the anti-digoxin antibody 26-10 that is 15N-labeled exclusively in the VL domain. Comparison of a two-dimensional 1H-15N heteronuclear single-quantum correlation (HSQC) spectrum of the reconstituted Fv with a HSQC spectrum of a fully 15N-labeled Fv sample reveals that all 1H-15N correlations of the VL domain align precisely in both spectra. Assignments for 105 of the 106 backbone HN groups of the VL domain within the reconstituted Fv have been obtained by analysis of three-dimensional nuclear Overhauser effect spectroscopy-HSCQ and total correlation spectroscopy-HSQC spectra with reference to assignments previously reported for the isolated VL domain. Chemical shift differences between the isolated VL domain and the VL domain within the Fv are moderately correlated with proximity to the surfaces of the VH domain and bound hapten (ouabain) as defined by X-ray crystallography and molecular modeling. These results demonstrate that nuclear magnetic resonance studies of reconstituted antibody Fv fragments, in conjunction with investigations of isolated antibody domains, can yield extensive resonance assignments for the Fv. This will facilitate detailed studies of antigen-antibody and domain-domain interactions.

Characterization of the backbone dynamics of an anti-digoxin antibody VL domain by inverse detected 1H-15N NMR: comparisons with X-ray data for the Fab.

The dynamic behavior of the polypeptide backbone of a recombinant antidigoxin antibody VL domain has been characterized by measurements of 15NT1 and T2 relaxation times, 1H-15N NOE values, and 1H-2H exchange rates. These data were acquired with 2D inverse detected heteronuclear 1H-15N NMR methods. The relaxation data are interpreted in terms of model free spectral density functions and exchange contributions to transverse relaxation rates R2 (= 1/T2). All characterized residues display low-amplitude picosecond time-scale librational motions. Fifteen residues undergo conformational changes on the nanosecond timescale, and 24 residues have significant R2 exchange contributions, which reflect motions on the microsecond to millisecond time-scale. For several residues, microsecond to millisecond motions of nearby aromatic rings are postulated to account for some or all of their observed R2 exchange contributions. The measured 1H-2H exchange rates are correlated with hydrogen bonding patterns and distances from the solvent accessible surface. The degree of local flexibility indicated by the NMR measurements is compared to crystallographic B-factors derived from X-ray analyses of the native Fab and the Fab/digoxin complex. In general, both the NMR and X-ray data indicate enhanced flexibility in the turns, hypervariable loops, and portions of beta-strands A, B, and G. However, on a residue-specific level, correlations among the various NMR data, and between the NMR and X-ray data, are often absent. This is attributed to the different dynamic processes and environments that influence the various observables. The combined data indicate that certain regions of the VL domain, including the three hypervariable loops, undergo dynamic changes upon VL:VH association and/or complexation with digoxin. Overall, the 26-10 VL domain exhibits relatively low flexibility on the ps-ns timescale. The possible functional consequences of this result are considered.

Aliphatic 1H and 13C resonance assignments for the 26-10 antibody VL domain derived from heteronuclear multidimensional NMR spectroscopy.

Extensive 1H and 13C assignments have been obtained for the aliphatic resonances of a uniformly 13C- and 15N-labeled recombinant VL domain from the anti-digoxin antibody 26-10. Four-dimensional triple resonance NMR data acquired with the HNCAHA and HN(CO)CAHA pulse sequences [Kay et al. (1992) J. Magn. Reson., 98, 443-450] afforded assignments for the backbone HN, N, H alpha and C alpha resonances. These data confirm and extend HN, N and H alpha assignments derived previously from three-dimensional 1H-15N NMR studies of uniformly 15N-labeled VL domain [Constantine et al. (1992), Biochemistry, 31, 5033-5043]. The identified H alpha and C alpha resonances provided a starting point for assigning the side-chain aliphatic 1H and 13C resonances using three-dimensional HCCH-COSY and HCCH-TOCSY experiments [Clore et al. (1990), Biochemistry, 29, 8172-8184]. The C alpha and C beta chemical shifts are correlated with the VL domain secondary structure. The extensive set of side-chain assignments obtained will allow a detailed comparison to be made between the solution structure of the isolated VL domain and the X-ray structure of the VL domain within the 26-10 Fab.

Sequential 1H and 15N NMR assignments and secondary structure of a recombinant anti-digoxin antibody VL domain.

A uniformly 15N-labeled recombinant light-chain variable (VL) domain from the anti-digoxin antibody 26-10 has been investigated by heteronuclear two-dimensional (2D) and three-dimensional (3D) NMR spectroscopy. Complementary homonuclear 2D NMR studies of the unlabeled VL domain were also performed. Sequence-specific assignments for 97% of the main-chain and 70% of the side-chain proton resonances have been obtained. Patterns of nuclear Overhauser effects observed in 2D NOESY, 3D NOESY-HSQC, and 3D NOESY-TOCSY-HSQC spectra afford a detailed characterization of the VL domain secondary structure in solution. The observed secondary structure--a nine-stranded antiparallel beta-barrel--corresponds to that observed crystallographically for VL domains involved in quaternary associations. The locations of slowly exchanging amide protons have been discerned from a 2D TOCSY spectrum recorded after dissolving the protein in 2H2O. Strands B, C, E, and F are found to be particularly stable. The possible consequences of these results for domain-domain interactions are discussed.

Hemopexin is a developmentally regulated, acute-phase plasma protein in the chicken.

Identity has been established between chicken hemopexin and alpha 1-globulin "M," a plasma known for the hormone responsiveness of its synthesis in monolayer cultures of embryonic chicken hepatocytes (Grieninger, G., Plant, P. W., Liang, T. J., Kalb, R. G., Amrani, D., Mosesson, M. W., Hertzberg, K. M., and Pindyk, J. (1983) Ann. N. Y. Acad. Sci. 408, 469-489). Identification was based on immunological cross-reactivity, electrophoretic behavior on sodium dodecyl sulfate-polyacrylamide gels, heme-binding capacity, and pattern of cleavage by proteolytic enzymes. Electroimmunoassays were used to investigate plasma protein levels, particularly those of hemopexin, in the acute-phase response and embryonic development. Acute-phase plasma protein production, elicited by injection of chickens with turpentine, bore many similarities to the pattern of hepatocellular plasma protein synthesis produced in response to the addition of specific hormones in culture. The response of the stressed chickens included elevated levels of hemopexin and fibrinogen (5- and 2-fold, respectively) accompanied by a 50% drop in albumin. Hemopexin levels of developing chick embryos were measured for several days before and after hatching. Onset of hemopexin production occurred around the time of hatching, and was followed by a steep increase (more than 1000-fold over 4 days). Similarly, it was not until the 12th h of culture that hepatocytes isolated from both early and late stage chicken embryos began to produce hemopexin, although, from their initiation in culture, they secreted a number of other plasma proteins in quantity. After 12 h, hepatocellular output of hemopexin rapidly accelerated. This precocious induction ex vivo required no hormonal or macromolecular medium supplements. These observations indicate that the embryonic chicken hepatocyte culture system will provide a useful model for studying the regulation of hemopexin biosynthesis in hepatic development and the acute-phase response.

An avian serum alpha 1-glycoprotein, hemopexin, differing significantly in both amino acid and carbohydrate composition from mammalian (beta-glycoprotein) counterparts.

We report here on physicochemical characteristics of chicken hemopexin, which can be isolated by heme-agarose affinity chromatography [Tsutsui, K., & Mueller, G. C. (1982) J. Biol. Chem. 257, 3925-3931], in comparison with representative mammalian hemopexins of rat, rabbit, and human. The avian polypeptide chain appears to be slightly longer (52 kDa) than the human, rat, or rabbit forms (49 kDa), and also the glycoprotein differs from the mammalian hemopexins in being an alpha 1-glycoprotein instead of a beta 1-glycoprotein. This distinct electrophoretic mobility probably arises from significant differences in the amino acid composition of the chicken form, which, although lower in serine and particularly in lysine, has a much higher glutamine/glutamate and arginine content, and also a higher proline, glycine, and histidine content, than the mammalian hemopexins. Compositional analyses and 125I concanavalin A and 125I wheat germ agglutinin binding suggest that chicken hemopexin has a mixture of three fucose-free N-linked bi- and triantennary oligosaccharides. In contrast, human hemopexin has five N-linked oligosaccharides and an additional O-linked glycan blocking the N-terminal threonine residue [Takahashi, N., Takahashi, Y., & Putnam, F. W. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 2021-2025], while the rabbit form has four N-linked oligosaccharides [Morgan, W. T., & Smith, A. (1984) J. Biol. Chem. 259, 12001-12006]. In keeping with the finding of a simpler carbohydrate structure, the avian hemopexin exhibits only a single band on polyacrylamide gel electrophoresis under both nondenaturing and denaturing conditions, whereas the hemopexins of the three mammalian species tested show several bands.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial characterization of intra- and extracellular forms of the glycoprotein hemopexin in liver cell culture and cell-free translation.

Primary cultures of rat hepatocytes produce extracellular and intracellular species of hemopexin. We examined the presence of high-mannose oligosaccharides and neuraminic acid residues in these species by comparing their electrophoretic mobility on SDS-PAGE before and after digestion by endoglycosidase H and neuraminidase. The predominant intracellular form was not susceptible to digestion by neuraminidase but was sensitive to endoglycosidase treatment, which digested it to a species with a molecular weight comparable to that of the sole hemopexin species produced by tunicamycin-treated hepatocytes and that produced by in vitro translation. By contrast, both the minor intracellular and the extracellular species of hemopexin were neuraminidase-, yet not endoglycosidase H-sensitive, and may be identical. It can be concluded that the intracellular precursor contains high-mannose type oligosaccharides which are processed to complex type oligosaccharides shortly before secretion of hemopexin.

Synthesis and secretion of hemopexin in primary cultures of rat hepatocytes. Demonstration of an intracellular precursor of hemopexin.

Secretion of hemopexin (20% carbohydrate) and its dependence on glycosylation was studied in primary rat hepatocyte cultures in comparison to the secretion of transferrin (5% carbohydrate). In pulse-chase experiments with [35S]methionine half of the labeled hemopexin was secreted in 30 min. By contrast, it took approximately 50 min for secretion of half of the transferrin. Tunicamycin treatment of cultures significantly delayed the secretion of hemopexin but not that of transferrin. During the pulse period a prominent intracellular precursor of hemopexin, smaller than the mature protein, was evident. It is concluded that the extent of glycosylation of a secretory protein is not necessarily a determinant of the transit time required for intracellular processing and secretion. In the case of hemopexin the glycosylation apparently facilitates the secretion although it is not an absolute prerequisite for the exocytosis of this protein.

ATPase activities in partially purified membranes of Acetabularia.

Partly purified membranes (with plasmalemma material) of Acetabularia mediterranea were studied with respect to ATPase activity in alkali- and Ca(++)-free media and its sensitivity to pH (5 - 9), oligomycin (200 ώg/mg protein), 100 ώM N-N'-dicyclohexylcarbodiimide (DCCD), and 50 ώM vanadate. Besides activities which may originate from mitochondrial H(+) ATPase (oligomycin-sensitive, alkaline pH optimum) and tonoplast H(+) ATPase (DCCD-sensitive, pH optimum 7.5), there is ATPase activity with a pH optimum around pH 6.5, sensitive to vanadate and insensitive to DCCD. These results strongly suggest that the electrogenic Cl(-) pump in the plasmalemma of Acetabularia is an ATPase. Effects of Mg(++), Mg-ATP, ADP, GTP, UTP, CTP and HCO3 (-) versus Cl(-) on this ATPase activity are described.

Electrogenic Cl- pump in Acetabularia.

Measurements of this transmembrane potential difference (V) under various conditions have demonstrated the operation of an electrogenic Cl- pump in the outer plasma membrane (plasmalemma) of the unicellular marine alga Acetabularia. In preparations of partly purified membranes (containing plasmalemma), there is Cl- stimulated, N,N'-dicyclohexylcarbodiimide-insensitive, vanadate-sensitive ATPase activity with a pH optimum around pH 6.5. These properties are consistent with the assumption that the electrogenic Cl- pump is an ATPase. In order to investigate electrical details of the "Mitchellian" type of charge-translocating enzyme, steady-state current-voltage curves of the electrogenic pump (Ip(V)) were measured in vivo under dark and light conditions and analysed by two-state reaction kinetic model. This model with the resulting parameters predicts V-sensitive, undirectional Cl- effluxes through the pump. The predictions of this model agree with the experimental results. Green light causes a fast decrease of V, which is explained as a disturbance of the pump cycle. Relaxation studies on this effect and reaction kinetic analysis of Ip(V) under different external Cl- concentrations are used to develop a consistent three-state model of the pump that includes the order of and absolute rate constants of individual reactions, states of charge, stoichiometry, voltage-sensitivity and density of the pump molecules in the membrane.

Effect of derivatives of 3-(beta-aminoethyl)-1,2,4-triazole on the histamine H1- and H2-receptors.

Derivatives of 3-(beta-aminoethyl)-1,2,4-triazole similar to histamine have been resynthesized and studied. Four well-known effects of histamine have been considered (the contractions of an isolated guinea pig terminal ileum, dog gastric acid secretion, contractile frequency of an isolated guinea pig right atrium, blood pressure lowering effect in cats). Newly synthesized compounds IEM-813 and IEM-759 exerted some selective effects of H1 and H2-receptors: IEM-813 showed greater affinity for gut-isolated H1-receptors than for stomach and atrium H2-receptors, while IEM-759 mainly influenced H2-receptors. Accordingly, the influence of 1,2,4-triazole analogues on H1- and H2-receptors is subject to structural demands.