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.

Characterizing the use of perdeuteration in NMR studies of large proteins: 13C, 15N and 1H assignments of human carbonic anhydrase II.

Perdeuteration of all non-exchangeable proton sites can significantly increase the size of proteins and protein complexes for which NMR resonance assignments and structural studies are possible. Backbone 1H, 15N, 13CO, 13C alpha and 13C beta chemical shifts and aliphatic side-chain 13C and 1H(N)/15N chemical shifts for human carbonic anhydrase II (HCA II), a 259 residue 29 kDa metalloenzyme, have been determined using a strategy based on 2D, 3D and 4D heteronuclear NMR experiments, and on perdeuterated 13C/15N-labeled protein. To date, HCA II is one of the largest monomeric proteins studied in detail by high-resolution NMR. Of the backbone resonances, 85% have been assigned using fully protonated 15N and 3C/15N-labeled protein in conjunction with established procedures based on now standard 2D and 3D NMR experiments. HCA II has been perdeuterated both to complete the backbone resonance assignment and to assign the aliphatic side-chain 13C and 1H(N)/15N resonances. The incorporation of 2H into HCA II dramatically decreases the rate of 13C and 1H(N)T2 relaxation. This, in turn, increases the sensitivity of several key 1H/13C/15N triple-resonance correlation experiments. Many otherwise marginal heteronuclear 3D and 4D correlation experiments, which are important to the assignment strategy detailed herein, can now be executed successfully on HCA II. Further analysis suggests that, from the perspective of sensitivity, perdeuteration should allow other proteins with rotational correlation times significantly longer than HCA II (tau c = 11.4 ns) to be studied successfully with these experiments. Two different protocols have been used to characterize the secondary structure of HCA II from backbone chemical-shift data. Secondary structural elements determined in this manner compare favorably with those elements determined from a consensus analysis of the HCA II crystal structure. Finally, having outlined a general strategy for assigning backbone and side-chain resonances in a perdeuterated large protein, we propose a strategy whereby this information can be used to glean more detailed structural information from the partially or fully protonated protein equivalent.

The three-dimensional solution structure of the SH2 domain from p55blk kinase.

Signal transduction in B cells is mediated, in part, by the interaction of the cytoplasmic components of the antigen receptor complex and various members of the src family tyrosine kinases. Key to this process appears to be the interaction of the tyrosine kinase SH2 domains with the tyrosine-phosphorylated cytoplasmic domain of Ig-alpha, a disulfide-bonded heterodimeric (with Ig-beta or Ig-gamma) transmembrane protein that noncovalently associates with the antigen receptor immunoglobin chains. In addition to binding to the phosphorylated cytoplasmic domains of Ig-alpha and Ig-beta, blk and fyn(T), two members of the src family kinases, have been shown to bind overlapping but distinct sets of phosphoproteins [Malek & Desiderio (1993) J. Biol. Chem. 268. 22557-22565]. A comparison of their three-dimensional structures may elucidate the apparently subtle differences required for phosphoprotein discrimination. To begin characterizing the blk/fyn/phosphosphoprotein interactions, we have determined the three-dimensional solution structure of the SH2 domain of blk kinase by nuclear magnetic resonance (NMR) spectroscopy. 1H, 13C, and 15N resonances of the SH2 domain of blk kinase were assigned by analysis of multidimensional, double- and triple-resonance NMR experiments. Twenty structures of the blk SH2 domain were refined with the program X-PLOR using a total of 2080 experimentally derived conformational restraints. The structures converged to a root-mean-squared (rms) distance deviation of 0.51 and 0.95 A for the backbone atoms and for the non-hydrogen atoms, respectively. The blk SH2 domain adopts the prototypical SH2 fold. Structurally, blk SH2 is most similar to the crystal structure of the v-src SH2 domain [Waksman et al. (1993) Nature 358.646-653] and superimposes on the crystal structure with an rmsd of 1.52 A for the backbone atoms. The largest deviations occur in the four loops interconnecting beta-strands A-E, which are the least well-defined regions in the NMR structure. Exclusion of these loops lowers this rmsd to 0.82 A. The conformation of the BC loop in the blk SH2 domain is similar to the open conformation in the apo lck SH2 domain, suggesting that, like the lck SH2 domain, the blk SH2 domain may have a gated phosphopeptide binding site. Finally, it is proposed that the amino acid substitution of Lys 88 (blk) for Glu [fyn(T)] is important for the observed differences in specificity between blk and fyn(T) SH2 domains.

Assignment of aliphatic side-chain 1HN/15N resonances in perdeuterated proteins.

The perdeuteration of aliphatic sites in large proteins has been shown to greatly facilitate the process of sequential backbone and side-chain 13C assignments and has also been utilized in obtaining long-range NOE distance restraints for structure calculations. To obtain the maximum information from a 4D 15N/15N-separated NOESY, as many main-chain and side-chain 1HN/15N resonances as possible must be assigned. Traditionally, only backbone amide 1HN/15N resonances are assigned by correlation experiments, whereas slowly exchanging side-chain amide, amino, and guanidino protons are assigned by NOEs to side-chain aliphatic protons. In a perdeuterated protein, however, there is a minimal number of such protons. We have therefore developed several gradient-enhanced and sensitivity-enhanced pulse sequences, containing water-flipback pulses, to provide through-bond correlations of the aliphatic side-chain 1HN/15N resonances to side-chain 13C resonances with high sensitivity: NH2-filtered 2D 1H-15N HSQC(H2N-HSQC), 3D H2N(CO)C gamma/beta and 3D H2N(COC gamma/beta)C beta/alpha for glutamine and asparagine side-chain amide groups; 2D refocused H(N epsilon/zeta)C delta/epsilon and H(N epsilon/zeta C delta/epsilon)C gamma/delta for arginine side-chain amino groups and non-refocused versions for lysine side-chain amino groups; and 2D refocused H(N epsilon)C zeta and nonrefocused H(N epsilon, eta)C zeta for arginine side-chain guanidino groups. These pulse sequences have been applied to perdeuterated 13C-/15N-labeled human carbonic anhydrase II (2H-HCA II). Because more than 95% of all side-chain 13C resonances in 2H-HCA II have already been assigned with the C(CC)(CO)NH experiment, the assignment of the side-chain 1HN/15N resonances has been straightforward using the pulse sequences mentioned above. The importance of assigning these side-chain HN protons has been demonstrated by recent studies in which the calculation of protein global folds was simulated using only 1HN-1HN NOE restraints. In these studies, the inclusion of NOE restraints to side-chain HN protons significantly improved the quality of the global fold that could be determined for a perdeuterated protein [R.A. Venters et al. (1995) J. Am. Chem. Soc., 117, 9592-9593].

Triple resonance HNCCCH experiments for correlating exchangeable and nonexchangeable cytidine and uridine base protons in RNA.

A set of triple resonance experiments is presented, providing through-bond H2N/HN to H6 connectivities in uridines and cytidines in 13C-/15N-labeled RNAs. These connectivities provide an important link between the sequential assignment pathways for the exchangeable and nonexchangeable proton resonances in nucleic acids. Both 2D and pseudo-3D HNCCCH experiments were applied to a 30-nucleotide lead-dependent ribozyme, known as the leadzyme. The HN to H6 connectivities for three uridines in the leadzyme were identified from one 2D H(NCCC)H experiment, and the H2N to H6 connectivities were identified for seven of the eight cytidines from the combination of a 2D H(NCCC)H and a pseudo-3D H(NCC)CH experiment.

High-level 2H/13C/15N labeling of proteins for NMR studies.

The protein human carbonic anhydrase II (HCA II) has been isotopically labeled with 2H, 13C and 15N for high-resolution NMR assignment studies and pulse sequence development. To increase the sensitivity of several key 1H/13C/15N triple-resonance correlation experiments, 2H has been incorporated into HCA II in order to decrease the rates of 13C and 1HN T2 relaxation. NMR quantities of protein with essentially complete aliphatic 2H incorporation have been obtained by growth of E. coli in defined media containing D2O, [1,2-13C2, 99%] sodium acetate, and [15N, 99%] ammonium chloride. Complete aliphatic deuterium enrichment is optimal for 13C and 15N backbone NMR assignment studies, since the 13C and 1HN T2 relaxation times and, therefore, sensitivity are maximized. In addition, complete aliphatic deuteration increases both resolution and sensitivity by eliminating the differential 2H isotopic shift observed for partially deuterated CHnDm moieties.

Refined solution structure of human profilin I.

Profilin is a ubiquitous eukaryotic protein that binds to both cytosolic actin and the phospholipid phosphatidylinositol-4,5-bisphosphate. These dual competitive binding capabilities of profilin suggest that profilin serves as a link between the phosphatidyl inositol cycle and actin polymerization, and thus profilin may be an essential component in the signaling pathway leading to cytoskeletal rearrangement. The refined three-dimensional solution structure of human profilin I has been determined using multidimensional heteronuclear NMR spectroscopy. Twenty structures were selected to represent the solution conformational ensemble. This ensemble of structures has root-mean-square distance deviations from the mean structure of 0.58 A for the backbone atoms and 0.98 A for all non-hydrogen atoms. Comparison of the solution structure of human profilin to the crystal structure of bovine profilin reveals that, although profilin adopts essentially identical conformations in both states, the solution structure is more compact than the crystal structure. Interestingly, the regions that show the most structural diversity are located at or near the actin-binding site of profilin. We suggest that structural differences are reflective of dynamical properties of profilin that facilitate favorable interactions with actin. The global folding pattern of human profilin also closely resembles that of Acanthamoeba profilin I, reflective of the 22% sequence identity and approximately 45% sequence similarity between these two proteins.

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)

Simultaneous acquisition of [13C,15N]- and [15N,15N]-separated 4D gradient-enhanced NOESY spectra in proteins.

The simultaneous acquisition of a 4D gradient-enhanced and sensitivity-enhanced [13C,15N]/[15N,15N]-separated NOESY is presented for the 74-residue [13C,15N]-labeled N-terminal SH3 domain of mGrb2 complexed with a peptide fragment from mSOS-2 in 90% H2O. The method readily accommodates different 13C and 15N spectral widths, but requires that the same number of increments be collected for both 13C and 15N in the simultaneous dimension (F2). For purposes of display and analysis, the two 4D spectra can be deconvolved during the processing stage by the appropriate linear combination of separately stored FIDs. Compared to collecting each of these two 4D data sets separately, the presented method is a factor (2)1/2 more efficient in sensitivity per unit acquisition time. The interleaved nature of this method may also lead to improved peak registration between the two 4D spectra.

Unambiguous through-bond sugar-to-base correlations for purines in 13C,15N-labeled nucleic acids: the HsCsNb,HsCs(N)bCb, and HbNbCb experiments.

A set of three 3D (1H,13C,15N) triple-resonance correlation experiments has been designed to provide H1'-H8 intraresidue sugar-to-base correlations in purines in an unambiguous and efficient manner. Together, the HsCsNb, HsCs(N)bCb, and HbNbCb experiments correlate the H1' sugar proton to the H8 proton of the attached base by means of the (H1', C1', N9, C8, H8) heteronuclear scalar coupling network. The assignment strategy presented here allows for unambiguous H1'-H8 intraresidue correlations, provided that no two purines have both the same H1' and C1' chemical shifts and the same C8 and N9 chemical shifts. These experiments have yielded H1'-H8 intraresidue sugar-to-base correlations for all five guanosines in the [13C,15N] isotopically labeled RNA duplex r(GGCGCUUGCGUC)2.

Generation of impossible cross-peaks between bulk water and biomolecules in solution NMR.

Intermolecular multiple-quantum coherences between bulk water and a glycoprotein fragment at modest concentration (20 mM) have been experimentally produced and detected, although such coherences are inconceivable in the normal theoretical framework of nuclear magnetic resonance. A density matrix treatment explains these results by including the long-range dipolar interaction between spins and by discarding the high-temperature approximation. These results imply that peak intensities (critical for structural determinations) can be distorted in many gradient experiments, and show that magic-angle gradients provide substantial improvements with reduced gradient strengths. They also suggest methods for contrast enhancement in magnetic resonance imaging.

A refocused and optimized HNCA: increased sensitivity and resolution in large macromolecules.

A 3D optimized, refocused HNCA experiment is described. It is demonstrated to yield a dramatic increase in sensitivity when applied to [13C, 15N]-labeled human carbonic anhydrase II, a 29-kDa protein. The reasons for the gain in sensitivity are discussed, and 3 distinct areas for further development are indicated.

Protein hydration studied with homonuclear 3D 1H NMR experiments.

Homonuclear 3D 1H NOESY-TOCSY and 3D 1H ROESY-TOCSY experiments were used to resolve and assign nuclear Overhauser effect (NOE) cross peaks between the water signal and individual polypeptide proton resonances in H2O solutions of the basic pancreatic trypsin inhibitor. Combined with a novel, robust water-suppression technique, positive and negative intermolecular NOEs were detected at 4 degrees C. The observation of positive NOEs between water protons and protein protons enables more precise estimates of the very short residence times of the water molecules in the hydration sites on the protein surface.

ESR studies of the erythrocyte membrane skeletal protein network: influence of the state of aggregation of spectrin on the physical state of membrane proteins, bilayer lipids, and cell surface carbohydrates.

The stability of the human erythrocyte membrane skeletal network is reported to be dependent on the state of aggregation of spectrin and decreased or increased by polyphosphate anions or the polyamine, spermine, respectively. We have employed polyacrylamide gel electrophoresis and electron spin resonance (ESR) utilizing spin labels specific for membrane proteins, bilayer lipids, or cell-surface sialic acid in order to gain insight into these observations and into the reliability of the ESR spectra of the protein-specific spin label used to correctly report the interactions of the skeletal protein network. The major findings are: (1) We confirm previous reports that the preferred state of spectrin aggregation in the skeletal network is tetrameric and that spectrin can be reversibly transformed to dimeric spectrin and back to tetrameric spectrin on the membrane. (2) The ESR spectra of the protein specific maleimide spin label employed accurately reflect the state of aggregation of spectrin. (3) As dimeric spectrin is increased on the membrane or when 2,3-bisphosphoglycerate was added to spin-labeled membranes, increased segmental motion of protein spin label binding sites reflecting decreased protein-protein interactions in the skeletal network is observed (P less than 0.002 and P less than 0.005, respectively). (4) Conversely, as protein-protein interactions between skeletal proteins or between skeletal proteins and the bilayer are increased by spermine (reflected in the total inability to extract spectrin from the membrane in contrast to control membranes), highly decreased segmental motion of the protein specific spin label binding site is observed (P less than 0.005). (5) The dimeric-tetrameric state of spectrin aggregation on the membrane does not have influence on the order or motion of bilayer lipids nor on the rotational rate of spin-labeled, cell-surface sialic acid, a result also observed when protein-protein interactions were decreased by 2,3-bisphosphoglycerate. In contrast, increased protein-protein interactions by addition of spermine produced a small, but significant, increase in order and decrease in motion of bilayer lipids near the membrane surface as well as a nearly 40% decrease in the apparent rotational correlation time of spin labeled, cell surface sialic acid (P less than 0.002). These latter observations are discussed with reference to possible associations of phospholipids and the major, transmembrane sialoglycoprotein with the skeletal protein network.

The alteration of membrane proteins in human erythrocyte membranes induced by quinolinic acid, an endogenous neurotoxin. Correlation of effect with structure.

Quinolinic acid (2,3-pyridinedicarboxylic acid), an endogenous metabolite of L-tryptophan, reportedly via the kynurenine pathway, has been previously shown to possess neurotoxic properties when injected into rat striatum (Schwarcz, R., Whetsell, W.O., Jr. and Mangano, R.M. (1983) Science 219, 316-318) and to alter the physical state of human erythrocyte membrane proteins, as judged by ESR spectroscopy (Farmer, B.T., II and Butterfield, D.A. (1984) Life Sci. 35, 501-509). Both the morphologic and ESR studies employed nicotinic acid as one comparative control and found that the effect of quinolinic acid is significantly different from that of nicotinic acid. In the present study, we report that the effects of several structural analogues and positional isomers of quinolinic acid on the ESR parameter associated with the physical state of membrane proteins in human erythrocyte membranes suggest the following conclusions concerning the structure-effect relationship of quinolinic acid: The alteration in the conformation of membrane proteins: (1) requires the presence of two carboxylic acid groups; (2) is independent of their relationship to one another on the pyridine ring; (3) is slightly dependent on the presence of the pyridine nitrogen atom but is independent of the positional relationship of the two carboxylic acid moieties to the heteroatom; and (4) seems to depend upon the presence of restricted internal motion derived from the aromaticity in these compounds.

Selective spin labeling of terminal galactose and N-acetylgalactosamine residues on the membrane surface of erythrocytes.

A method to selectively spin label galactose and N-acetylgalactosamine residues of erythrocyte membrane glycoconjugates is described. The method is based on the activation of the C-6 CH2OH group of these two sugars by galactose oxidase followed by reductive amination with 2,2,6,6-tetramethyl-4-aminopiperidine-1-oxyl in the presence of a mild reducing agent, NaBH3CN. The extent and distribution of the spin labeling suggest that the major sialoglycoprotein, PAS-1, incorporates the greatest amount of spin label while the glycolipids incorporate less than 10% of the spin label.