Presence of asparagine-linked N-acetylglucosamine and chitobiose in Pyrus pyrifolia S-RNases associated with gametophytic self-incompatibility.

S-RNases encoded by the S-locus of rosaceous and solanaceous plants discriminate between the S-alleles of pollen in gametophytic self-incompatibility reactions, but it is not clear how. We report the structures of N-glycans attached to each of the N-glycosylation sites of seven S-RNases in Pyrus pyrifolia of the Rosaceae. The structures were identified by chromatographic analysis of pyridylaminated sugar chains prepared from S4-RNase and by liquid chromatography/electrospray ionization-mass spectrometric analysis of the protease digests of reduced and S-carboxymethylated S-RNases. S4-RNase carries various types of sugar chains, including plant-specific ones with beta1-->2-linked xylose and alpha1-->3-linked fucose residues. More than 70% of the total N-glycans of S4-RNase are, however, an N-acetylglucosamine or a chitobiose (GlcNAcbeta1-->4GlcNAc), which has not been found naturally. The N-acetylglucosamine and chitobiose are mainly present at the N-glycosylation sites within the putative recognition sites of the S-RNase, suggesting that these sugar chains may interact with pollen S-product(s).     

Ribonuclease structure and catalysis: crystal structure of sulfate-free native ribonuclease A at 1.5-A resolution

The structure of native bovine pancreatic ribonuclease A, without the inhibitory sulfate anion normally bound at the active site, has been determined by X-ray diffraction at 1.53-A resolution. Treatment of a crystal of ribonuclease containing sulfate with an alkaline buffer released most of the sulfate anions. On return to active pH, few of the side chains moved, and the backbone structure remained unchanged. The active site conformation was essentially unchanged except for the replacement of the sulfate anion by a water molecule, which is hydrogen-bonded to histidine-12 and to another water, and for a small movement of the side chain of lysine-41. Histidines-12 and -119, the catalytic basic and acidic residues, have not moved. Thus the distance between them, and the presence of an intervening water, prohibits the possibility of their being hydrogen-bonded together. The structure has been refined by restrained least squares to an R factor of 0.17. Analysis of individual atomic temperature factors indicates that the molecule has become less rigid in general but that some regions were particularly affected by loss of the sulfate, while others were relatively unaffected. The active site geometry of native ribonuclease A supports the original in-line mechanism of Rabin and co-workers and is in disagreement with the adjacent mechanism of Witzel and co-workers.     

Crystal structure of a major secreted protein of Mycobacterium tuberculosis-MPT63 at 1.5-A resolution

MPT63 is a small, major secreted protein of unknown function from Mycobacterium tuberculosis that has been shown to have immunogenic properties and has been implicated in virulence. A BLAST search identified that MPT63 has homologs only in other mycobacteria, and is therefore mycobacteria specific. As MPT63 is a secreted protein, mycobacteria specific, and implicated in virulence, MPT63 is an attractive drug target against the deadliest infectious disease, tuberculosis (TB). As part of the TB Structural Genomics Consortium, the X-ray crystal structure of MPT63 was determined to 1.5-Angstrom resolution with the hope of yielding functional information about MPT63. The structure of MPT63 is an antiparallel beta-sandwich immunoglobulin-like fold, with the unusual feature of the first beta-strand of the protein forming a parallel addition to the small antiparallel beta-sheet. MPT63 has weak structural similarity to many proteins with immunoglobulin folds, in particular, Homo sapiens beta2-adaptin, bovine arrestin, and Yersinia pseudotuberculosis invasin. Although the structure of MPT63 gives no conclusive evidence to its function, structural similarity suggests that MPT63 could be involved in cell-host interactions to facilitate endocytosis/phagocytosis.     

The crystal structure of ribonuclease B at 2.5-A resolution

The glycosylated form of bovine pancreatic ribonuclease, RNase B, was crystallized from polyethylene glycol 4000 at low ionic strength in space group C2 with unit cell dimensions of a = 101.81 A, b = 33.36 A, c = 73.60 A, and beta = 90.4 degrees. The crystals, which contained two independent molecules of RNase B as the asymmetric unit, were solved by a combination of multiple isomorphous replacement and molecular replacement approaches. The structures of the two molecules were refined to 2.5-A resolution and a conventional R factor of 0.22 using a constrained-restrained least squares procedure (CORELS). Complexes were also investigated of RNase B plus ruthenium pentaamine and between RNase B and a substrate analogue iodouridine. The polypeptide backbones of the two molecules of RNase B in the asymmetric unit were found to be statistically identical and their differences from RNase A to be statistically insignificant. The carbohydrate chains of both molecules extended into solvent cavities in the crystal lattice and appear to be disordered for the most part. The oligosaccharides appear to exert no influence on the structure of the protein. Iodouridine was observed to bind identically in the pyrimidine site of both RNase B molecules and in a way apparently the same as that previously observed for RNase A. Ruthenium pentaamine bound at histidine 105 of both RNase B molecules in the asymmetric unit, but at a number of secondary sites as well. An array of bound ions was observed by Fo-Fc difference Fourier syntheses. These ions were proximal to lysine and arginine residues at the surface of the proteins while a pair of strong ion binding sites were seen to fall exactly in the active site clefts of both RNase B molecules in the asymmetric unit.     

Crystal structure of ribonuclease T1 complexed with adenosine 2'-monophosphate at 1.8-A resolution.

Ribonuclease T1 was purified from an Escherichia coli overproducing strain and co-crystallized with adenosine 2'-monophosphate (2'-AMP) by microdialysis against 50% (v/v) 2-methyl-2,4-pentanediol in 20 mM sodium acetate, 2 mM calcium acetate, pH 4.2. The crystals have orthorhombic space group P2(1)2(1)2(1), with cell dimensions a = 48.93(1), b = 46.57(4), c = 41.04(2) A; Z = 4 and V = 93520 A3. The crystal structure was determined on the basis of the isomorphous structure of uncomplexed RNase T1 (Martinez-Oyanedel et al. (1991) submitted for publication) and refined by least squares methods using stereochemical restraints. The refinement was based on Fhkl of 7,445 reflections with Fo greater than or equal to 1 sigma (Fo) in the resolution range of 10-1.8 A, and converged at a crystallographic R factor of 0.149. The phosphate group of 2'-AMP is tightly hydrogen-bonded to the side chains of the active site residues Tyr38, His40, Glu58, Arg77, and His92, comparable with vanadate binding in the respective complex (Kostrewa, D., Choe, H.-W., Heinemann, U., and Saenger, W. (1989) Biochemistry 28, 7592-7600) and different from the complex with guanosine 2'-monophosphate (Arni, R., Heinemann, U., Tokuoka, R., and Saenger, W. (1988) J. Biol. Chem. 263, 15358-15368) where the phosphate does not interact with Arg77 and His92. The adenosine moiety is not located in the guanosine recognition site but stacked on Gly74 carbonyl and His92 imidazole, which serve as a subsite, as shown previously (Lenz, A., Cordes, F., Heinemann, U., and Saenger, W. (1991) J. Biol. Chem. 266, 7661-7667); in addition, there are hydrogen bonds adenine N6H . . . O Gly74 (minor component of three-center hydrogen bond) and adenosine O5' . . . O delta Asn36. These binding interactions readily explain why RNase T1 has some affinity for 2'-AMP. The molecular structure of RNase T1 is only marginally affected by 2'-AMP binding. Its "empty" guanosine-binding site features a flipped Asn43-Asn44 peptide bond and the side chains of Tyr45, Glu46 adopt conformations typical for RNase T1 not involved in guanosine binding. The side chains of amino acids Leu26, Ser35, Asp49, Val78 are disordered. The disorder of Val78 is of interest since this amino acid is located in a hydrophobic cavity, and the disorder appears to be correlated with an "empty" guanosine-binding site. The two Asp15 carboxylate oxygens and six water molecules coordinate a Ca2+ ion 8-fold in the form of a square antiprism.     

Different positively selected sites at the gametophytic self-incompatibility pistil S-RNase gene in the Solanaceae and Rosaceae (Prunus, Pyrus, and Malus)

In this work we perform a comparative study on the location of positively selected sites (those likely responsible for defining specificity differences) at the S-RNase gene, the pistil component of the gametophytic self-incompatibility system. For Plantaginaceae and Rosaceae (Prunus and Pyrus/Malus) this is the first study of this kind. A clear sign of positive selection was observed for 13, 17, and 27 amino acid sites in Solanaceae, Prunus, and Pyrus/Malus, respectively, using two different methodologies. In Plantaginaceae no clear positively selected sites were identified. Possible reasons for this result are discussed. Indirect experimental evidence suggests that the identified positively selected amino acid sites play a role in specificity determination. The percentage of positively selected sites is similar in Solanaceae and Rosaceae but the location of those sites is different. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. Reviewing Editor: Dr. Martin Kreitman     

Sequence diversity of pistil S-proteins associated with gametophytic self-incompatibility in Nicotiana alata

Summary In order to study the extent and nature of differences among various S-allele-associated proteins in N. alata, we carried out comparative studies of seven such proteins. We first isolated and sequenced cDNA clones for the S_z-, S_F11-, S₁-, and S_a-alleles, and then we compared the deduced amino acid sequences both of these four S-proteins and of three previously published S₂-, S₃-, and S₆-proteins. This comparison revealed (1) an average homology of 53.8% among the seven proteins and (2) two homology classes, with S_z and S_F11 in one class and S₁, S₂, S₃, and S₆ in the other class. There are 60 conserved residues, including 9 cysteines. Of the 144 variable residues, 50 were identified as hypervariable based on a calculation of their Similarity Indices. Although conserved, variable, and hypervariable residues are dispersed throughout the protein, some are clustered to form five conserved, five hypervariable, and a number of variable regions. Those variable sites which contain residues conserved within one class of S-proteins but different between classes might provide a clue to the evolutionary relationship of these two classes of S-proteins. The hypervariable residues, which account for sequence variability, may contribute to allelic specificity.     

A cascade signal pathway occurs in self-incompatibility of Pyrus pyrifolia

Pear (Pyrus pyrifolia L.) possesses an S-RNase-based gametophytic self-incompatibility (GSI) system and S-RNase, the self-incompatibility (SI) determinant in the pistil, has also been implicated in the rejection of self-pollen and genetically identical pollen. We have demonstrated that S-RNase depolymerises actin cytoskeleton, triggers mitochondrial alteration and DNA degradation in the incompatible pollen tube, which indicates programmed cell death (PCD) may occur in SI response of Pyrus pyrifolia. Recently, we have identified that S-RNase specifically disrupted tip-localized reactive oxygen species (ROS) of incompatible pollen tube via arrest of ROS formation in mitochondria and cell walls in Pyrus pyrifolia. Furthermore, tip-localized ROS disruption not only decreased the Ca²⁺ current and depolymerised the actin cytoskeleton, but it also induced nuclear DNA degradation in the pollen tube. The results mentioned above indicate that a cascade signal pathway may occur in SI of Pyrus pyrifolia and PCD is used to terminate the incompatible pollen tubes growth. In this addendum, we review the cascade signal pathway of Pyrus pyrifolia SI.Key words: S-RNase, programmed cell death, reactive oxygen species, actin cytoskeleton, Ca²⁺ current, nuclear DNA     

Crystal structure of recombinant rabbit interferon-gamma at 2.7-A resolution

The crystal structure of recombinant rabbit interferon-gamma was solved by the multiple isomorphous replacement technique at 2.7-A resolution and refined to a crystallographic R-factor of 26.2%. The interferon crystallizes with one-half of the functional dimer in the asymmetric unit, with the two polypeptide chains of the dimer related by a crystallographic 2-fold symmetry axis. The structure is predominantly alpha-helical with extensive interdigitation of the alpha-helical segments of the two polypeptide chains.     

Three-dimensional structure of the elastase of Pseudomonas aeruginosa at 1.5-A resolution

Pseudomonas aeruginosa elastase (PAE) is a zinc metalloprotease with 301 amino acids. We have crystallized and solved the three-dimensional structure of PAE, using data to 1.5-A resolution, and have refined the native molecular structure to R = 0.188. The overall tertiary structure of the PAE molecule is similar to that of thermolysin, with which it shares 28% amino acid sequence identity. Nearly all of the active site residues that might potentially interact with substrates are identical in the two proteins. However, the active site cleft is significantly more "open" in PAE than in thermolysin.     

X-ray crystallographic structure of the Norwalk virus protease at 1.5-A resolution

Norwalk virus (NV), a member of the Caliciviridae family, is the major cause of acute, epidemic, viral gastroenteritis. The NV genome is a positive sense, single-stranded RNA that encodes three open reading frames (ORFs). The first ORF produces a polyprotein that is processed by the viral cysteine protease into six nonstructural proteins. We have determined the structure of the NV protease to 1.5 and 2.2 A from crystals grown in the absence or presence, respectively, of the protease inhibitor AEBSF [4-(2-aminoethyl)-benzenesulfonyl fluoride]. The protease, which crystallizes as a stable dimer, exhibits a two-domain structure similar to those of other viral cysteine proteases with a catalytic triad composed of His 30, Glu 54, and Cys 139. The native structure of the protease reveals strong hydrogen bond interactions between His 30 and Glu 54, in the favorable syn configuration, indicating a role of Glu 54 during proteolysis. Mutation of this residue to Ala abolished the protease activity, in a fluorogenic peptide substrate assay, further substantiating the role of Glu 54 during proteolysis. These observations contrast with the suggestion, from a previous study of another norovirus protease, that this residue may not have a prominent role in proteolysis (K. Nakamura, Y. Someya, T. Kumasaka, G. Ueno, M. Yamamoto, T. Sato, N. Takeda, T. Miyamura, and N. Tanaka, J. Virol. 79:13685-13693, 2005). In the structure from crystals grown in the presence of AEBSF, Glu 54 undergoes a conformational change leading to disruption of the hydrogen bond interactions with His 30. Since AEBSF was not apparent in the electron density map, it is possible that these conformational changes are due to subtle changes in pH caused by its addition during crystallization.     

Refined crystal structure of calcium-liganded carp parvalbumin 4.25 at 1.5-A resolution

The crystal structure of carp parvalbumin (pI = 4.25) has been refined by restrained least-squares analysis employing X-ray diffractometer data to 1.5-A resolution. The final residual for 12,653 reflections between 10 and 1.5 A with I(hkl) greater than 2 sigma(I) is 0.215. A total of 74 solvent molecules were included in the least-squares analysis. The root mean square deviation from ideality of bond lengths is 0.024 A. The model has a root mean square difference of 0.59 A from the positions of the main-chain atoms in a previously reported structure [Moews, P. C., & Kretsinger, R. H. (1975) J. Mol. Biol. 91, 201-228], which was refined by difference Fourier syntheses using data collected by film to 1.9 A. Although the overall features of the two models are very similar, there are significant differences in the amino-terminal region, which was extensively refit, and in the number of oxygen atoms liganding calcium in the CD and EF sites, which increased from six to seven in the CD site and decreased from eight to seven in the EF site.     

Crystal structure of a phycourobilin-containing phycoerythrin at 1.90-A resolution.

The structure of R-phycoerythrin (R-PE) from the red alga Griffithsia monilis was solved at 1.90-A resolution by molecular replacement, using the atomic coordinates of cyanobacterial phycocyanin from Fremyella diplosiphon as a model. The crystallographic R factor for the final model is 17.5% (Rfree 22.7%) for reflections in the range 100-1.90 A. The model consists of an (alphabeta)2 dimer with an internal noncrystallographic dyad and a fragment of the gamma-polypeptide. The alpha-polypeptide (164 amino acid residues) has two covalently bound phycoerythrobilins at positions alpha82 and alpha139. The beta-polypeptide (177 residues) has two phycoerythrobilins bound to residues beta82 and beta158 and one phycourobilin covalently attached to rings A and D at residues beta50 and beta61, respectively. The electron density of the gamma-polypeptide is mostly averaged out by threefold crystallographic symmetry, but a dipeptide (Gly-Tyr) and one single Tyr could be modeled. These two tyrosine residues of the gamma-polypeptide are in close proximity to the phycoerythrobilins at position beta82 of two symmetry-related beta-polypeptides and are related by the same noncrystallographic dyad as the (alphabeta)2 dimer. Possible energy transfer pathways are discussed briefly.     

Crystal structure of Escherichia coli CheY refined at 1.7-A resolution

The three-dimensional structure of wild-type CheY from Escherichia coli has been refined by stereochemically restrained least squares minimization to a crystallographic R-factor of 15.1% at 1.7-A resolution. The structure contains 1165 atoms, including all atoms of the protein, 147 water molecules, and three sulfate ions. The final model has root mean square deviations of 0.018 and 0.049 A from idealized bond lengths and angle distances, respectively. Seven amino acid side chains have been modeled in dual conformations. CheY folds as a compact (beta/alpha)5 globular protein, with the phosphorylation region contained in a cavity on one face of the molecule. This active site area is bordered by the carboxyl termini of the three central beta-strands, by alpha 1, and by the loop connecting beta 5 to alpha 5. The Lys-109 side chain of this loop extends into the active site by virtue of its cis peptide bond conformation preceding Pro-110. The epsilon-amino group of Lys-109 is in close bonding contact with the carboxyl group of Asp-57, the residue that is phosphorylated in the activation process of CheY. The details of the hydrogen bonding network in the phosphorylation region indicate that structural rearrangements must accompany the phosphorylation of Asp-57.     

The refined crystal structure of a fully active semisynthetic ribonuclease at 1.8-A resolution

A fully active, semisynthetic analog of bovine ribonuclease A, comprised of residues 1-118 of the molecule in a noncovalent complex with the synthetic peptide analog of residues 111-124, has been crystallized in space group P3(2)21 from a solution of 1.3 M ammonium sulfate and 3.0 M cesium chloride at pH 5.2. The crystallographic structure was determined by rotation and translation searches utilizing the coordinates for ribonuclease A reported by Wlodawer and Sjolin (Wlodawer, A., and Sjolin, L. (1983) Biochemistry 22, 2720-2728) and has been refined at 1.8-A resolution to an agreement factor of 0.204. Most of the structure of the semisynthetic enzyme closely resembles that found in ribonuclease A with the synthetic peptide replacing the C-terminal elements of the naturally occurring enzyme. No redundant structure is seen; residues 114-118 of the larger chain and residues 111-113 of the peptide do not appear in our map. The positions of those residues at or near the active site are very similar to, if not identical with, those previously reported by others, except for histidine 119, which occupies predominantly the B position seen as a minor site by Borkakoti et al. (Borkakoti, N., Moss, D. S., and Palmer, R. A. (1982) Acta Crystallogr. Sect. B Struct. Crystallogr. Cryst. Chem. 38,2210-2217) and not at all by Wlodawer and Sjolin (1983).     

Three-dimensional structure of the ribonuclease T1 2'-GMP complex at 1.9-A resolution

The complex formed between the enzyme ribonuclease T1 (EC 3.1.27.3) and its specific inhibitor 2'-guanylic acid (2'-GMP) has been refined to R = 0.180 using x-ray diffraction data to 1.9-A resolution. The protein molecule displays a compact fold; a 4.5 turn alpha-helix packed over an antiparallel beta-pleated sheet shields most of the hydrophobic interior of the protein against the solvent. The extended pleated sheet structure of ribonuclease T1 is composed of three long and four short strands building up a two-stranded minor beta-sheet near the amino terminus and a five-stranded major sheet in the interior of the protein molecule. In the complex with ribonuclease T1, the inhibitor 2'-guanylic acid adopts the syn-conformation and C2'-endo sugar pucker. Binding of the nucleotide is mainly achieved through amino acid residues 38-46 of the protein. The catalytically active amino acid residues of ribonuclease T1 (His40, Glu58, Arg77, and His92) are located within the major beta-sheet which, as evident from the analysis of atomic temperature factors, provides an environment of minimal local mobility. The geometry of the active site is consistent with a mechanism for phosphodiester hydrolysis where, in the transesterification step, His40 and/or Glu58 act as a general base toward the ribose 2'-hydroxyl group and His92, as a general acid, donates a proton to the leaving 5'-hydroxyl group.     

Crystal structure of yeast cytochrome c peroxidase refined at 1.7-A resolution

The crystal structure of cytochrome c peroxidase (EC 1.11.1.5) has been refined to an R factor of 0.20 computed for all reflections to 1.7 A. The refined molecular model includes 263 bound water molecules and allows for x-ray scattering by amorphous solvent. The mean positional error in atomic coordinates is estimated to lie between 0.12 and 0.21 A. Two factors are identified which may account for the ability of the enzyme to stabilize high-oxidation states of the heme iron during catalysis: 1) the proximal histidine forms a hydrogen bond with a buried aspartic acid side chain, Asp-235; and 2) the heme environment is more polar than in the cytochromes c or globins, owing to the presence of the partially buried side-chain of Arg-48 and five water molecules bound in close proximity to the heme. Two of these occupy the presumed peroxide-binding site. Two candidates are likely for the side chain that is oxidized to a free radical during formation of Compound I: 1) Trp-51, which rests 3.3 A above the heme plane in close proximity (2.7 A) to the sixth coordination position; and 2) Met-172, which is 3.7 A from the heme. Nucleophilic stabilization of the methionyl cation radical may be possible via Asp-235. His-181 is found to lie coplanar with the heme in a niche between the two propionates near the suspected cytochrome c-binding site. A network of hydrogen bonds involving this histidine may provide a preferred pathway for electron transfer between hemes.     

Evidence for rare recombination at the gametophytic self-incompatibility locus

The gametophytic self-incompatibility locus has been thought to be a nonrecombining genomic region. Inferences have been made, however, about the functional importance of different parts of the S-locus, based on differences in the levels of variability along the gene, and this is valid only if recombination occurs. It is thus important to test whether recombination occurs within and near the S-locus. Several recent attempts to test this have reached conflicting conclusions. In this study, we examine a large data set on sequence variation at the S-locus in several species with gametophytic self-incompatibility systems, in the Solanaceae, Rosaceae and Scrophulariaceae. We use the longest sequences available to test for recombination based on linkage disequilibrium between polymorphic sites in the S-locus. The relationship between linkage disequilibrium and physical distance between the sites suggests rare intragenic exchange in the evolutionary history of four species of Solanaceae and two species of Rosaceae.     

Crystal structure of Escherichia coli cystathionine gamma-synthase at 1.5 A resolution.

The transsulfuration enzyme cystathionine gamma-synthase (CGS) catalyses the pyridoxal 5'-phosphate (PLP)-dependent gamma-replacement of O-succinyl-L-homoserine and L-cysteine, yielding L-cystathionine. The crystal structure of the Escherichia coli enzyme has been solved by molecular replacement with the known structure of cystathionine beta-lyase (CBL), and refined at 1.5 A resolution to a crystallographic R-factor of 20.0%. The enzyme crystallizes as an alpha4 tetramer with the subunits related by non-crystallographic 222 symmetry. The spatial fold of the subunits, with three functionally distinct domains and their quaternary arrangement, is similar to that of CBL. Previously proposed reaction mechanisms for CGS can be checked against the structural model, allowing interpretation of the catalytic and substrate-binding functions of individual active site residues. Enzyme-substrate models pinpoint specific residues responsible for the substrate specificity, in agreement with structural comparisons with CBL. Both steric and electrostatic designs of the active site seem to achieve proper substrate selection and productive orientation. Amino acid sequence and structural alignments of CGS and CBL suggest that differences in the substrate-binding characteristics are responsible for the different reaction chemistries. Because CGS catalyses the only known PLP-dependent replacement reaction at Cgamma of certain amino acids, the results will help in our understanding of the chemical versatility of PLP.