The role of zinc in selective neuronal death after transient global cerebral ischemia

Zinc is present in presynaptic nerve terminals throughout the mammalian central nervous system and likely serves as an endogenous signaling substance. However, excessive exposure to extracellular zinc can damage central neurons. After transient forebrain ischemia in rats, chelatable zinc accumulated specifically in degenerating neurons in the hippocampal hilus and CA1, as well as in the cerebral cortex, thalamus, striatum, and amygdala. This accumulation preceded neurodegeneration, which could be prevented by the intraventricular injection of a zinc chelating agent. The toxic influx of zinc may be a key mechanism underlying selective neuronal death after transient global ischemic insults.

Importance of zinc in the central nervous system: the zinc-containing neuron

Zinc is essential to the structure and function of myriad proteins, including regulatory, structural and enzymatic. It is estimated that up to 1% of the human genome codes for zinc finger proteins. In the central nervous system, zinc has an additional role as a neurosecretory product or cofactor. In this role, zinc is highly concentrated in the synaptic vesicles of a specific contingent of neurons, called "zinc-containing" neurons. Zinc-containing neurons are a subset of glutamatergic neurons. The zinc in the vesicles probably exceeds 1 mmol/L in concentration and is only weakly coordinated with any endogenous ligand. Zinc-containing neurons are found almost exclusively in the forebrain, where in mammals they have evolved into a complex and elaborate associational network that interconnects most of the cerebral cortices and limbic structures. Indeed, one of the intriguing aspects of these neurons is that they compose somewhat of a chemospecific "private line" of the mammalian cerebral cortex. The present review outlines (1) the methods used to discover, define and describe zinc-containing neurons; (2) the neuroarchitecture and synaptology of zinc-containing neural circuits; (3) the physiology of regulated vesicular zinc release; (4) the "life cycle" and molecular biology of vesicular zinc; (5) the importance of synaptically released zinc in the normal and pathological processes of the cerebral cortex; and (6) the role of specific and nonspecific stressors in the release of zinc.

Crystal structure of Thermus aquaticus DNA polymerase

The DNA polymerase from Thermus aquaticus (Taq polymerase), famous for its use in the polymerase chain reaction, is homologous to Escherichia coli DNA polymerase I (pol I) Like pol I, Taq polymerase has a domain at its amino terminus (residues 1-290) that has 5' nuclease activity and a domain at its carboxy terminus that catalyses the polymerase reaction. Unlike pol I, the intervening domain in Taq polymerase has lost the editing 3'-5' exonuclease activity. Although the structure of the Klenow fragment of pol I has been known for ten years, that of the intact pol I has proved more elusive. The structure of Taq polymerase determined here at 2.4 A resolution shows that the structures of the polymerase domains of the thermostable enzyme and of the Klenow fragment are nearly identical, whereas the catalytically critical carboxylate residues that bind two metal ions are missing from the remnants of the 3'-5' exonuclease active site of Taq polymerase. The first view of the 5' nuclease domain, responsible for excising the Okazaki RNA in lagging-strand DNA replication, shows a cluster of conserved divalent metal-ion-binding carboxylates at the bottom of a cleft. The location of this 5'-nuclease active site some 70 A from the polymerase active site in this crystal form highlights the unanswered question of how this domain works in concert with the polymerase domain to produce a duplex DNA product that contains only a nick.

Histochemically-reactive zinc in amyloid plaques, angiopathy, and degenerating neurons of Alzheimer's diseased brains

Excess brain zinc has been implicated in Alzheimer's neuropathology. Here we evaluated that hypothesis by searching the brains of Alzheimer's patients for abnormal zinc deposits. Using histochemical methods, we found vivid Zn2+ staining in the amyloid deposits of dense-core (senile) plaques, in the amyloid angiopathy surrounding diseased blood vessels, and in the somata and dendrites of neurons showing the characteristic neurofibrillary tangles (NFT) of Alzheimer's. In contrast, brains from age-matched, non-demented subjects showed only occasional staining for Zn2+ in scattered neurons and possible plaques. A role of abnormal zinc metabolism in Alzheimer's neuropathology is suggested.

Evidence that synaptically-released zinc contributes to neuronal injury after traumatic brain injury

Prior evidence indicates that synaptically-released zinc enters postsynaptic neurons in toxic excess during ischemia and seizures. In addition, prevention of this zinc translocation has been shown to be neuroprotective in both ischemia and seizures. Here we show evidence that the same translocation of zinc from presynaptic boutons into postsynaptic neurons occurs after mechanical injury to the brain. Specifically, using a rat model of traumatic brain injury, we show that trauma is associated with (i) loss of zinc from presynaptic boutons (ii) appearance of zinc in injured neurons, and (iii) neuroprotection by intraventricular administration of a zinc chelator just prior to brain impact. The possible use of zinc chelators for neuroprotection after head trauma is considered.

The crystal structure of a triacylglycerol lipase from Pseudomonas cepacia reveals a highly open conformation in the absence of a bound inhibitor

BACKGROUND

. Lipases, a family of enzymes which catalyze the hydrolysis of triglycerides, are widely distributed in many organisms. True lipases are distinguished from esterases by the characteristic interfacial activation they exhibit at an oil-water interface. Lipases are one of the most frequently used biocatalysts for organic reactions performed under mild conditions. Their biotechnological applications include food and oil processing and the preparation of chiral intermediates for the synthesis of enantiomerically pure pharmaceuticals. Recent structural studies on several lipases have provided some clues towards understanding the mechanisms of hydrolytic activity, interfacial activation, and stereoselectivity. This study was undertaken in order to provide structural information on bacterial lipases, which is relatively limited in comparison to that on the enzymes from other sources.

RESULTS

. We have determined the crystal structure of a triacylglycerol lipase from Pseudomonas cepacia (PcL) in the absence of a bound inhibitor using X-ray crystallography. The structure shows the lipase to contain an alpha/beta-hydrolase fold and a catalytic triad comprising of residues Ser87, His286 and Asp264. The enzyme shares several structural features with homologous lipases from Pseudomonas glumae (PgL) and Chromobacterium viscosum (CvL), including a calcium-binding site. The present structure of PcL reveals a highly open conformation with a solvent-accessible active site. This is in contrast to the structures of PgL and PcL in which the active site is buried under a closed or partially opened 'lid', respectively.

CONCLUSIONS

. PcL exhibits some structural features found in other lipases. The presence of the Ser-His-Asp catalytic triad, an oxyanion hole, and the opening of a helical lid suggest that this enzyme shares the same mechanisms of catalysis and interfacial activation as other lipases. The highly open conformation observed in this study is likely to reflect the activated form of the lipase at an oil-water interface. The structure suggests that the interfacial activation of bacterial lipases involves the reorganization of secondary structures and a large movement of the lid to expose the active site. This is similar to the mechanism described for other well characterized fungal and mammalian lipases.

Rapid translocation of Zn(2+) from presynaptic terminals into postsynaptic hippocampal neurons after physiological stimulation

Zn(2+) is found in glutamatergic nerve terminals throughout the mammalian forebrain and has diverse extracellular and intracellular actions. The anatomical location and possible synaptic signaling role for this cation have led to the hypothesis that Zn(2+) is released from presynaptic boutons, traverses the synaptic cleft, and enters postsynaptic neurons. However, these events have not been directly observed or characterized. Here we show, using microfluorescence imaging in rat hippocampal slices, that brief trains of electrical stimulation of mossy fibers caused immediate release of Zn(2+) from synaptic terminals into the extracellular microenvironment. Release was induced across a broad range of stimulus intensities and frequencies, including those likely to induce long-term potentiation. The amount of Zn(2+) release was dependent on stimulation frequency (1-200 Hz) and intensity. Release of Zn(2+) required sodium-dependent action potentials and was dependent on extracellular Ca(2+). Once released, Zn(2+) crosses the synaptic cleft and enters postsynaptic neurons, producing increases in intracellular Zn(2+) concentration. These results indicate that, like a neurotransmitter, Zn(2+) is stored in synaptic vesicles and is released into the synaptic cleft. However, unlike conventional transmitters, it also enters postsynaptic neurons, where it may have manifold physiological functions as an intracellular second messenger.

High-resolution crystal structure of the non-specific lipid-transfer protein from maize seedlings

BACKGROUND

The movement of lipids between membranes is aided by lipid-transfer proteins (LTPs). Some LTPs exhibit broad specificity, transferring many classes of lipids, and are termed non-specific LTPs (ns-LTPs). Despite their apparently similar mode of action, no sequence homology exists between mammalian and plant ns-LTPs and no three-dimensional structure has been reported for any plant ns-LTP.

RESULTS

We have determined the crystal structure of ns-LTP from maize seedlings by multiple isomorphous replacement and refined the structure to 1.9 A resolution. The protein comprises a single compact domain with four alpha-helices and a long C-terminal region. The eight conserved cysteines form four disulfide bridges (assigned as Cys4-Cys52, Cys14-Cys29, Cys30-Cys75, and Cys50-Cys89) resolving the ambiguity that remained from the chemical determination of pairings in the homologous protein from castor bean. Two of the bonds, Cys4-Cys52 and Cys50-Cys89, differ from what would have been predicted from sequence alignment with soybean hydrophobic protein. The complex between maize ns-LTP and hexadecanoate (palmitate) has also been crystallized and its structure refined to 1.8 A resolution.

CONCLUSIONS

The fold of maize ns-LTP places it in a new category of all-alpha-type structure, first described for soybean hydrophobic protein. In the absence of a bound ligand, the protein has a tunnel-like hydrophobic cavity, which is large enough to accommodate a long fatty acyl chain. In the structure of the complex with palmitate, most of the acyl chain is buried inside this hydrophobic cavity.

Kunitz-type soybean trypsin inhibitor revisited: refined structure of its complex with porcine trypsin reveals an insight into the interaction between a homologous inhibitor from Erythrina caffra and tissue-type plasminogen activator

The Kunitz-type trypsin inhibitor from soybean (STI) consists of 181 amino acid residues with two disulfide bridges. Its crystal structures have been determined in complex with porcine pancreatic trypsin in two crystal forms (an orthorhombic form at 1.75 A resolution and a tetragonal form at 1.9 A) and in the free state at 2.3 A resolution. They have been refined to crystallographic R-values of 18.9%, 21.6% and 19.8%, respectively. The three models of STI reported here represent a significant improvement over the partial inhibitor structure in the complex, which was previously determined at a nominal resolution of 2.6 A by the multiple isomorphous replacement method. This study provides the first high-resolution picture of the complex between a Kunitz-type proteinase inhibitor with its cognate proteinase. Many of the external loops of STI show high B-factors, both in the free and the complexed states, except the reactive site loop whose B-factors are dramatically reduced upon complexation. The reactive site loop of STI adopts a canonical conformation similar to those in other substrate-like inhibitors. The P1 carbonyl group displays no out-of-plane displacement and thus retains a nominal trigonal planar geometry. Modeling studies on the complex between a homologous Kunitz-type trypsin inhibitor DE-3 from Erythrina caffra and the human tissue-type plasminogen activator reveal a new insight into the specific interactions which could play a crucial role in their binding.

Crystal structure and functional analysis of the SurE protein identify a novel phosphatase family

Homologs of the Escherichia coli surE gene are present in many eubacteria and archaea. Despite the evolutionary conservation, little information is available on the structure and function of their gene products. We have determined the crystal structure of the SurE protein from Thermotoga maritima. The structure reveals the dimeric arrangement of the subunits and an active site around a bound metal ion. We also demonstrate that the SurE protein exhibits a divalent metal ion-dependent phosphatase activity that is inhibited by vanadate or tungstate. In the vanadate- and tungstate-complexed structures, the inhibitors bind adjacent to the divalent metal ion. Our structural and functional analyses identify the SurE proteins as a novel family of metal ion-dependent phosphatases.

The galactan-binding immunoglobulin Fab J539: an X-ray diffraction study at 2.6-A resolution

The crystal structure of the Fab of the galactan-binding immunoglobulin J539 (a mouse IgA,kappa) has been determined at a resolution of approximately 2.6 A by X-ray diffraction. The starting model was that obtained from the real space search described previously (Navia, M.A., Segal, D.M., Padlan, E.A., Davies, D.R., Rao, D.N., Rudikoff, S. and Potter, M. "Crystal structure of galactan-binding mouse immunoglobulin J539 Fab at 4.5 A resolution." Proc. Nat. Acad. Sci. USA, 76:4071-4074, 1979). This Fab structure has now been refined by restrained least-squares procedures to an R-value of 19% for the 11,690 unique reflections between 8.0 A and 2.6 A. The rms deviation from ideal bond lengths is 0.025 A. The overall structure differs from McPC603 Fab, another mouse IgA,kappa antibody, in that the elbow bend, relating the variable and constant parts of the molecule, is 145 degrees vs. 133 degrees for McPC603. The region of the molecule expected to be the antigen binding site contains a large cavity with two clefts leading away from it. This has been fitted with a model of an oligo-galactan.

Crystal structure of NAD(+)-dependent DNA ligase: modular architecture and functional implications

DNA ligases catalyze the crucial step of joining the breaks in duplex DNA during DNA replication, repair and recombination, utilizing either ATP or NAD(+) as a cofactor. Despite the difference in cofactor specificity and limited overall sequence similarity, the two classes of DNA ligase share basically the same catalytic mechanism. In this study, the crystal structure of an NAD(+)-dependent DNA ligase from Thermus filiformis, a 667 residue multidomain protein, has been determined by the multiwavelength anomalous diffraction (MAD) method. It reveals highly modular architecture and a unique circular arrangement of its four distinct domains. It also provides clues for protein flexibility and DNA-binding sites. A model for the multidomain ligase action involving large conformational changes is proposed.

Structural basis of non-specific lipid binding in maize lipid-transfer protein complexes revealed by high-resolution X-ray crystallography

Non-specific lipid-transfer proteins (nsLTPs) are involved in the movement of phospholipids, glycolipids, fatty acids, and steroids between membranes. Several structures of plant nsLTPs have been determined both by X-ray crystallography and nuclear magnetic resonance. However, the detailed structural basis of the non-specific binding of hydrophobic ligands by nsLTPs is still poorly understood. In order to gain a better understanding of the structural basis of the non-specific binding of hydrophobic ligands by nsLTPs and to investigate the plasticity of the fatty acid binding cavity in nsLTPs, seven high-resolution (between 1.3 A and 1.9 A) crystal structures have been determined. These depict the nsLTP from maize seedlings in complex with an array of fatty acids.A detailed comparison of the structures of maize nsLTP in complex with various ligands reveals a new binding mode in an nsLTP-oleate complex which has not been seen before. Furthermore, in the caprate complex, the ligand binds to the protein cavity in two orientations with equal occupancy. The volume of the hydrophobic cavity in the nsLTP from maize shows some variation depending on the size of the bound ligands. The structural plasticity of the ligand binding cavity and the predominant involvement of non-specific van der Waals interactions with the hydrophobic tail of the ligands provide a structural explanation for the non-specificity of maize nsLTP. The hydrophobic cavity accommodates various ligands from C10 to C18. The C18:1 ricinoleate with its hydroxyl group hydrogen bonding to Ala68 possibly mimics cutin monomer binding which is of biological importance. Some of the myristate binding sites in human serum albumin resemble the maize nsLTP, implying the importance of a helical bundle in accommodating the non-specific binding of fatty acids.

Structural and mechanistic conservation in DNA ligases

DNA ligases are enzymes required for the repair, replication and recombination of DNA. DNA ligases catalyse the formation of phosphodiester bonds at single-strand breaks in double-stranded DNA. Despite their occurrence in all organisms, DNA ligases show a wide diversity of amino acid sequences, molecular sizes and properties. The enzymes fall into two groups based on their cofactor specificity, those requiring NAD(+) for activity and those requiring ATP. The eukaryotic, viral and archael bacteria encoded enzymes all require ATP. NAD(+)-requiring DNA ligases have only been found in prokaryotic organisms. Recently, the crystal structures of a number of DNA ligases have been reported. It is the purpose of this review to summarise the current knowledge of the structure and catalytic mechanism of DNA ligases.

Tertiary structure of plant RuBisCO: domains and their contacts

The three-dimensional structure of ribulose-1,5-biphosphate carboxylase-oxygenase (RuBisCO), has been determined at 2.6 A resolution. This enzyme initiates photosynthesis by combining carbon dioxide with ribulose bisphosphate to form two molecules of 3-phosphoglycerate. In plants, RuBisCO is built from eight large (L) and eight small (S) polypeptide chains, or subunits. Both S chains and the NH2-terminal domain (N) of L are antiparallel beta, "open-face-sandwich" domains with four-stranded beta sheets and flanking alpha helices. The main domain (B) of L is an alpha/beta barrel containing most of the catalytic residues. The active site is in a pocket at the opening of the barrel that is partly covered by the N domain of a neighboring L chain. The domain contacts of the molecule and its conserved residues are discussed in terms of this structure.

Malignant obstruction of gastric outlet and duodenum: palliation with flexible covered metallic stents

PURPOSE

To assess the usefulness of flexible covered metallic stents in the palliation of malignant obstruction of the gastric outlet and duodenum.

MATERIALS AND METHODS

Twenty-four consecutive patients with malignant obstruction of the gastric outlet (n = 22) or duodenum (n = 2) underwent palliative treatment with self-expandable flexible covered metallic stents. Fourteen patients had advanced gastric carcinoma at the antrum and/or pylorus, and eight had obstruction at the anastomosis site of previous gastrojejunostomy. Complications and clinical status were investigated during the study period.

RESULTS

The technical success rate was 75% (18 of 24 patients). Twenty-one stents were placed in 18 patients by using an introducer 6 (n = 7) or 8 mm (n = 14) in diameter. The mean follow-up period was 3.4 months (range, 1 week to 9 months). Symptoms improved in 12 (67%) patients after the procedure. There was no change in symptoms in five and a decrease in one. Twelve patients died during the follow-up period (mean survival, 4.3 months). The complication rate was 25% (six of 24 patients), including stent migration (n = 5) and fracture (n = 3).

CONCLUSION

Flexible covered metallic stent placement can be useful for palliation in patients with malignant obstruction of the gastric outlet or duodenum.

Crystal structure of Escherichia coli CyaY protein reveals a previously unidentified fold for the evolutionarily conserved frataxin family

Friedreich ataxia is an autosomal recessive neurodegenerative disease caused by defects in the FRDA gene, which encodes a mitochondrial protein called frataxin. Frataxin is evolutionarily conserved, with homologs identified in mammals, worms, yeast, and bacteria. The CyaY proteins of gamma-purple bacteria are believed to be closely related to the ancestor of frataxin. In this study, we have determined the crystal structure of the CyaY protein from Escherichia coli at 1.4-A resolution. It reveals a protein fold consisting of a six-stranded antiparallel beta-sheet flanked on one side by two alpha-helices. This fold is likely to be shared by all members of the conserved frataxin family. This study also provides a framework for the interpretation of disease-associated mutations in frataxin and for understanding the possible functions of this protein family.

Malignant colorectal obstruction: treatment with a flexible covered stent

PURPOSE

To evaluate the usefulness of flexible covered stents for treatment of acute colorectal obstruction secondary to malignant colorectal carcinoma.

MATERIALS AND METHODS

Twenty patients with acute colorectal obstruction secondary to malignant colorectal carcinoma were treated by means of intubation of a flexible stent with fluoroscopic guidance with occasional endoscopic assistance. Two types of stents were placed (type 1, completely covered, type 2, two-thirds of proximal part uncovered). Of 15 patients with primary colorectal carcinoma, 12 underwent placement of a stent for presurgical decompression of colorectal obstruction; three, for palliative decompression. In three patients with rectosigmoid seeding from advanced gastric carcinoma and two patients with recurrent colonic carcinoma, stents were placed for palliative decompression.

RESULTS

Stent placement was successful in 18 (90%) of 20 patients. Symptoms of obstruction resolved within 24 hours in 15 (75%) patients. Eight patients underwent elective single-stage surgery without complications 5-7 days after stent placement. Two patients underwent tumor resection and colostomy. In eight patients, stents provided palliative decompression of the colon. Type 1 stents migrated in four (50%) of eight patients; type 2 stents were used thereafter.

CONCLUSION

Flexible stents effectively relieved acute colonic obstruction secondary to malignant rectosigmoid neoplasm. Stent placement allowed patients to undergo single-stage surgery in most cases and provided palliative decompression in cases of inoperable or disseminated disease.

Rice non-specific lipid transfer protein: the 1.6 A crystal structure in the unliganded state reveals a small hydrophobic cavity

This study describes the high-resolution X-ray structure of the non-specific lipid transfer protein (ns-LTP) from rice seeds in the unliganded state. The model has been refined to a crystallographic R-factor of 0.186 for 8.0 to 1.6 A data (with Fo > 2 sigma F). It accounts for all 91 amino acid residues, 68 water molecules, one sulfate ion, and two molecules of 3-[cyclohexylamino]-1-propanesulfonic acid. The root-mean-square deviations from ideal bond lengths and angles are 0.017 A and 1.76 degrees, respectively. The overall fold of rice ns-LTP is very similar to that of maize ns-LTP. A superposition of 91 common C alpha atoms in rice and maize ns-LTPs, both in the unliganded state, gives a root-mean-square deviation of 1.2 A. Large structural differences from the crystal structure of maize ns-LTP are observed in two regions: the loop between two alpha-helices H1 and H2, where one residue deletion (Gln21 of maize sequence) occurs, and the C-terminal region around Tyr79. The C-terminal region of rice protein is somewhat collapsed into the hydrophobic cavity. As a consequence, its hydrophobic cavity is considerably smaller than that of maize protein (144 A3 versus 408 A3 for van der Waals cavity volumes), despite a high level of sequence identity (79%) between them. In the rice ns-LTP structure, the side-chain of Arg44 partially blocks the mouth of the cavity, while the side-chain of Ile81 effectively closes the other end by protruding into the cavity. And the side-chain of Tyr79 divides the cavity into two parts, with the larger part being shielded from the solvent. The present study illuminates the structure-function relationship of rice ns-LTP and allows a detailed structural comparison with other plant ns-LTPs.

Crystal structure of carboxylesterase from Pseudomonas fluorescens, an alpha/beta hydrolase with broad substrate specificity

BACKGROUND

A group of esterases, classified as carboxylesterases, hydrolyze carboxylic ester bonds with relatively broad substrate specificity and are useful for stereospecific synthesis and hydrolysis of esters. One such carboxylesterase from Pseudomonas fluorescens is a homodimeric enzyme, consisting of 218-residue subunits. It shows a limited sequence similarity to some members of the alpha/beta hydrolase superfamily. Although crystal structures of a number of serine esterases and lipases have been reported, structural information on carboxylesterases is very limited. This study was undertaken in order to provide such information and to understand a structural basis for the substrate specificity of this carboxylesterase.

RESULTS

In this study, the crystal structure of carboxylesterase from P. fluorescens has been determined by the isomorphous replacement method and refined to 1.8 A resolution. Each subunit consists of a central seven-stranded beta sheet flanked by six alpha helices. The structure reveals the catalytic triad as Ser 114-His 199-Asp 168. The structure of the enzyme in complex with the inhibitor phenylmethylsulfonyl fluoride has also been determined and refined to 2.5 . The inhibitor is covalently attached to Ser 114 of both subunits, with the aromatic ring occupying a hydrophobic site defined by the aliphatic sidechains of Leu23, Ile58, Ile70, Met73 and Val170. No large structural changes are observed between the free and inhibitor-bound structures.

CONCLUSIONS

Carboxylesterase from P. fluorescens has the alpha/beta hydrolase fold and the Ser-His-Asp catalytic triad. The active-site cleft in each subunit is formed by the six loops covering the catalytic serine residue. Three of the active-site loops in each subunit are involved in a head-to-head subunit interaction to form a dimer; it may be these extra structural elements, not seen in other esterases, that account for the inability of carboxylesterase to hydrolyze long chain fatty acids. As a result of dimerization, the active-site clefts from the two subunits merge to form holes in the dimer. The active-site clefts are relatively open and thus the catalytic residues are exposed to the solvent. An oxyanion hole, formed by nitrogen atoms of Leu23 and Gln115, is present in both the free and inhibitor-bound structures. An open active site, as well as a large binding pocket for the acid part of substrates, in P. fluorescens carboxylesterase may contribute to its relatively broad substrate specificity.

The TB structural genomics consortium: a resource for Mycobacterium tuberculosis biology

The TB Structural Genomics Consortium is an organization devoted to encouraging, coordinating, and facilitating the determination and analysis of structures of proteins from Mycobacterium tuberculosis. The Consortium members hope to work together with other M. tuberculosis researchers to identify M. tuberculosis proteins for which structural information could provide important biological information, to analyze and interpret structures of M. tuberculosis proteins, and to work collaboratively to test ideas about M. tuberculosis protein function that are suggested by structure or related to structural information. This review describes the TB Structural Genomics Consortium and some of the proteins for which the Consortium is in the progress of determining three-dimensional structures.

Crystal structure of the ribosome recycling factor from Escherichia coli

We have determined the crystal structure of the Escherichia coli ribosome recycling factor (RRF), which catalyzes the disassembly of the termination complex in protein synthesis. The L-shaped molecule consists of two domains: a triple-stranded antiparallel coiled-coil and an alpha/beta domain. The coil domain has a cylindrical shape and negatively charged surface, which are reminiscent of the anticodon arm of tRNA and domain IV of elongation factor EF-G. We suggest that RRF binds to the ribosomal A-site through its coil domain, which is a tRNA mimic. The relative position of the two domains is changed about an axis along the hydrophobic cleft in the hinge where the alkyl chain of a detergent molecule is bound. The tRNA mimicry and the domain movement observed in RRF provide a structural basis for understanding the role of RRF in protein synthesis.

Sliding-layer conformational change limited by the quaternary structure of plant RuBisCO

RuBisCO, D-ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39), converts carbon dioxide to sugar in the first step of photosynthesis. In plants and some bacteria, this enzyme has an L8S8 structure, where L is the large catalytic subunit and S is the small subunit of unknown function. The molecule resembles a keg 105 A along the 4-fold axis and 132 A in diameter at the widest point of the keg. Here we describe the quaternary structure of RuBisCO from N. tabacum, the first L8S8 type known from an X-ray crystallographic study at near-atomic resolution (3 A). The structure shows that all eight L subunits are elongated along the 4-fold axis so that the molecule cannot be simply described as layers of subunits, as it had been from studies by electron microscopy. The structure, with its elongated and interdigitated L subunits, is evidence against a large, sliding-layer conformational change in plant RuBisCO, as proposed recently in Nature for the same enzyme from Alcaligenes eutrophus.

Trigger digits in children

Seven thousand, seven hundred newborn children were examined prospectively to determine the congenital incidence of trigger thumb and finger. No cases were found. The case histories of 43 trigger digit cases (35 trigger thumbs and eight trigger fingers) noted in 40 children diagnosed at our center between 1995 and 1998 were reviewed with special reference to the spontaneous recovery rate, treatment outcome, and age at presentation. Of the 35 thumb cases, 23 underwent surgical release and all responded satisfactorily to surgical treatment. Spontaneous recovery was noted in 12 trigger thumb cases and in all eight trigger finger cases. Trigger finger developed earlier in life than trigger thumb and the spontaneous recovery rate was higher in trigger finger than trigger thumb.

Nitric oxide causes apparent release of zinc from presynaptic boutons

One of us showed previously [Cuajungco and Lees (1998) Brain Res. 799, 188-129] that nitric oxide injected into the cerebrum in vivo causes zinc staining to appear in the somata of neurons and suggested that this staining of somata might be accompanied by a depletion (release) of zinc from axon terminals. In the present study, we confirm earlier results and report that there is a dramatic loss (apparent release) of histologically reactive zinc from the boutons of zinc-containing axons induced by infusion of nitric oxide into the brain in vivo. Rats were anesthetized with halothane and a cannula was inserted into the hippocampus. Either nitric oxide donor (spermineNONOate, 100 mM/2 microl) or control (spermine, 100 mM/2 l) was infused into the hippocampus or the cerebellar cortex. Two hours after infusion, N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ) staining for zinc in the brains revealed that sperminenitric oxide, but not control (spermine only) produced up to 95% depletion of zinc staining from the zinc-containing boutons. TSQ-positive neurons were also conspicuous throughout injection sites, in both the cerebral cortex and in the cerebellar cortex, where the Purkinje neurons were especially vivid, despite the scarcity of zinc-containing axonal boutons. It is suggested that the TSQ-stainable zinc in somata might represent intracellular stores mobilized from within or permeating extracellular stores.