Preferential localization of glutamate receptors opposite sites of high presynaptic release

BACKGROUND: The localization of glutamate receptors is essential for the formation and plasticity of excitatory synapses. These receptors cluster opposite neurotransmitter release sites of glutamatergic neurons, but these release sites have heterogeneous structural and functional properties. At the Drosophila neuromuscular junction, receptors expressed in a single postsynaptic cell are confronted with an array of hundreds of apposed active zones. Hence, this is an ideal preparation for the investigation of whether receptor clustering is sensitive to the morphological and physiological properties of the apposed active zones. RESULTS: To investigate the relationship between the localization of glutamate receptors and the properties of the apposed active zones, we investigated receptor localization in mutants in which receptors are limited. We find that receptors are not uniformly distributed opposite the full array of active zones but that some active zones have a disproportionately large share of receptors as assayed by receptor levels and response to transmitter. The active zones at which receptors preferentially cluster are larger and have a higher neurotransmitter release probability than the average active zone. We find a similar relationship between glutamate receptor clusters and active-zone size at wild-type synapses. CONCLUSIONS: When confronted with an array of active zones, glutamate receptors preferentially cluster opposite the largest and most physiologically active sites. These results suggest an activity-dependent matching of pre- and postsynaptic function at the level of a single active zone.     

Site-directed mutagenesis of the beta subunit of tryptophan synthase from Salmonella typhimurium. Role of active site glutamic acid 350.

To investigate the functional role of glutamic acid 350 in the active site of the beta subunit of tryptophan synthase from Salmonella typhimurium, we have replaced this residue by glutamine or alanine by use of site-directed mutagenesis. The mutant alpha 2 beta 2 complexes were expressed, purified, crystallized, and characterized by spectroscopic and kinetic studies with several substrates. We find large alterations in the substrate and reaction specificity of each mutant form of the alpha 2 beta 2 complex. Since the two mutant enzymes are virtually inactive in reactions with L-serine but are active in reactions with beta-chloro-L-alanine, glutamic acid 350 may facilitate the beta-elimination of the weak hydroxyl leaving group of L-serine. The mutant alpha 2 beta 2 complexes are more active than the wild type enzyme in the beta-elimination reaction with beta-chloro-L-alanine. These enzymes are irreversibly inactivated by beta-chloro-L-alanine, whereas the wild type enzyme is not. These altered properties may result from a change in the conformation of the active site, from a change in the orientation of the coenzyme relative to active site residues, or from a change in the solvent accessibility of the active site. The alteration in the active site may enhance the release of amino acrylate from the Schiff base intermediate by hydrolysis or by transamination.     

Isolation of a peroxidatically active product from the peptic digest of ox-liver catalase and some of its properties

1. A homogeneous peroxidatically active product has been isolated from a peptic digest of ox-liver catalase. 2. The nitrogen and the iron contents of the ;active product' are 15.3% and 0.21% respectively. 3. The S(0) (20,w) and molecular weight of the ;active product' are 2.6s and 27500 respectively. 4. The ;active product' contains 248 amino acid residues/mol. assuming a mol.wt. of 27500. 5. The properties of the ;active product' and catalase are compared and the relationship between their structures and enzyme activity is discussed.     

A kinetic model for determining the consequences of electrogenic active transport in cardiac muscle

When active transport is electrogenic in a tissue that is continuously active, such as cardiac muscle, the active transport current is as important in the generation of the action potential as are the passive currents. A thermodynamically constrained kinetic model of electrogenic active transport of sodium and potassium ions has been developed in which the influences of voltage and chemical composition are explicitly defined. This model is coupled to a system of passive permeabilities, of the minimum degree of complexity, to simulate the integrated activity of active and passive ion transport in the generation of the cardiac action potential. Results of preliminary simulations indicate that electrogenic active transport provides a mechanism for slowly changing currents both within the time scale of an action potential as well as of many action potentials. The presence of active transport also complicates the interpretation of isotopic flux measurements and the separation of currents.     

Analysis of the structure and function of creatine kinase active sites using affinity modification

Data of studies of creatine kinase from rabbit skeletal muscle (EC 2.7.3.2) by affinity labelling and affinity chromatography are reviewed. Efficiencies of these techniques are demonstrated for analysis of cooperative interactions of the enzyme's active sites, nature of non-equivalence of enzyme subunits, distances between active sites which are situated on different subunits, dynamics of enzyme-substrate interactions and usefulness of affinity labelling for localization of amino acid residues in the enzyme active sites.     

The presynaptic active zones in three different types of fibres in frog muscle

Presynaptic active zones were studied in slow, fast and intermediate types of frog muscle fibres in freeze-fracture replicas. In fast fibres, the double rows of paired particles are present on active zone ridges perperdicular to the longitudinal axis of the nerve whereas in slow fibres active zone ridges are rudimentary or absent and double rows of particles occur in all directions, mostly paired, sometimes single. In the intermediate type of muscle fibres both types of active zone deployment coexist on a single muscle fibre.     

Guanine nucleotide activation of, and competition between, RAS proteins from Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae, yeast RAS proteins are potent activators of adenylate cyclase. In the present work we measured the activity of adenylate cyclase in membranes from Saccharomyces cerevisiae which overexpress this enzyme. The response of the enzyme to added RAS2 proteins bound with various guanine nucleotides and their analogs suggests that RAS2 proteins are active in their GTP-bound form and are virtually inactive in their GDP-bound form. Also, active RAS2 protein is not inhibited by inactive RAS2, suggesting that the inactive form does not compete with the active form in binding to its effector.     

Deficient repair of the transcribed strand of active genes in Cockayne's syndrome cells.

Removal of ultraviolet light induced cyclobutane pyrimidine dimers (CPD) from active and inactive genes was analyzed in cells derived from patients suffering from the hereditary disease Cockayne's syndrome (CS) using strand specific probes. The results indicate that the defect in CS cells affects two levels of repair of lesions in active genes. Firstly, CS cells are deficient in selective repair of the transcribed strand of active genes. In these cells the rate and efficiency of repair of CPD are equal for the transcribed and the nontranscribed strand of the active ADA and DHFR genes. In normal cells on the other hand, the transcribed strand of these genes is repaired faster than the nontranscribed strand. However, the nontranscribed strand is still repaired more efficiently than the inactive 754 gene and the gene coding for coagulation factor IX. Secondly, the repair level of active genes in CS cells exceeds that of inactive loci but is slower than the nontranscribed strand of active genes in normal cells. Our results support the model that CS cells lack a factor which is involved in targeting repair enzymes specifically towards DNA damage located in (potentially) active DNA.     

Active site identification through geometry-based and sequence profile-based calculations: burial of catalytic clefts

Electrostatics calculations with proteins that are uniformly charged over volume can aid enzyme/non-enzyme discrimination. For known enzymes, such methods locate active sites to within 5% on the enzyme surface, in 77% of a test set. We now report that removing the dielectric boundary improves active site location to 80%, with optimal discrimination between enzymes and non-enzymes of around 80% specificity and 80% sensitivity. This calculation quantifies burial of solvent-accessible regions. Many of the true enzymes incorrectly assigned as non-enzymes have active sites at subunit boundaries. These are missed in monomer-based calculations. Catalytic and non-catalytic antibodies are studied in this context of active/binding site burial. Whilst catalytic antibodies, on average, have marginally higher active site burial than non-catalytic antibodies, these values are generally smaller than for non-antibody enzymes, possibly contributing to their relatively low turnover. Prediction of active site location improves further when sequence profile-based weights replace the uniform charge distribution, so that a combination of burial and amino acid conservation is assessed. Accuracy rises to 93% of active sites to within 5%, in the test set, for the optimal profile weights scheme. The equivalent value in a separate validation set is 89% to within 5%. Enzyme/non-enzyme and enzyme functional site predictions are made for structural genomics proteins, suggesting that a substantial majority of these are non-enzymes.     

Nuclease sensitivity of active chromatin.

The active regions of chicken erythrocyte nuclei were labeled using the standard DNase I directed nick translation reaction. These nuclei were then used to study the characteristics and, in particular, the nuclease sensitivity of active genes. Although DNase I specifically attacks active genes, micrococcal nuclease solubilizes these regions to about the same degree as the total DNA. On the other hand micrococcal nuclease does selectively cut the internucleosomal regions of active genes resulting in the appearance of mononucleosomal fraction which is enriched in active gene DNA. A small percentage of the active chromatin is also released from the nucleus by low speed centrifugation following micrococcal nuclease treatment. The factors which make active genes sensitive to DNase I were shown to reside on individual nucleosomes from these regions. This was established by showing that isolated active mononucleosomes were preferentially sensitive to DNase I digestion. Although the high mobility group proteins are essential for the maintenance of DNase I sensitivity in active regions, these proteins are not necessary for the formation of the conformation which makes these genes preferentially accessible to micrococcal nuclease. The techniques employed in this paper enable one to study the chromatin structure of the entire population of actively expressed genes. Previous studies have elucidated the structure of a few special highly prevalent genes such as ovalbumin and hemoglobin. The results of this paper show that this special conformation is a general feature of all active genes irregardless of the extent of expression.     

Inhibitory effect of membrane active compounds on the uptake of 14C-alpha-aminoisobutyric acid (AIB) in cultured chick embryo liver cells.

High concentrations of beta-adrenoceptor blocking drugs with membrane active properties and of the membrane active compounds quinidine and lidocaine inhibit the uptake of ∝-aminoisobutyric acid by chick embryo liver cells in culture. Beta-adrenoceptor blockers without membrane active properties were without effect. These results are in accordance with previous findings which showed partial inhibition of incorporation of amino acids into proteins caused by membrane active drugs in this system.     

Conformational changes of active site of copper zinc superoxide dismutase can be detected sensitively by electron-transfer reaction

The electron-transfer (ET) reaction between Fe(CN)64- and copper zinc superoxide dismutase (CuZn-SOD) occurs at the active site of the enzyme. The ET parameters which are sensitive to the denaturation have been used to determine the conformational changes of the active site induced by guanidine hydrochloride and thermal denaturation. The decreases of ET rates for all the denatured enzyme samples reflect the collapse of the active cavity of enzyme in the unfolding processes. The interesting changes of ET amplitude for the enzyme denatured at different pH values suggest that electrostatic interaction plays an important role in the conformational changes of active site. From the results of the kinetic analyses, it is concluded that the conformational changes of the active site are parallel with the inactivation.     

Study of the mechanism of initiation of enzymatic NADPH-dependent lipid peroxidation in membranes of the endoplasmic reticulum

The localization and mechanism of generation of active oxygen species in the enzymatic NADPH-dependent lipid peroxidation system in liver microsomes were studied. Using the spin-trapping method, the key role of active oxygen species in the initiation of NADPH-dependent enzymatic lipid peroxidation was confirmed. It was shown that active oxygen species are generated via consecutive one-electron reduction of the oxygen molecule by NADPH-cytochrome P-450 reductase.     

The Effect of Life History on Retroviral Genome Invasions

Endogenous retroviruses (ERV), or the remnants of past retroviral infections that are no longer active, are found in the genomes of most vertebrates, typically constituting approximately 10% of the genome. In some vertebrates, particularly in shorter-lived species like rodents, it is not unusual to find active endogenous retroviruses. In longer-lived species, including humans where substantial effort has been invested in searching for active ERVs, it is unusual to find them; to date none have been found in humans. Presumably the chance of detecting an active ERV infection is a function of the length of an ERV epidemic. Intuitively, given that ERVs or signatures of past ERV infections are passed from parents to offspring, we might expect to detect more active ERVs in species with longer generation times, as it should take more years for an infection to run its course in longer than in shorter lived species. This means the observation of more active ERV infections in shorter compared to longer-lived species is paradoxical. We explore this paradox using a modeling approach to investigate factors that influence ERV epidemic length. Our simple epidemiological model may explain why we find evidence of active ERV infections in shorter rather than longer-lived species.     

Mutagenic exploration of the active site of lactate dehydrogenase from Plasmodium falciparum

Several site-directed mutations of residues around the active site of the lactate dehydrogenase from Plasmodium falciparum are described. These include changes to three highly, but not completely, conserved residues in the pocket of the active site and also three changes (including deletions) to the active site loop. Changes to residues in the active-site pocket resulted in little or no over-production of protein and no enzymic activity. Likewise, a five residue deletion from the active site loop gave no over-produced protein, while a two residue deletion and changes of residue type in this loop were tolerated. The results are discussed in the light of this protein being a suitable target for novel anti-malarials.