Moving right along: how PP1 helps clear the checkpoint

Spindle checkpoint silencing is crucial for cell-cycle progression, but mechanisms underlying this process remain mysterious. Two papers, one in this issue of Developmental Cell (Meadows et?al., 2011) and one in Current Biology (Rosenberg et?al., 2011), begin to show how phosphatase PP1-gamma connects chromosome-microtubule attachment with anaphase entry.     

Studies of the structure--function relationships of Neurospora crassa pyruvate kinase: interaction with blue dextran--sepharose and Cibacron blue 3G-A

Blue dextran--Sepharose and Cibacron blue 3G-A interact with pyruvate kinase of Neurospora crassa. The enzyme is readily released from the substituted Sepharose column by elution with 0.17 M potassium phosphate buffer (pH 7.9), or 2 mM fructose 1,6-diphosphate (FDP), but not with either of the substrates, ADP and phosphoenolpyruvate (PEP), at 2 mM. Cibacron blue 3G A is a noncompetitive inhibitor of pyruvate kinase with respect to both substrates. It appears to compete with the allosteric effector, FDP, for binding to the enzyme surface. A lack of elution of the enzyme from the immobilized blue dextran matrix by adenine nucleotides and the absence of a difference spectrum in the 650- to 700-nm range suggest that a "dinucleotide-fold" substructure is not implicated in the dye binding sites on pyruvate kiase. The interaction of Cibacron blue 3G-A and this enzyme can be followed fluorometrically; incremental additon of the dye to the enzyme solution results in a progressive decrease in the fluorescence of surface tryptophanyl residues. The quenching of fluorescence of exposed aromatic groups is subject to reversal following addition of FDP to the pyruvte kinase--Cibacron blue complex.     

Extracellular cGMP phosphodiesterase related to the rod outer segment phosphodiesterase isolated from bovine and monkey retinas

A phosphodiesterase (PDE) has been characterized in the interphotoreceptor matrix (IPM) of light-adapted fresh bovine retinas. It is obtained through a gentle rinsing of the retinal surface under conditions where the light-activated rod outer segment (ROS) enzyme remains attached. The enzyme has an apparent native molecular weight of 350 000 by gel filtration and appears as a doublet at Mr 47 000 and 45 000 on sodium dodecyl sulfate-polyacrylamide gels. It has an apparent Km value for cGMP of 33 microM and an apparent Km value for cAMP of 2200 microM. It is activated 3-6-fold by protamine and over 40-fold by trypsin. Protamine has no effect on the Km for cGMP while trypsin decreases the Km for cGMP by a factor of 2. The enzyme occurs in at least two forms as evidenced by two distinct peaks of activity after gel electrophoresis under nondenaturing conditions. A heat-stable inhibitor is tightly bound to the enzyme. The inhibitor obtained from the IPM PDE inhibits 98% of the activity of the trypsin-activated ROS PDE: conversely, the inhibitor obtained by boiling the ROS PDE completely inhibits the trypsin-activated IPM enzyme. A high-affinity monoclonal antibody to the active site of the ROS PDE, ROS 1 [Hurwitz, R., Bunt-Milan, A.H., & Beavo, J. (1984) J. Biol. Chem. 259, 8612-8618], quantitatively absorbs the IPM PDE. These observations indicate a clear relationship between these two PDEs even though their location, sizes, and specific functions in the retina appear to be distinct.     

High resolution crystal structures of human kynurenine aminotransferase‐I bound to PLP cofactor,and in complex with aminooxyacetate

In this study, we report two high‐resolution structures of the pyridoxal 5′ phosphate (PLP)‐dependent enzyme kynurenine aminotransferase‐I (KAT‐I). One is the native structure with the cofactor in the PLP form bound to Lys247 with the highest resolution yet available for KAT‐I at 1.28 Å resolution, and the other with the general PLP‐dependent aminotransferase inhibitor, aminooxyacetate (AOAA) covalently bound to the cofactor at 1.54 Å. Only small conformational differences are observed in the vicinity of the aldimine (oxime) linkage with which the PLP forms the Schiff base with Lys247 in the 1.28 Å resolution native structure, in comparison to other native PLP‐bound structures. We also report the inhibition of KAT‐1 by AOAA and aminooxy‐phenylpropionic acid (AOPP), with IC50s of 13.1 and 5.7 μM, respectively. The crystal structure of the enzyme in complex with the inhibitor AOAA revealed that the cofactor is the PLP form with the external aldimine linkage. The location of this oxime with the PLP, which forms in place of the native internal aldimine linkage of PLP of the native KAT‐I, is away from the position of the native internal aldimine, with the free Lys247 substantially retaining the orientation of the native structure. Tyr101, at the active site, was observed in two conformations in both structures.     

Depurination of ribosomal RNA and inhibition of viral RNA translation by an antiviral protein of Celosia cristata

An antiviral protein (25 kD) isolated from leaves of Celosia cristata (CCP 25) was tested for depurination study on ribosomal RNA from yeast. Ribosomal RNA yielded 360 nucleotide base fragment after treatment with CCP 25 indicating that CCP 25 was a ribosome inactivating protein. CCP 25 also inhibited translation of brome mosaic virus (BMV) and pokeweed mosaic virus (PMV) RNAs in rabbit reticulocyte translation system. The radioactive assay showed that incorporation of [35S]-methionine was less in translation proteins of BMV nucleic acid when CCP 25 was added to translation system. This indicated that antiviral protein from Celosia cristata not only depurinated ribosomal RNA but also inhibited translation of viral RNA in vitro.     

Modulation by gibberellic acid and adenosine-3′,5′-cyclic monophosphate of starch hydrolysing activity of cowpea seedlings

In cowpea seedlings starch hydrolysing activity increases 35–50 fold on germination for 4 days. This increase in enzyme activity was inhibited by the in vivo addition of 1% glucose but this inhibition was completely overcome by the addition of gibberellic acid (GA₃) (10^?5 M) and adenosine-3′,5′-cyclic monophosphate (cAMP) (10^?5 M). At 5% glucose, GA₃ and cAMP were only partially effective. Structural analogues of cAMP failed to relieve the inhibitory effect of glucose. The inhibition by glucose is not direct but RNA and protein synthesis may be involved. Glucose appears to reduce the internal pool of cAMP which causes inhibition of RNA synthesis and decrease in starch hydrolysing activity. Exogenous application of cAMP may replenish the endogenous pool of cyclic nucleotide and thus overcome inhibition of RNA synthesis and enzyme activity.     

Changes in the levels of peroxidase and phenylalanine ammonia-lyase inBrassica napus cultivars showing variable resistance toLeptosphaeria maculans

Peroxidase and phenylalanine ammonia-lyase activities (PAL) were determined in leaves of healthy and inoculatedBrassica napus cultivars, showing differential disease reaction towards a virulent and a weakly virulent strain ofLeptosphaeria maculans, the black leg pathogen. Both enzymes showed increased activities as the result of inoculation, PAL activity increasing as early as 12 h after inoculation. The most significant increase in both peroxidase and PAL activity was observed when the moderately resistant cultivar, Cresor, was challenged with the weakly virulent strain. Highest activity of the two enzymes was detected 2 d after inoculation. Very low peroxidase activity was detected in both strains ofL. maculans, while no PAL activity was detectable in either of the strains. Cytochemical tests revealed increased peroxidase activity following inoculation, mainly in the epidermal and guard cells.