Purification and characterization of two sucrose synthase isoforms from Japanese pear fruit

Two isoforms (SS I and SS II) of sucrose synthase (SS; EC 2.54.1.13) were purified from Japanese pear fruit and their properties were compared. SS I mainly appeared in young fruit and SS II mainly in mature fruit. SS I and SS II were purified to the specific activity of 3.37 and 4.26 (units (mg protein)(-1)), respectively. The MW of native and subunit proteins of SS I and SS II were almost the same and both SSs seemed to be a tetramer composed of an 83 kDa polypeptide. However, the ionic charges of the native proteins and the kinetic parameters of SSs were different. Specifically, the Km value for UDP-glucose in SS I was the same as that for UDP, while the Km value for UDP-glucose in SS II was less than that for UDP. SS II easily reacted for sucrose synthesis than sucrose cleavage compared with SS I. Therefore, it is considered that SS I and SS II play different roles in the utilization of carbohydrate in young and mature fruit, respectively.     

Protein phosphorylation and membrane association of sucrose synthase in developing tomato fruit.

Calcium-dependent protein kinase (CDPK) activities were detected both in the soluble and the membrane fraction of various tomato (Lycopersicon esculentum Mill.) organs, using a synthetic peptide mimicking the serine 11 phosphorylation site of a tomato sucrose synthase (SS, EC 2.4.1.13) isoform as substrate. The levels of membrane and soluble Ser-CDPK activities were differentially regulated during fruit development. The membrane Ser-CDPK activity was maximal in young fruit but decreased as the fruit developed, suggesting a specific role during fruit growth. Using an in gel assay with purified tomato SS as substrate, we showed that partially purified soluble and membrane Ser-CDPK preparations both contained a SS-kinase polypeptide of 55 kDa. The membrane and soluble Ser-CDPK activities were largely inactivated in the absence of calcium or when MgCl(2) was replaced by MnCl(2). Both soluble and membrane Ser-CDPK activities were very sensitive to staurosporine. Using Fe(III)-immobilized metal chromatography to determine the apparent phosphorylation status of the enzyme in vivo, we showed that soluble SS was largely dephosphorylated in fruits fed EGTA or staurosporine, compared to fruits fed water or sucrose. Moreover, the level of SS increased by about two-fold in the membrane fraction of fruits fed the Ser-CDPK inhibitors, compared to the control. The level of SS protein in the membrane and soluble fractions of tomato fruit was developmentally regulated, the membrane form being specifically detected in actively growing fruits. Together, our results suggest that a mechanism involving protein phosphorylation/dephosphorylation and/or calcium would in part control the association of SS isoforms with membranes in developing tomato fruit.     

Analysis of protein complexes in wheat amyloplasts reveals functional interactions among starch biosynthetic enzymes

Protein-protein interactions among enzymes of amylopectin biosynthesis were investigated in developing wheat (Triticum aestivum) endosperm. Physical interactions between starch branching enzymes (SBEs) and starch synthases (SSs) were identified from endosperm amyloplasts during the active phase of starch deposition in the developing grain using immunoprecipitation and cross-linking strategies. Coimmunoprecipitation experiments using peptide-specific antibodies indicate that at least two distinct complexes exist containing SSI, SSIIa, and either of SBEIIa or SBEIIb. Chemical cross linking was used to identify protein complexes containing SBEs and SSs from amyloplast extracts. Separation of extracts by gel filtration chromatography demonstrated the presence of SBE and SS forms in protein complexes of around 260 kD and that SBEII forms may also exist as homodimers. Analysis of cross-linked 260-kD aggregation products from amyloplast lysates by mass spectrometry confirmed SSI, SSIIa, and SBEII forms as components of one or more protein complexes in amyloplasts. In vitro phosphorylation experiments with gamma-(32)P-ATP indicated that SSII and both forms of SBEII are phosphorylated. Treatment of the partially purified 260-kD SS-SBE complexes with alkaline phosphatase caused dissociation of the assembly into the respective monomeric proteins, indicating that formation of SS-SBE complexes is phosphorylation dependent. The 260-kD SS-SBEII protein complexes are formed around 10 to 15 d after pollination and were shown to be catalytically active with respect to both SS and SBE activities. Prior to this developmental stage, SSI, SSII, and SBEII forms were detectable only in monomeric form. High molecular weight forms of SBEII demonstrated a higher affinity for in vitro glucan substrates than monomers. These results provide direct evidence for the existence of protein complexes involved in amylopectin biosynthesis.     

Phosphorylation of protein phosphatase inhibitor-1 by protein kinase C

Inhibitor-1 becomes a potent inhibitor of protein phosphatase 1 when phosphorylated by cAMP-dependent protein kinase at Thr(35). Moreover, Ser(67) of inhibitor-1 serves as a substrate for cyclin-dependent kinase 5 in the brain. Here, we report that dephosphoinhibitor-1 but not phospho-Ser(67) inhibitor-1 was efficiently phosphorylated by protein kinase C at Ser(65) in vitro. In contrast, Ser(67) phosphorylation by cyclin-dependent kinase 5 was unaffected by phospho-Ser(65). Protein kinase C activation in striatal tissue resulted in the concomitant phosphorylation of inhibitor-1 at Ser(65) and Ser(67), but not Ser(65) alone. Selective pharmacological inhibition of protein phosphatase activity suggested that phospho-Ser(65) inhibitor-1 is dephosphorylated by protein phosphatase 1 in the striatum. In vitro studies confirmed these findings and suggested that phospho-Ser(67) protects phospho-Ser(65) inhibitor-1 from dephosphorylation by protein phosphatase 1 in vivo. Activation of group I metabotropic glutamate receptors resulted in the up-regulation of diphospho-Ser(65)/Ser(67) inhibitor-1 in this tissue. In contrast, the activation of N-methyl-d-aspartate-type ionotropic glutamate receptors opposed increases in striatal diphospho-Ser(65)/Ser(67) inhibitor-1 levels. Phosphomimetic mutation of Ser(65) and/or Ser(67) did not convert inhibitor-1 into a protein phosphatase 1 inhibitor. On the other hand, in vitro and in vivo studies suggested that diphospho-Ser(65)/Ser(67) inhibitor-1 is a poor substrate for cAMP-dependent protein kinase. These observations extend earlier studies regarding the function of phospho-Ser(67) and underscore the possibility that phosphorylation in this region of inhibitor-1 by multiple protein kinases may serve as an integrative signaling mechanism that governs the responsiveness of inhibitor-1 to cAMP-dependent protein kinase activation.     

Phosphorylation of the tubulin-binding protein, stathmin, by Cdk5 and MAP kinases in the brain

Regulation of cytoskeletal dynamics is essential to neuronal plasticity during development and adulthood. Dysregulation of these mechanisms may contribute to neuropsychiatric and neurodegenerative diseases. The neuronal protein kinase, cyclin-dependent kinase 5 (Cdk5), is involved in multiple aspects of neuronal function, including regulation of cytoskeleton. A neuroproteomic search identified the tubulin-binding protein, stathmin, as a novel Cdk5 substrate. Stathmin was phosphorylated by Cdk5 in vitro at Ser25 and Ser38, previously identified as mitogen-activated protein kinase (MAPK) and p38 MAPKdelta sites. Cdk5 predominantly phosphorylated Ser38, while MAPK and p38 MAPKdelta predominantly phosphorylated Ser25. Stathmin was phosphorylated at both sites in mouse brain, with higher levels in cortex and striatum. Cdk5 knockout mice exhibited decreased phospho-Ser38 levels. During development, phospho-Ser25 and -Ser38 levels peaked at post-natal day 7, followed by reduction in total stathmin. Inhibition of protein phosphatases in striatal slices caused an increase in phospho-Ser25 and a decrease in total stathmin. Interestingly, the prefrontal cortex of schizophrenic patients had increased phospho-Ser25 levels. In contrast, total and phospho-Ser25 stoichiometries were decreased in the hippocampus of Alzheimer's patients. Thus, microtubule regulatory mechanisms involving the phosphorylation of stathmin may contribute to developmental synaptic pruning and structural plasticity, and may be involved in neuropsychiatric and neurodegenerative disorders.     

The role of sucrose-metabolizing enzymes in pear fruit that differ in sucrose accumulation

In the paper, the soluble sugar composition and activities of enzymes metabolizing sucrose: invertase (β-fructosidase, EC 3.2.1.26), sucrose synthase (SS; EC 2.4.1.13) and sucrose-phosphate synthase (SPS; EC 2.4.1.14) were investigated during fruit development of two pear species: Pyrus bretschneideri Rehd. cv. ‘Yali’ and P. pyrifolia Nakai cv. ‘Aikansui’, characterized as low and high sucrose types, respectively. It was found that, at the end of fruit development of ‘Aikansui’, the level of sucrose was five times higher than in ‘Yali’ in the same period. It was coincident with the significantly higher activities of SS (synthesis) and SPS and lower activities of invertase (vacuolar and cell wall-bound acid invertase and neutral invertase). The high correlation was found between sucrose level and SS (synthesis) and SPS activities in ‘Aikansui’ pears.     

Purification and characterization of two soluble acid invertase isozymes from Japanese pear fruit

Two isozymes (AIV I and AIV II) of soluble acid invertase (EC 3.2.1.26) were purified from Japanese pear fruit through procedures including (NH(4))(2)SO(4) precipitating, DEAE-Sephacel column chromatography, Concanavalin A (ConA)-Sepharose affinity chromatography, hydroxyapatite column chromatography and Mono Q HR 5/5 column chromatography. The specific activities of purified AIV I and AIV II were 2670 and 2340 (nkat/mg protein), respectively. AIV I was a monomeric enzyme of 80 kDa, while AIV II may be also a monomeric enzyme, which is easy to be cleaved to 52 kDa and 34 kDa polypeptide during preparation by SDS-PAGE. The Km values for sucrose of AIV I and AIV II were 3.33 and 4.58 mM, respectively, and optimum pH of both enzyme activities was pH 4.5.     

Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance.

Experiments were conducted to determine whether sucrose synthase (SuSy) was phosphorylated in the elongation zone of maize (Zea mays L.) leaves. The approximately 90-kD subunit of SuSy was 32P-labeled on seryl residue(s) when excised shoots were fed [32P]orthophosphate. Both isoforms of SuSy (the SS1 and SS2 proteins) were phosphorylated in vivo, and tryptic peptide-mapping analysis suggested a single, similar phosphorylation site in both proteins. A combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and automated Edman sequencing analysis unequivocally identified the phosphorylation site in the maize SS2 protein as serine-15. This site was phosphorylated in vitro by endogenous protein kinase(s) in a strictly Ca(2+)-dependent manner. A synthetic peptide, based on the phosphorylation site sequence, was used to identify and partially purify an endogenous Ca(2+)-dependent protein kinase(s) from the maize leaf elongation zone and expanding spinach leaves. Phosphorylation of SuSy in vitro selectively activates the cleavage reaction by increasing the apparent affinity of the enzyme for sucrose and UDP, suggesting that phosphorylation may be of regulatory significance. Conservation of the phosphorylation site, and the sequences surrounding it, among plant species suggests that phosphorylation of SuSy may be widespread, if not universal, in plants.     

Sucrose Synthase in Developing Maize Leaves: Regulation of Activity by Protein Level during the Import to Export Transition

The maize (Zea mays) leaf is a valuable system to study the sucrose import to sucrose export transition at the cellular level. Rapidly growing and fully heterotrophic cells in the basal part of the young leaf showed a high sucrose synthase (SS) activity. Leaf SS has been purified to homogeneity. By comparison with purified kernel SS isozymes, the leaf SS has been identified as SS₂. SS₁ protein and SS₂ protein were clearly separated by electrophoresis and the two monomers differed in size by 6 kilodaltons. Nevertheless, kinetic parameters of both enzymes were very similar. Immunodetection of SS protein showed that in young heterotrophic tissues SS₂ was a major protein accounting for 3% of the total protein. Concurrent with greening, SS activity decreased and the change of activity was explained by regulation of the protein level. In mature green tissues, which are synthetizing sucrose as evidenced by the presence of sucrose phosphate synthase activity, SS activity was almost completely absent. Results suggested that down regulation of SS₂ enzyme protein level was an early event in the transition from import to export status of the leaf.     

Phosphorylation of RNA polymerase I augments its interaction with autoantibodies of systemic lupus erythematosus patients

Purified RNA polymerase I was phosphorylated by the endogenous protein kinase or dephosphorylated by alkaline phosphatase and used as antigen in a radioimmunoassay with sera from systemic lupus erythematosus patients or serum from an immunized rabbit. Enzyme incubated in the absence of ATP or phosphatase served as control. Three to seven times more of the autoantibodies in the patients' sera reacted with phosphorylated RNA polymerase I than with control enzyme. The reactivity of the dephosphorylated enzyme with lupus autoantibodies was only 50-60% of that observed with control enzyme. Neither phosphorylation nor dephosphorylation of the enzyme had an effect on its reaction with the rabbit antibodies. The effect of phosphorylation on the reaction of each RNA polymerase I subunit (S1-S8; Mr = 190,000-17,000) with the patients' antibodies was determined by an immunoblot procedure following resolution of the subunits on polyacrylamide gels. Prior phosphorylation of the enzyme resulted in a dramatic increase in binding of each patient's antibodies to all polymerase subunits with the exception of S4. Anti-S4 antibody was not detected with either phosphorylated or control enzyme. Strikingly, antibodies in each patients' sera reacted with S6 only after its phosphorylation. Similarly, anti-S5 antibodies in the serum of one patient were only detected with phosphorylated RNA polymerase I. The present data suggest that at least a significant fraction of the anti-RNA polymerase I autoantibodies in the sera of systemic lupus erythematosus patients might be directed against phosphorylated sites on the enzyme and that phosphorylation may have a role in the production of this and other autoimmunogenic nuclear components which are hallmarks of this disease.     

Heterogeneity in the Phosphorylation of Micro tubule-Associated Protein MAP 1B During Rat Brair Development

Abstract: The patterns of isoforms and of immunoreactivity of the microtubule-associated protein MAP1 B toward a panel of antibodies to phosphorylation-sensitive epitopes are different in distinct rat brain regions and change during development. This suggests the occurrence of a considerable degree of heterogeneity in the phosphorylation state of rat brain MAP1 B. It appears that MAP1 B can be phosphorylated at multiple sites that may be conventionally classified into at least two modes of phosphorylation. Mode I of phosphorylation induces significant upward shifts in the electrophoretic mobility of the protein, giving rise to "high" MAP1B isoforms, whereas the mode II of MAP1B phosphorylation does not greatly affect the electrophoretic mobility of the protein. These MAP1B phosphorylation modes are differentially regulated throughout development and show some regional specificity. Cytosolic MAP1 B is highly phosphorylated both at mode I and mode II sites in the developing rat brain, as well as in the adult olfactory bulb, where axonal growth takes place. In most adult rat brain regions, cytosolic MAP1B is highly phosphorylated at mode II sites but largely dephosphorylated at certain mode I sites. However, MAP1 B present in the particulate fraction of most rat brain region homogenates may be partially dephosphorylated at certain mode II sites, although it contains some phosphorylated mode I sites. These data are compatible with the view that different protein kinases, possibly including casein kinase II and proline-directed protein kinases, might regulate the state of phosphorylation of MAP1B in distinct localizations along the development of different neuronal populations in the brain.     

套袋对梨果实发育过程中糖组分及其相关酶活性的影响

以翠冠和黄金梨为试材,测定套袋和未套袋(对照)梨果实发育时期果实中蔗糖、葡萄糖、果糖和山梨醇含量以及蔗糖代谢相关酶酸性转化酶(AI)、中性转化酶(NI)、蔗糖合成酶(SS)和蔗糖磷酸合成酶(SPS)的活性,并对果实中糖组分积累与酶活性的关系进行了分析.结果表明:(1)两梨品种套袋果实在发育过程中蔗糖、葡萄糖、果糖、山梨醇和糖代谢相关酶活性变化趋势与对照基本一致,套袋果实糖含量均低于对照但差异不显著,而各相关酶活性在两类果实间差异表现各异.(2)在梨果实发育早期,果实中以分解酶类为主,糖分积累低;发育后期以合成酶类为主,糖分积累多.(3)两品种套袋和对照果实AI活性与葡萄糖含量均呈显著或极显著正相关,SS合成方向活性与蔗糖含量均为极显著正相关,且翠冠对照果SPS活性与蔗糖含量呈极显著正相关.可见,套袋通过提高果实发育早期转化酶(Inv)活性,降低果实后期蔗糖磷酸合成酶(SPS)、蔗糖合成酶(SS)的活性来影响糖分积累,从而影响梨果品质.     

Changes in tonoplast protein and density with the development of pear fruit

Protoplasts isolated from pear fruit at the end of the cell‐division stage, 30 days after flowering (DAF), had already formed a large central vacuole and the vacuole occupied most of the protoplast. The changes in protein composition and density of the tonoplast (vacuolar membrane) were investigated during fruit development. After a linear sucrose density gradient centrifugation, the distribution of tonoplasts at 30 and 48 DAF was broad and began to narrow with further fruit development. This suggests that the tonoplast of young fruit is heterogeneous and becomes homogeneous with fruit development. The apparent density of the tonoplast at 30 DAF was approximately 1.12 g ml⁻¹; it decreased with fruit development and was finally 1.09 g ml⁻¹ in mature fruit. The phospholipid amount on the basis of tonoplast protein was 0.80 mg mg⁻¹ at 30 DAF. It increased with fruit development, and finally reached 7.49 mg mg⁻¹. This result indicates that the decrease in the density of the tonoplast was caused by the increase in the ratio of phospholipid to membrane protein. The protein composition of the tonoplast at each stage was quite different. The level of polypeptides of 94, 70, 61, 52, 48 and 41 kDa was low in young fruit and high in the middle or later stages of fruit development. In contrast, the level of a 76‐kDa polypeptide was high in young fruit and decreased with fruit development. Although their functions are still unclear, these tonoplast proteins may play important roles in fruit development.     

Phosphorylation of neurofilament H subunit at the tail domain by CDC2 kinase dissociates the association to microtubules

We sought the mammalian neurofilament tail domain-specific kinase. Several well known kinases including cAMP-dependent protein kinase, protein kinase C, Ca(2+)-calmodulin-dependent protein kinase II, casein kinase I, and casein kinase II phosphorylated the high (NF-H) and middle molecular mass subunit (NF-M) of bovine neurofilaments, but they did not reduced the electrophoretic mobility of the dephosphorylated form of NF-M and NF-H by phosphorylation nor was the amount of phosphorylation increased by dephosphorylation of NF proteins, indicating that the phosphorylation sites by these kinases are not major in vivo phosphorylation sites at the tail domain. In contrast, cdc2 kinase phosphorylated specifically the dephosphorylated form of NF-H. 4 mol of phosphates were incorporated per mol of NF-H and this phosphorylation returned the electrophoretic mobility of the dephosphorylated form of NF-H to the position of the isolated, fully phosphorylated form of NF-H. Furthermore, the phosphorylation by cdc2 kinase dissociated the binding of dephosphorylated NF-H to microtubules. Phosphorylation sites were located at the carboxyl-terminal tail domain. The KSPXK motif, but not KSPXX, in the repetitive sequence was suggested to be the phosphorylation site by using synthetic peptides.     

Suppression of calpain-dependent cleavage of the CDK5 activator p35 to p25 by site-specific phosphorylation

Cdk5 is a proline-directed Ser/Thr protein kinase predominantly expressed in postmitotic neurons together with its activator, p35. N-terminal truncation of p35 to p25 by calpain results in deregulation of Cdk5 and contributes to neuronal cell death associated with several neurodegenerative diseases. Previously we reported that p35 occurred as a phosphoprotein, phospho-p35 levels changed with neuronal maturation, and that phosphorylation of p35 affected its vulnerability to calpain cleavage. Here, we identify the p35 residues Ser(8) and Thr(138) as the major sites of phosphorylation by Cdk5. Mutagenesis of these sites to unphosphorylatable Ala increased susceptibility to calpain in cultured cells and neurons while changing them to phosphomimetic glutamate-attenuated cleavage. Furthermore, phosphorylation state-specific antibodies to these sites revealed that Thr(138) was dephosphorylated in adult rat, although both Ser(8) and Thr(138) were phosphorylated in prenatal brains. In cultured neurons, inhibition of protein phosphatases converted phosho-Ser(8) p35 to dual phospho-Ser(8)/Thr(138) p35 and conferred resistance to calpain cleavage. These results suggest phosphorylation of Thr(138) predominantly defines the susceptibility of p35 to calpain-dependent cleavage and that dephosphorylation of this site is a critical determinant of Cdk5-p25-induced cell death associated with neurodegeneration.     

Increased Membrane/Nuclear Translocation and Phosphorylation of p90 KD Ribosomal S6 Kinase in the Brain of Hypoxic Preconditioned Mice

Our previous studies have demonstrated that hypoxic precondition (HPC) increased membrane translocation of protein kinase C isoforms and decreased phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the brain of mice. The goal of this study was to determine the involvement of p90 KD ribosomal S6 kinase (RSK) in cerebral HPC of mice. Using Western-blot analysis, we found that the levels of membrane/nuclear translocation, but not protein expression of RSK increased significantly in the frontal cortex and hippocampus of HPC mice. In addition, we found that the phosphorylation levels of RSK at the Ser227 site (a PDK1 phosphorylation site), but not at the Thr359/Ser363 sites (ERK1/2 phosphorylated sites) increased significantly in the brain of HPC mice. Similar results were confirmed by an immunostaining study of total RSK and phospho-Ser227 RSK. To further define the cellular populations to express phospho-Ser227 RSK, we found that the expression of phospho-Ser227 RSK co-localized with neurogranin, a neuron-specific marker, in cortex and hippocampus of HPC mice by using double-labeled immunofluorescent staining method. These results suggest that increased RSK membrane/nuclear translocation and PDK1 mediated neuron-specific phosphorylation of RSK at Ser227 might be involved in the development of cerebral HPC of mice.     

网纹甜瓜发育果实糖分积累与蔗糖代谢参与酶的关系

随着网纹甜瓜果实的发育,果实中葡萄糖和果糖的含量增加,蔗糖的快速积累发生在果实发育的中后期,高蔗糖积累型果实中蔗糖积累速率明显快于低蔗糖积累型.蔗糖磷酸合成酶活性在果实发育的前期短暂下降, 而后稳步上升,在果实发育的中后期高蔗糖积累型果实中该酶的活性显著高于低蔗糖积累型果实;随着果实发育,蔗糖合成酶的分解活性降低而合成活性升高.酸性和中性转化酶在未成熟果实中活性较高,而在成熟果实中很低; 高蔗糖积累型果实中酸性转化酶活性显著低于同期低蔗糖积累型果实.合成蔗糖的酶活性小于分解蔗糖的酶活性时蔗糖几乎没有积累.根据这些结果推测,转化酶活性的下降、蔗糖磷酸合成酶活性的增加以及蔗糖合成酶分解活性的下降和合成活性的增加,是引起果实蔗糖积累的主要内在因子.