Regulation of sedoheptulose-1,7-bisphosphatase by sedoheptulose-7-phosphate and glycerate,and of fructose-1,6-bisphosphatase by glycerate in spinach chloroplasts

Using partially purified sedoheptulose-1,7-bisphosphatase from spinach (Spinacia oleracea L.) chloroplasts the effects of metabolites on the dithiothreitoland Mg²⁺-activated enzyme were investigated. A screening of most of the intermediates of the Calvin cycle and the photorespiratory pathway showed that physiological concentrations of sedoheptulose-7-phosphate and glycerate specifically inhibited the enzyme by decreasing its maximal velocity. An inhibition by ribulose-1,5-bisphosphate was also found. The inhibitory effect of sedoheptulose-7-phosphate on the enzyme is discussed in terms of allowing a control of sedoheptulose-1,7-bisphosphate hydrolysis by the demand of the product of this reaction. Subsequent studies with partially purified fructose-1,6-bisphosphatase from spinach chloroplasts showed that glycerate also inhibited this enzyme. With isolated chloroplasts, glycerate was found to inhibit CO₂ fixation by blocking the stromal fructose-1,6-bisphosphatase. It is therefore possible that the inhibition of the two phosphatases by glycerate is an important regulatory factor for adjusting the activity of the Calvin cycle to the ATP supply by the light reaction.Abbreviations DTT dithiothreitol - FBPase fructose-1,6-bisphosphatase - Fru-1,6-P₂ fructose-1,6-bisphosphate - Fru-6-P fructose-6-phosphate - 3-PGA 3-phosphoglycerate - Ru-1,5-P₂ ribulose-1,5-bisphosphate - Ru-5-P ribulose-5-phosphate - SBPase sedoheptulose-1,7-bisphosphatase - Sed-1,7-P₂ sedoheptulose-1,7-bisphosphate - Sed-7-P sedoheptulose-7-phosphate This work was supported by the Deutsche Forschungsgemein-schaft.     

Metabolite levels in the stroma of spinach chloroplasts exposed to osmotic stress: Effects of the pH of the medium and exogenous dihydroxyacetone phosphate

The levels of stromal photosynthetic intermediates were measured in isolated intact spinach (Spinacia oleracea L.) chloroplasts exposed to reduced osmotic potentials. Stressed chloroplasts showed slower rates of metabolite accumulation upon illumination than controls. Relative to other metabolites sedoheptulose-1,7-bisphosphate (SBP) and fructose-1,6-bisphosphate (FBP) accumulated in the stroma in the stressed treatments. Under these conditions 3-phosphoglycerate (3-PGA) efflux to the medium was restricted. Chloroplasts previously incubated with [³²P]KH₂PO₄ and [³²P]dihydroxyacetone phosphate ([³²P]DAP) in the dark were characterized by very high FBP and SBP levels prior to illumination. Metabolism of these pools upon illumination increased with increasing pH of the medium but was consistently inhibited in osmotically stressed chloroplasts. The responses of stromal FBP and SBP pools under hypertonic conditions are discussed in terms of both inhibited light activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) and sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37), and likely increases in stromal ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) active-site concentrations.Abbreviations and symbols DAP dihydroxyacetone phosphate - FBP fructose-1,6-bisphosphate - PGA 3-phosphoglycerate - RuBP ribulose-1,5-bisphosphate - SBP sedoheptulose-1,7-bisphosphate - _s osmotic potential     

Light activation of fructose bisphosphatase in isolated spinach chloroplasts and deactivation by hydrogen peroxide

Spinach chloroplast fructose bisphosphatase (EC 3.1.3.11.) exists in both oxidised and reduced forms. Only the latter has the kinetic properties that allow it to function at physiological concentrations of fructose 1,6-bisphosphate and Mg²⁺. Illumination of freshly prepared type A chloroplasts causes a conversion of oxidised to reduced enzyme. The rate of this conversion does not limit the rate of CO₂ fixation. In the dark the reduced enzyme partially reverts back to the oxidised form. If catalase is omitted from the reaction medium the rate of CO₂ fixation by chloroplasts is decreased and seems to be limited by the rate of conversion of the enzyme to the reduced form. The physiological significance of the light dependent generation of dithiol compounds (such as thioredoxin) within chloroplasts is discussed.     

Overexpression of SBPase enhances photosynthesis against high temperature stress in transgenic rice plants

Activity of the Calvin cycle enzyme sedoheptulose-1,7-bisphosphatase (SBPase) was increased by overexpression of a rice plants 9,311 (Oryza sativa L.) cDNA in rice plants zhonghua11 (Oryza sativa L.). The genetic engineering enabled the plants to accumulate SBPase in chloroplasts and resulted in enhanced tolerance to high temperature stress during growth of young seedlings. Moreover, CO₂ assimilation of transgenic plants was significantly more tolerant to high temperature than that of wild-type plants. The analyses of chlorophyll fluorescence and the content and activation of SBPase indicated that the enhancement of photosynthesis to high temperature was not related to the function of photosystem II but to the content and activation of SBPase. Western blotting analyses showed that high temperature stress led to the association of SBPase with the thylakoid membranes from the stroma fractions. However, such an association was much more pronounced in wild-type plants than that in transgenic plants. The results in this study suggested that under high temperature stress, SBPase maintained the activation of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) by preventing the sequestration of Rubisco activase to the thylakoid membranes from the soluble stroma fraction and thus enhanced the tolerance of CO₂ assimilation to high temperature stress. The results suggested that overexpression of SBPase might be an effective method for enhancing high temperature tolerance of plants.     

Photosynthesis of isolated chloroplasts and protoplasts under osmotic stress

1. Isolated intact spinach chloroplasts respond to changes of the sorbitol concentration of the suspending medium as near-perfect osmometers within a large range of osmotic potentials. Under isotonic conditions (=9–10 bar), their average osmotic volume is 24 m³ and the total volume 36 m³. The osmotic volume can be increased to 63 m³ by lowering the sorbitol concentration until a critical osmotic potential of =4 bar is reached. Below that value chloroplasts rupture. Between 10 bar and 4 bar, volume changes are reversible. 2. Increasing the chloroplast volume above 24 m³ causes inhibition of photosynthesis, with 50% inhibition occurring at an osmotic potential of =5–6 bar. This corresponds to an osmotic volume of 45–55 m³. Depending on the duration of hypotonic treatment, inhibition of photosynthesis is more or less reversible. 3. Between 4 and 10 bar, the chloroplast envelope exhibits a very low permeability for ferricyanide, many metabolites, and soluble stroma proteins. 4. Electron transport is not inhibited by swelling of chloroplasts. Also, the ATP/ADP-ratio remains unchanged. 5. The solute concentration in the chloroplasts appears to be optimal for photosynthesis at 10 bar. Increasing the chloroplast volume causes inhibition of photosynthesis by dilution effects.     

Photosynthesis under osmotic stress

The reversibility of the inhibition of photosynthetic reactions by water stress was examined with four systems of increasing complexity—stromal enzymes, intact chloroplasts, mesophyll protoplasts, and leaf slices. The inhibition of soluble chloroplast enzymes by high solute concentrations was instantly relieved when solutes were properly diluted. In contrast, photosynthesis was not restored but actually more inhibited when isolated chloroplasts exposed to hypertonic stress were transferred to conditions optimal for photosynthesis of unstressed chloroplasts. Upon transfer, chloroplast volumes increased beyond the volumes of unstressed chloroplasts, and partial envelope rupture occurred. In protoplasts and leaf slices, considerable and rapid, but incomplete restoration of photosynthesis was observed during transfer from hypertonic to isotonic conditions. Chloroplast envelopes did not rupture in situ during water uptake. It is concluded that inhibition of photosynthesis by severe water stress is at the biochemical level brought about in part by reversible inhibition of chloroplast enzymes and in part by membrane damage which requires repair mechanisms for reversibility. Both soluble enzymes and membranes appear to be affected by the increased concentration of internal solutes, which is caused by dehydration.     

Regulation of fructose-1,6-bisphosphatase activity in intact chloroplasts. Studies of the mechanism of inactivation

1. The aim of this work was to investigate the mechanism of dark inactivation of fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) in isolated intact chloroplasts of Triticum aestivum.

2. Dark inactivation of the enzyme, which was rapid under aerobic conditions, was prevented under anaerobic conditions when chloroplasts were incubated in the absence of an electron acceptor. Electron acceptors such as oxaloacetate readily brought about inactivation under anaerobic conditions whether chloroplasts were illuminated or in the dark. Inactivation of the enzyme also occurred if illuminated or darkened anaerobic chloroplasts were exposed to oxygen.

3. Pyocyanine, which catalyses a cyclic electron flow around Photosystem I, also caused inactivation of the enzyme in illuminated, anaerobic chloroplasts.

4. It is proposed that the activity of fructose-1,6-bisphosphatase is regulated by the availability of electrons, and thus by electron acceptors, and that dark inactivation may occur by a direct reversal of the activation process.     

Control of CO2 fixation regulation of stromal fructose-1,6-bisphosphatase in spinach by pH and Mg2+ concentration

The effect of pH and of Mg²⁺ concentration on the light activated form of stromal fructose-1,6-bisphosphatase (FBPase) was studied using the enzyme rapidly extracted from illuminated spinach chloroplasts. The (fructose-1,6-bisphosphate^4-)(Mg²⁺) complex has been identified as the substrate of the enzyme. Therefore, changes of pH and Mg²⁺ concentrations have an immediate effect on the activity of FBPase by shifting the pH and Mg²⁺ dependent equilibrium concentration of the substrate. In addition, changes of pH and Mg²⁺ concentration in the assay medium have a delayed effect on FBPase activity. A correlation of the activities observed using different pH and Mg²⁺ concentrations indicates, that the effect is not a consequence of the pH and Mg²⁺ concentration as such, but is caused by a shift in the equilibrium concentration of a hypothetical inhibitor fructose-1,6-bisphosphate^3- (uncomplexed), resulting in a change of the activation state of the enzyme. The interplay between a rapid effect on the concentration of the substrate and a delayed effect on the activation state enables a rigid control of stromal FBPase by stromal Mg²⁺ concentrations and pH. Fructose-1,6-bisphosphatase is allosterically inhibited by fructose-6-phosphate in a sigmoidal fashion, allowing a fine control of the enzyme by its product.Abbreviations Fru1,6 bis P fructose-1,6-bisphosphate - Fru6P fructose-6-phosphate - FBPase fructose-1,6-bisphosphatase Some of these results have been included in a preliminary report (Heldt et al. 1984)     

Molecular design of photosynthesis-elevated chloroplasts for mass accumulation of a foreign protein

In order to increase production of a useful protein by the chloroplast transformation technique, it seems to be necessary to determine the upper limit for the accumulation of a biologically active foreign protein in chloroplasts and then improve photosynthetic capacity and plant productivity. Here we show that the stromal fractions of tobacco chloroplasts could accommodate an additional 200-260 mg ml(-1) of green fluorescent protein in the stroma without any inhibition of gas exchange under various light intensity and growth conditions. The minimum amount of fructose-1,6-/sedoheptulose-1,7-bisphosphatase (FBP/SBPase) limiting photosynthesis was then calculated. Analyses of the photosynthetic parameters and the metabolites of transformants into which FBP/SBPase was introduced with various types of promoter (PpsbA, Prrn, Prps2 and Prps12) indicated that a 2- to 3-fold increase in levels of FBPase and SBPase activity is sufficient to increase the final amount of dry matter by up to 1.8-fold relative to the wild-type plants. Their increases were equivalent to an increase of <1 mg ml(-1) of the FBP/SBPase protein in chloroplasts and were calculated to represent <1% of the protein accumulated via chloroplast transformation. Consequently, >99% of the additional 200-260 mg ml(-1) of protein expressed in the chloroplasts could be used for the production of useful proteins in the photosynthesis-elevated transplastomic plants having FBP/SBPase.     

Photosynthetic induction in wheat protoplasts and chloroplasts. Autocatalysis and light activation of enzymes

Abstract Unlike wheat chloroplasts, wheat protoplasts showed a pronounced restoration of the induction phase after a short period of darkness. This difference was used to investigate the relative roles of light-induced reductive activation of enzymes and the auto-catalytic increase in the level of substrates in the control of the rate of photosynthesis during induction. Light activation and dark inactivation of ribulose 5-phosphate kinase, fructose 1,6-biphosphatase and NADP⁺-specific glyceraldehydephosphate dehydrogenase were measured. In this respect there was no appreciable difference between protoplasts and chloroplasts. In contrast, the level of photosynthetic intermediates remained constant in darkened isolated chloroplasts, but declined rapidly in chloroplasts isolated from darkened protoplasts. When fructose 1,6-bisphosphatase was pre-activated by treating protoplasts with dithiothreitol the lag was only slightly shortened. These results are discussed in terms of control of the rate of the photosynthesis during the lag by substrates rather than limitation imposed by activity of any of the enzymes measured.     

Effect of leaf position on levels of the chloroplast and cytosolic fructose bisphosphatase isozymes in the pea leaf nucleus

Summary. Immunocytolocalization experiments indicate that nuclear levels of the pea leaf cytosolic fructose bisphosphatase are higher in leaves located near the base of the plant and lower in expanded leaves at the apex. It seems possible that the cytosolic isozyme in the nucleus has a role in tissue aging. In contrast, there is no indication that leaf position or tissue age affects levels of the chloroplastic enzyme in the nucleus. The density of the chloroplast fructose bisphosphatase is higher in the nucleolus than in the nucleoplasm. Conversely, the density of the cytosolic isozyme is slightly higher in the nucleoplasm. Analysis of double immunolabeling experiments indicates that both isozymes are distributed nonrandomly with respect to DNA, and therefore colocalized with DNA, in the nucleus, and that the chloroplast isozyme is also distributed nonrandomly with respect to DNA in the chloroplast. Correspondence and reprints: Department of Biological Sciences m/c 066, University of Illinois-Chicago, 845 West Taylor, Chicago, Illinois 60607-7060, U.S.A.     

Sugar accumulation,photosynthesis and growth of two indica rice varieties in response to salt stress

Sugar, a final product of photosynthesis, is reported to be involved in the defense mechanisms of plants against abiotic stresses such as salinity, water deficiency, extreme temperature and mineral toxicity. Elements involved in photosynthesis, sugar content, water oxidation, net photosynthetic rate, activity of enzyme and gene expression have therefore been studied in Homjan (HJ), salt-tolerant, and Pathumthani 1 (PT1), salt-sensitive, varieties of rice. Fructose-1,6-biphosphatase (FBP) and fructokinase (FK) genes were rapidly expressed in HJ rice when exposed to salt stress for 1–6 h and to a greater degree than in PT1 rice. An increase in FBP enzyme activity was found in both roots and leaves of the salt-tolerant variety after exposure to salt stress. A high level of sugar and a delay in chlorophyll degradation were found in salt-tolerant rice. The total sugar content in leaf and root tissues of salt-tolerant rice was 2.47 and 2.85 times higher, respectively, than in the salt-sensitive variety. Meanwhile, less chlorophyll degradation was detected. Salt stress may promote sugar accumulation, thus preventing the degradation of chlorophyll. Water oxidation by the light reaction of photosynthesis in the salt-tolerant variety was greater than that in the salt-sensitive variety, indicated by a high maximum quantum yield of PSII (F _v/F _m) and quantum efficiency of PSII (Φ_PSII) with low nonphotochemical quenching (NPQ), leading to a high net photosynthetic rate. In addition, the overall growth performances in the salt-tolerant variety were higher than those in the salt-sensitive variety. The FBP gene expression and enzyme activity, sugar accumulation, pigment stabilization, water oxidation and net photosynthetic rate parameters in HJ rice should be further investigated as multivariate salt-tolerant indices for the classification of salt tolerance in rice breeding programs.     

Cloning,structure and expression of a pea cDNA clone coding for a photosynthetic fructose-1,6-bisphosphatase with some features different from those of the leaf chloroplast enzyme

A positive clone against pea (Pisum sativum L.) chloroplast fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) antibodies was obtained from a copy DNA (cDNA) library in λgt11. The insert was 1261 nucleotides long, and had an open reading frame of 1143 base pairs with coding capability for the whole FBPase subunit and a fragment of a putative processing peptide. An additional 115 base pairs corresponding to a 3′-untranslated region coding for an mRNA poly(A)⁺ tail were also found in the clone. The deduced sequence for the FBPase subunit was a 357-amino-acid protein of molecular mass 39253 daltons (Da), showing 82–88% absolute homology with four chloroplastic FBPases sequenced earlier. The 3.1-kilobase (kb)KpnI-SacI fragment of the λgt11 derivative was subcloned between theKpnI-SacI restriction sites of pTZ18R to yield plasmid pAMC100. Lysates ofEscherichia coli (pAMC100) showed FBPase activity; this was purified as a 170-kDa protein which, upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, displayed a 44-kDa band. As occurs with native FBPases, this indicates a homotetrameric structure for the expressed FBPase. When assayed under excess Mg²⁺ (10 mM), the expressed enzyme had a higher affinity for the substrate than the native pea leaf FBPase; this parameter appears to be substantiated by a tenfold higher specific activity than that of the native enzyme. However, when activated with dithiothreitol plus saturating concentrations of pea thioredoxin (Td) f, both FBPase had similar activities, with a 4:1 Td f-FBPase stoichiometry. In contrast to the native pea chloroplast FBPase, theE. coli-expressed enzyme did not react with the monoclonal antibody GR-PB5. It also had a higher heat sensitivity, with 42% residual activity after heating for 30 min at 60°C, conditions which preserved the native enzyme in a fully active state. These results show the existence of some difference(s) in the conformation of the two FBPases; this could be a consequence of a different expression of the genomic and cDNA clones, or be due to the need for some factor for the correct assembly of the oligomeric structure of the native chloroplast enzyme. Accession number for pea chloroplast FBPase coding sequence: X68826 in the European Molecular Biology Laboratory (EMBL)     

Purification and properties of pea (Pisum sativum L.) thioredoxin f,a plant thioredoxin with unique features in the activation of chloroplast fructose-1,6-bisphosphatase

Thioredoxin (Td) f from pea (Pisum sativum L.) leaves was purified by a simple method, which provided a high yield of homogeneous Td f. Purified Td f had an isoelectric point of 5.4 and a relative molecular mass (M_r) of 12 kilodaltons (kDa) when determined by filtration through Superose 12, but an M_r of 15.8 kDa when determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified protein remained fully active for several months when conserved frozen at — 20° C. The pea protein was able to activate fructose1,6-bisphosphatase (FBPase; EC 3.1.3.11), but in contrast to other higher-plant Td f proteins, was not functional in the modulation of NADP⁺-malate dehydrogenase activity. In spite of the absence of immunological cross-reactions of pea and spinach Td f proteins with the corresponding antibodies, pea Td f activated not only the homologous FBPase, but also the spinach enzyme. The saturation curves for pea FBPase, either with fructose-1,6-bisphosphate in the presence of different concentrations of homologous Td f, or with pea Td f in the presence of excess substrate, showed sigmoid kinetics; this can be explained on the basis of a random distribution of fructose-1,6-bisphosphate, and of the oxidized and reduced forms of the activator, among the four Td f- and substrate-binding sites of this tetrameric enzyme. From the saturation curves of pea and spinach Td f proteins against pea FBPase, a 4:1 stoichiometry was determined for the Td f-enzyme binding. This is in contrast to the 2:1 stoichiometry found for the spinach FBPase. The UV spectrum of pea Td f had a maximum at 277 nm, which shifted to 281 nm after reduction with dithiothreitol (s at 280 nm for 15.8-kDa M_r = 6324 M^–1 · cm^–1). The fluorescence emission spectrum after 280-nm excitation had a maximum at 334 nm, related to tyrosine residues; after denaturation with guanidine isothiocyanate an additional maximum appeared at 350 nm, which is concerned with tryptophan groups. Neither the native nor the denatured form showed a significant increase in fluorescence after reduction by dithiothreitol, which means that the tyrosine and tryptophan groups in the reduced Td f are similarly exposed. Pea Td f appears to have one cysteine residue more than the three cysteines earlier described for spinach and Scenedesmus Td f proteins.Abbreviations DDT dithiothreitol - ELISA enzyme-linked immunosorbent assay - FBPase fructose- 1,6-bisphosphatase - kDa kilodalton - M_r relative molecular mass - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - Td thioredoxin The authors are grateful to Mrs. Francisca Castro and Mr. Narciso Algaba for skilful technical assistance. This work was supported by grant PB87-0431 of Dirección General de Investigación Cientifica y Técnica (DGICYT, Spain).     

Regulation of fructose-1,6-bisphosphatase activity in leaves

Fructose-1,6-bisphosphatase (EC 3.1.3.11) activity increased markedly (greater than 10-fold) upon illumination of wheat leaves. Darkening caused a relatively slow but complete reversal of light activation. The effects of O₂ and CO₂ concentration and light intensity on fructose-bisphosphatase activation were measured. In ratelimiting light, 2% O₂ stimulated enzyme activity, whereas varying the CO₂ concentration had little effect. In saturating light, lowering the oxygen tension had no effect, but CO₂ at near-saturating concentrations for photosynthesis inhibited enzyme activity. Dark inactivation of the enzyme was completely prevented by incubation of leaves in N₂, but was facilitated by O₂, indicating that O₂ is the major oxidant in darkened leaves. It is argued that while fructose bisphosphatase is redox-regulated in leaves, modulation of enzyme activity by this mechanism is unlikely to contribute to the regulation of CO₂ fixation in leaves.     

Bundle-sheath thylakoids from NADP-malic enzyme-type C4 plants require an exogenous electron donor for enzyme light activation

Light activation of either NADP-malate dehydrogenase (EC 1.1.1.82) or fructose-1,6-bisphosphate phosphatase (EC 3.1.3.11) was assayed in a reconstituted chloroplastic, system comprising the isolated proteins of the ferredoxin-thioredoxin light-activation system and thylakoids from either mesophyll or bundle-sheath tissues of different C₄ plants. While C₄-plant thylakoids functionned almost equally well with C₃-or C₄-plant proteins, the photosyntem-II-deficient bundle-sheath thylakoids from the NADP-malic enzyme type, were unable to perform enzyme photoactivation unless supplemented with an electron donor to photosystem I. Bundle-sheath thylakoids isolated from plants showing no photosystem-II deficiency did not require such an addition. The results are discussed with respect to a possible requirement for a physiological reductant of ferredoxin for enzyme light activation in bundle-sheath, tissues.Abbreviations Chl chlorophyll - DCMU 3-(3, 4-dichlorophenyl)-1,1-dimethylurea - DPIP dichlorophenolindophenol - FBPase fructose-1,6-bisphosphatase - FTR ferredoxin-thioredoxin reductase - NADP-MDH NADP-dependent malate dehydrogenase - PSI, II photosystems I, II     

Role of light in the in vivo and in vitro synthesis of spinach thioredoxin f

Upon continuous illumination of dark-grown spinach ( Spinacia oleracea L. cv. Winter Giant) seedlings, the thioredoxin f (Td f) content (ELISA) showed a steep rise, which can be evaluated after 3 and 36 h illumination as 3 times and 10 times the dark value, respectively. These figures correspond to 0.03% and 0.1% of total soluble protein, which means a higher biosynthetic rate for Td f compared to the average of total proteins in the earlier steps of plant development. After 40-50 h light the Td f level reached its highest value which remained stable for an additional 40 h and then decreased. Pulse-chase in vivo experiments with [³⁵S]-methionine also showed this sharp increase of Td f in the dark-light transition. From the pattern of decay of [³⁵S]-labelled Td f, a half-life of 7 h was determined for this chloroplast protein. In vitro translation experiments with poly(A)-mRNA isolated from illuminated young spinach seedlings, coupled to a wheat-germ synthesizing system, showed the appearance of a labelled fraction of ca 19 kDa molecular mass, recognizable by a specific Td f antiserum. When intact spinach chloroplasts were added to the translation assay medium, and then illuminated, the 19 kDa band disappeared, with a parallel increase of an internalized 13 kDa labelled polypeptide, also recognized by the Td f antiserum. These results are good evidence for a nuclear-coded synthesis of a Td f precursor, which travels through the chloroplast envelope, leaving the functional protein inside the organelle after the loss of a 6 kDa transit peptide.     

Beta-D-Galactoside transport in Escherichia coli: Mr determination of the transport protein in organic solvent

Parathyroid hormone stimulates [U-14C]glucose incorporation into glycogen of cultured osteoblast-like calvaria cells. This effect is detectable only several hours after the addition of PTH and it is mimicked by dibutyryl cyclic AMP. In contrast to insulin (in pharmacological concentrations), PTH enhances glycogen formation only in calvaria cells, but not in fibroblasts. Insulin-like growth factor I in physiological concentrations promotes glycogen-synthesis shortly after addition.