Role of uridine diphosphate glucose in the biosynthesis of starch. Mechanism of formation and enlargement of a glucoproteic acceptor

Oligomerization of the reversibly glycosylated polypeptide: its role during rice plant development and in the regulation of self-glycosylation

A multigenic family of self-glycosylating proteins named reversibly glycosylated polypeptides, designated as RGPs, have been usually associated with carbohydrate metabolism, although they are an enigma both at the functional, as well as at the structural level. In this work, we used biochemical approaches to demonstrate that complex formation is linked to rice plant development, in which class 1 Oryza sativa RGP (OsRGP) would be involved in an early stage of growing plants, while class 2 OsRGP would be associated with a late stage linked to an active polysaccharide synthesis that occurs during the elongation of plant. Here, a further investigation of the complex formation of the Solanum tuberosum RGP (StRGP) was performed. Results showed that disulfide bonds are at least partially responsible for maintaining the oligomeric protein structure, so that the nonreduced StRGP protein showed an apparent higher molecular weight and a lower radioglycosylation of the monomer with respect to its reduced form. Hydrophobic cluster analysis and secondary structure prediction revealed that class 2 RGPs no longer maintained the Rossman fold described for class 1 RGP. A 3D structure of the StRGP protein resolved by homology modeling supports the possibility of intercatenary disulfide bridges formed by exposed cysteines residues C79, C303 and C251 and they are most probably involved in complex formation occurring into the cell cytoplasm.

Mutation of the plastidial alpha-glucan phosphorylase gene in rice affects the synthesis and structure of starch in the endosperm

Plastidial phosphorylase (Pho1) accounts for approximately 96% of the total phosphorylase activity in developing rice (Oryza sativa) seeds. From mutant stocks induced by N-methyl-N-nitrosourea treatment, we identified plants with mutations in the Pho1 gene that are deficient in Pho1. Strikingly, the size of mature seeds and the starch content in these mutants showed considerable variation, ranging from shrunken to pseudonormal. The loss of Pho1 caused smaller starch granules to accumulate and modified the amylopectin structure. Variation in the morphological and biochemical phenotype of individual seeds was common to all 15 pho1-independent homozygous mutant lines studied, indicating that this phenotype was caused solely by the genetic defect. The phenotype of the pho1 mutation was temperature dependent. While the mutant plants grown at 30 degrees C produced mainly plump seeds at maturity, most of the seeds from plants grown at 20 degrees C were shrunken, with a significant proportion showing severe reduction in starch accumulation. These results strongly suggest that Pho1 plays a crucial role in starch biosynthesis in rice endosperm at low temperatures and that one or more other factors can complement the function of Pho1 at high temperatures.

Regulation of self-glycosylation of reversibly glycosylated polypeptides from Solanum tuberosum

Reversibly glycosylated polypeptides (RGPs) belong to a family of self-glycosylating proteins believed to be involved in plant polysaccharide synthesis. The precise function of these enzymes remains to be elucidated. Our results showed that the RGP 38-kDa subunit is phosphorylated in potato extracts (Solanum tuberosum L.). An increase in the self-glycosylation of Solanum tuberosum RGP (StRGP) 38-kDa subunit was observed after alkaline phosphatase (AP) treatment. Our results suggest that phosphorylation of StRGP appears to regulate its self-glycosylation. It was determined that when the StRGP reaction was carried out in the presence of UDP-[(14)C]Glc as the sugar donor and then 1 mM UDP was added in a chase-out experiment, radioactive UDP-Glc was obtained indicating that StRGP reaction seems to be reversible. The anomeric configuration of transferred sugars to StRGP protein was also studied.

Starch biosynthesis: mechanism for the elongation of starch chains

Starch granules from eight diverse plant sources all had active starch synthases and branching enzymes inside the granules. The enzymes synthesized both amylose and amylopectin from ADPGlc. Pulsing of the granules with ADP-[14C]Glc gave synthesis of starch that on reduction and glucoamylase hydrolysis gave 14C-labeled D-glucitol. The pulsed label could be chased by nonlabeled ADPGlc to give a significant decrease of 14C-label in D-glucitol. Evidence further indicated that the synthase forms a high-energy covalent complex with D-glucose and the growing starch chain, and that the D-glucopyranosyl group is added to the reducing end of the growing starch chain by a two-site insertion mechanism.

Sequence of the structural gene for granule-bound starch synthase of potato (Solanum tuberosum L.) and evidence for a single point deletion in the amf allele

The genomic sequence of the potato gene for starch granule-bound starch synthase (GBSS; "waxy protein") has been determined for the wild-type allele of a monoploid genotype from which an amylose-free (amf) mutant was derived, and for the mutant part of the amf allele. Comparison of the wild-type sequence with a cDNA sequence from the literature and a newly isolated cDNA revealed the presence of 13 introns, the first of which is located in the untranslated leader. The promoter contains a G-box-like sequence. The deduced amino acid sequence of the precursor of GBSS shows a high degree of identity with monocot waxy protein sequences in the region corresponding to the mature form of the enzyme. The transit peptide of 77 amino acids, required for routing of the precursor to the plastids, shows much less identity with the transit peptides of the other waxy preproteins, but resembles the hydropathic distributions of these peptides. Alignment of the amino acid sequences of the four mature starch synthases with the Escherichia coli glgA gene product revealed the presence of at least three conserved boxes; there is no homology with previously proposed starch-binding domains of other enzymes involved in starch metabolism. We report the use of chimeric constructs with wild-type and amf sequences to localize, via complementation experiments, the region of the amf allele in which the mutation resides. Direct sequencing of polymerase chain reaction products confirmed that the amf mutation is a deletion of a single AT basepair in the region coding for the transit peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

A 42,000-Da protein in rabbit tissues and in a glycogen synthase preparation cross-reacts with antibodies to glycogenin

Rabbit skeletal muscle glycogen previously has been shown to be covalently bound to a 40,000-Da protein ("glycogenin") via a novel glucosyl-tyrosine linkage [I.R. Rodriguez and W.J. Whelan (1985) Biochem. Biophys. Res. Commun. 132, 829-836]. Antibodies raised against rabbit skeletal muscle glycogenin cross-react with a similar protein present in rabbit heart and liver glycogens, as well as with a 42,000-Da "acceptor protein" present in high-speed supernatants of rabbit muscle, heart, retina, and liver. This 42,000-Da protein incorporates [U-14C]Glc when an ammonium sulfate fraction prepared from the tissue supernatants is incubated with UDP-[U-14C]Glc. The [U-14C]Glc incorporated can be removed quantitatively by treatment with amylolytic enzymes, indicating that the [U-14C]Glc incorporation represents elongation of a preexisting glucan attached to the acceptor protein. Furthermore, a commercial preparation of rabbit skeletal muscle glycogen synthase contains this 42,000-Da protein. We propose that the 42,000-Da protein represents the free form of glycogenin in tissues, with its covalently attached glucan chain(s) providing a "primed" elongation site for glycogen synthesis.

Synthesis of beta-glucans in Prototheca zopfii. Isolation and characterization of the glycoprotein primer

When Prototheca zopfii cells were pulse-labeled with 14C-containing amino acids and homogenized, 14C-labeled membranes were obtained. In vitro incubations with the previously labeled membranes and UDP-[3H]Glc produced a trichloroacetic-acid-insoluble fraction having both isotopes. A double-labeled glucoprotein was isolated and characterized. It has a relative molecular mass of 28,000-30,000 and a carbohydrate content of 10%. The oligosaccharide chain is linked to the protein through an O-glycosidic bond between hydroxyproline and glucose. The oligosaccharide has a polymerization degree ranging over 10-20 hexose units. Glucose is the only monosaccharide found; most of the glucose residues are beta-1,4-linked (90%) but some are beta-1,3-linked (10%).

Synthesis of beta-glucans in Prototheca zopfii. Evidence for the existence of a glycoprotein primer

Membrane preparations from the non-photosynthetic alga Prototheca zopfii incorporate glucose from UDP-[3H]glucose into the trichloroacetic-acid-insoluble fraction and the polysaccharides insoluble in hot alkali. Time course and pulse-chase experiments indicate that the acid-insoluble fraction was a precursor of the alkali-insoluble fraction. Isolation of 3H-labeled membrane or soluble fraction showed that only membrane fractions were able to transfer radioactivity into polysaccharides. Treatment of glucosylated membranes with trypsin or cellulase only partially affect their transfer ability, indicating that the precursor was internalized in vesicles. Analysis of the in vitro synthesized polysaccharides by enzymatic and acid hydrolysis showed that glucose and cellobiose were present as radioactive sugars. Permethylation of the polysaccharide indicates that 80% of the glucose was beta-1,4-bonded with 20% in beta-1,3-linkages. This polysaccharide was found to be identical with the cell-wall beta-glucan obtained in vivo [Rivas, L.A. & Pont Lezica, R. (1978) Planta (Berl.) 165, 348-353].

Alpha-glucan synthesis on a protein primer. A reconstituted system for the formation of protein-bound alpha-glucan

Reconstitution experiments with the DEAE-cellulose-treated enzymes, engaged in a two-step mechanism of synthesis of alpha-glucan bound to protein, are performed. Urea/sodium dodecyl sulfate/polyacrylamide gel electrophoretic analysis of the radioactive products synthesized by the reconstituted system shows highly glucosylated, labeled bands, whose apparent molecular masses change with the acrylamide concentration in the gels. The long carbohydrate chains synthesized during the second step arise from the sequential addition of glucosyl moieties to the glucoprotein formed during the first step. A deglucosylation experiment confirms that the product of the reconstituted system originates from the 38-kDa glucosylated component of the reaction 1 product by the addition of beta-amylase-sensitive glucosyl moieties. Our data suggest that specific phosphorylases and starch synthetases are found in potato tuber, which are capable of utilizing reaction 1 product as primer for the synthesis of protein-bound glucan.

alpha-Glucan synthesis on a protein primer, uridine diphosphoglucose: protein transglucosylase I. Separation from starch synthetase and phosphorylase and a study of its properties

It was found that the DEAE-cellulose-treated UDP-Glc:protein transglucosylase I catalyzing the first step (reaction 1) in the formation of alpha-glucan bound to protein in potato tuber is not only specific for the glucosyl donor but also for the endogenous acceptor. A single radioactive 38-kDa macromolecular component appeared during denaturing polyacrylamide gel electrophoresis of reaction 1 product. The labeled component is probably the polypeptide subunit of the endogenous acceptor which is being glucosylated. The radioactivity incorporated in reaction 1 product was isolated from a protease digest as a low-molecular-mass glucopeptide fraction. A beta-elimination reaction carried out in the presence of a reducing agent demonstrated that only one glucosyl moiety is transferred from UDP-Glc to the aminoacyl residue, thus forming an O-glucosidic linkage. 3H-labeled sodium borohydride showed that serine and threonine are involved in the peptide bond to glucose. Ion-exchange chromatography on DEAE-cellulose, affinity chromatography on concanavalin-A--Sepharose, gel filtration on Sephacryl S-300 and sucrose density gradient centrifugation failed to separate the enzyme catalyzing reaction 1 from the endogenous acceptor.

Intermediatry steps in Acetobacter xylinum cellulose synthesis: studies with whole cells and cell-free preparations of the wild type and a celluloseless mutant

Intermediatry steps in cellulose synthesis in Acetobacter xylinum were studied with resting cells and particulate-membranous preparations of the wild-type strain and of a celluloseless mutant. Exogenously supplied [1-14C]glucose was rapidly converted by resting cells of both types into glucose 6-phosphate, glucose 1-phosphate, and uridine glucose 5'-diphosphate (UDP)-glucose and incorporated into lipid-, water-, and alkali-soluble cellular fractions. The decrease in the level of labeled hexose-phosphates and UDP-glucose upon depletion of the exogenous substrate was accounted for by a continuous incorporation of [14C]glucose into cellulose in the wild type and into the above-mentioned cellular components in the mutant. [14C]glucose retained in the alkali- and water-soluble fractions of pulse-labeled wild-type cells was quantitatively chased into cellulose. Sonic extracts of both strains catalyzed the transfer of glucose from UDP-glucose into lipid-, water-, and alkali-soluble materials, as well as into an alkali-insoluble cellulosic beta-1,4-glucan. The results strongly support the sequence glucose leads to glucose 6-phosphate leads to glucose 1-phosphate leads to UDP-glucose leads to cellulose and indicate that lipid- and protein-linked cellodextrins may function as intermediates between UDP-glucose and cellulose in A. xylinum.

Streptococcal extracellular NAD+ nucleosidase. Characterization of changes occurring during purification

Purification of streptococcal extracellular NAD+ nucleosidase is associated with changes of kinetic properties. A high-molecular weight component is required for a full activity of the enzyme. The component is not produced by bacteria and is present in the Todd-Hewitt cultivation medium, the beef-heart extract serving primarily as its source. In its effect on the enzyme this component can be replaced with bovine albumin. It follows from the time relationship of the NAD decomposition that the mechanism of the studied enzymic reaction is not hydrolytic. In addition to nicotinamide, a product of polymeric character is formed.

A two-step enzymatic formation of a glucoprotein in potato tuber

Evidence is presented on the occurrence of a two-step mechanism for the synthesis of protein bound glucan in a potato tuber particulate preparation. Experiments carried out with a differently labeled sugar donor for each step enabled the isolation of a double-labeled glucoprotein. Smith periodate degradation of the product confirmed this hypothesis.

Initiation of glycogen biosynthesis in Escherichia coli. Studies of the properties of the enzymes involved

The properties of the enzymes involved in the initiation of glycogen biosynthesis in Escherichia coli were studied. It was found that the enzymic activities which transfer the glycosyl residues from UDPglucose or ADPglucose for the glucoprotein synthesis had differing stabilities upon storage at 4 degrees C. The small amount of glycogen and the saccharide firmly bound to the membrane preparation, were degraded during the storage period. The activity measured in fresh and in stored preparations gave different time dependence curves. The stored preparation had a lag period which could be due to the transfer of the first glucose units to the protein. Both UDPglucose and ADPglucose : protein glucosyltransferases were affected in different ways by detergents. Based on the results presented, it may be concluded that both enzymatic activities are due to different enzymes. Furthermore, both enzymatic activities are different from that which transfers glucose from ADPglucose to glycogen. The following mechanism for the de novo synthesis is suggested. Glycogen in E. coli could be initiated by two different enzymes which transfer glucose to a protein acceptor either from UDPglucose or ADPglucose. Once the saccharide linked to the protein has reached a certain size it is almost exclusively enlarged by another ADPglucose-dependent enzyme. The participation of branching enzyme will produce a polysaccharide with the characteristics of glycogen.

Synthesis of cellulose precursors. The involvement of lipid-linked sugars

Particulate preparations from the Chlorophyta Prototheca zopfi catalyze the incorporation of [14C]glucose from UDP-[14C]glucoe into lipids. These lipids have been characterized as lipid-P-glucose, lipid-PP-glucose, and lipid-PP-oligosaccharides. The lipid-linked oligosaccharides were a mixture ranging from a disaccharide to approximately a decasaccharide. Cellulase digestion and periodate oxidation showed that the oligosaccharides seem to be built of beta-1,4-linked glucoses. The lipid moiety had the properties of dolichol. The glucolipids described appeared as precursors of a water-soluble polymer. Treatments of this polymer with hydrolytic enzymes and periodate oxidation that it could be a glycoprotein containing beta-1,4-linked glucoses. When GDP-glucose was added to the incubation mixture, the 14C-labelled soluble polymer became insoluble in hot alkali. This insoluble polymer had the properties expected for cellulose. A scheme is proposed with the reactions involved in the initiation of cellulose biosynthesis.

Glucosylation of membrane-bound proteins by lipid-linked glucose

Particulate preparations from Pisum sativum. were able to incorporate [(14)C]glucose from UDP-[(14)C]glucose into oligosaccharide-linked lipids was formed by an oligosaccharide chain containing 7-8 glucose residues linked to dolichol, presumably via a pyrophosphate. The polymer was identified as a membrane-bound glucoprotein that could be solubilized by Triton X-100. SDS gel electrophoresis showed that a polypeptide with an apparent molecular weight of 13,000 could be glucosylated from dolichyl-phosphate-glucose. This was coincident with the electrophoretic mobility of the β subunit of the pea lectin in the same system. The glucosylated protein was solubilized from the membranes by sonication and showed the same carbohydrate-binding ability as pea lectins. These results strongly suggest that pea lectins can be glucosylated by the lipid intermediate pathway.

A primer independent activity of rabbit muscle phosphorylase b

Rabbit muscle phosphorylase b was found to be capable of forming protein bound alpha-1,4 glucosyl chains upon incubation of the enzyme with appropriate concentrations of glucose-1-phosphate with no primer addition (unprimed synthesis). This activity would only be present in a small fraction of the total muscle phosphorylase b activity, as judged from the high concentrations of enzyme which are required to demonstrate the occurrence of unprimed synthesis. Polyacrylamide gel electrophoresis shows the presence of a phosphorylase isoenzyme capable of accepting glucosyl moieties, giving rise to a glucosylated protein enzymatically active in the chain lengthening of its own glucan.

A protein-bound glycogen component of rat liver

It has long been claimed, but frequently disputed, that part of the glycogen in rat liver is insoluble in 10% trichloroacetic acid, and a physiological significance was ascribed to the existence of the two pools of glycogen, desmo-glycogen, the insoluble form, and lyo-glycogen, the soluble component. Desmo-glycogen was thought to owe its acid insolubility to a covalent binding to protein. Recent claims that glycogen, similarly insoluble in acid, can be synthesized in vitro have renewed the interest in desmo-glycogen. We have obtained trichloroacetic acid-insoluble glycogen from rat liver and find that, despite subjecting the glycogen to proteolysis, peptide material remains in close association with the glycogen through a number of purification procedures and is freed from glycogen only by enzymic decomposition of the latter. The tenacity with which the glycogen and peptide material remain in association with each other is suggestive of the occurrence of protein-bound glycogen.