The incorporation of glucose into alpha-1,4-glucan proteins of bovine retina membranes

From the seminal discovery of proteoglycogen and glycogenin to emerging knowledge and research on glycogen biology

Although the discovery of glycogen in the liver, attributed to Claude Bernard, happened more than 160 years ago, the mechanism involved in the initiation of glucose polymerization remained unknown. The discovery of glycogenin at the core of glycogen's structure and the initiation of its glucopolymerization is among one of the most exciting and relatively recent findings in Biochemistry. This review focuses on the initial steps leading to the seminal discoveries of proteoglycogen and glycogenin at the beginning of the 1980s, which paved the way for subsequent foundational breakthroughs that propelled forward this new research field. We also explore the current, as well as potential, impact this research field is having on human health and disease from the perspective of glycogen storage diseases. Important new questions arising from recent studies, their links to basic mechanisms involved in the de novo glycogen biogenesis, and the pervading presence of glycogenin across the evolutionary scale, fueled by high throughput -omics technologies, are also addressed.

Two glycogen synthase activities associated with proteoglycogen in retina

Glycogen synthase of bovine retina was found associated with the acid-insoluble and acid-soluble proteoglycogen fractions. The synthase associated with the acid-insoluble proteoglycogen precursor showed an 8-fold lower Km for UDP-glucose than the synthase associated with the acid-soluble fraction, and was inhibited by detergent. A short digestion with pronase resulted in conversion of the acid insoluble fraction into acid-soluble. The results lead us to postulate that the acid-insolubility of the proteoglycogen fraction and the association with retina membrane proposed before, is caused by glycogen synthase strongly associated to its polysaccharide moiety. The enlargement of the polysaccharide moiety during proteoglycogen biosynthesis, from glycogenin linked to a few 11 to 12 glucose units to the acid-insoluble proteoglycogen precursor (Mr 470,000) would be carried out, together with the branching enzyme, by the glycogen synthase showing a low Km for UDP-glucose. The glycogen synthase with the highest Km for UDP-glucose would participate in conversion of the precursor into mature acid-soluble proteoglycogen.

A glycogen precursor associated with membrane in retina

The incorporation of [14C]glucose from UDP-[14C]glucose into proteoglycogen fractions of a retinal microsomal preparation was studied. From the rate of labelling of acid-insoluble and -soluble proteoglycogen at different sugar-donor concentrations, and from the conversion of the labelled acid-insoluble into an acid-soluble form measured by a 'chase' with unlabelled UDP-glucose, it was concluded that acid-insoluble 42 kDa protein (p42)-bound glycogen of weight-average Mr 4.7 x 10(5) and acid-soluble p42-bound glycogen of weight-average Mr 7.0 x 10(5) [Miozzo, Lacoste & Curtino (1989) Biochem. J. 260, 287-289] are related as precursor and product respectively. About one-third of the acid-insoluble proteoglycogen was excluded from a Sephacryl S-500 column and was associated with large membrane vesicles. Proteoglycogen was not dissociated from the membranes by treatment with saline solutions or with SDS at a low detergent-to-protein ratio. It was dissociated by treatment with detergents under conditions which were shown to solubilize integral membrane sialoglycoconjugates of retina. These results lead us to postulate that the biogenesis of retina glycogen starts on membrane-associated p42 to form acid-insoluble proteoglycogen, which is then dissociated from membranes and converted into acid-soluble proteoglycogen by the 'growth' of its polysaccharide moiety.

Characterization of the proteoglycogen fraction non-extractable from retina by trichloroacetic acid

The trichloroacetic acid-insoluble 1,4-alpha-glucan fraction from bovine retina was purified and characterized. It is a proteoglycogen fraction containing a 42 kDa protein moiety similar in size to the protein moiety of the trichloroacetic acid-soluble proteoglycogen fraction. The apparent weight-average Mr of acid-insoluble and acid-soluble proteoglycogens are 4.7 x 10(5) and 7.0 x 10(5) respectively. The present results support suggestions from earlier studies indicating that acid-insoluble proteoglycogen is the precursor of the acid-soluble form.

A (1----4)-alpha-D-glucan-protein involved in liver glycogen biosynthesis

A macromolecular (1----4)-alpha-D-[14C]glucan-protein complex was synthesized with a rat liver preparation and uridine diphosphate D-[14C]glucose. The size of the complex is contributed by both the protein and the (1----4)-alpha-D-glucosyl-oligomer components. Iodoacetamide treatment did not change the migration properties on Bio-Gel A-50m. Therefore, disulfide bonds linking glucan-protein subunits seem not to be involved. The [14C]glucan-protein, precipitated by diluted trichloroacetic acid, was digested by alpha-amylase, phosphorylase a, and proteases. The extent of proteolysis was greater for a complex having fewer D-glucose units incorporated. After proteolytic digestion of that complex, the labeled fragments behaved on electrophoresis, and ion-exchange and gel chromatography as [14C]glucosylated peptides. These findings support previous conclusions that the primer for liver glycogen synthesis is a protein on which glycogen is built up by covalent attachment.

Evidence for the glycoprotein nature of retina glycogen

Incubation of a bovine retina membrane preparation with micromolar amounts of UDP-[14C]glucose resulted in the incorporation of [14C]glucose into endogenous (1----4)-alpha-glucan, insoluble in trichloroacetic acid, and acid-soluble ethanol-insoluble glycogen. The trichloroacetic-acid-insoluble glucan fraction of retina migrated in 2.6-3% acrylamide gels when subjected to sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) and was rendered acid-soluble by digestion with pronase. The solubility of the acid-insoluble glucan in acidified organic solvent was different from that of amylose or glycogen and similar to membrane proteins and glycoproteins. The glycogen fraction of retina contained 1.5-2.0 micrograms protein/100 micrograms glucose. When this fraction was analyzed by SDS-PAGE only one band, which moved near the top of 3% acrylamide gels, was stained with periodic acid Schiff reagent and Coomassie blue. The protein nature of the Coomassie-blue-stainable material was demonstrated by iodination of the glycogen fraction with [131I]iodide and identification of labeled monoiodotyrosine and diiodotyrosine. The bulk of the label comigrated with carbohydrate near the top of gels in SDS-PAGE and treatment with alpha- amylse decreased the molecular size of both labeled and stainable material. Physical dissociative conditions (7.5 M urea/0.83% SDS/0.83% mercaptoethanol) and the following chemical treatments failed to dissociate the iodinated protein from glycogen: (a) 0.1 M NaOH/0.1 M NaBH4 at room temperature for 24 h; (b) 1 M HCl in methanol at 50 degrees C for 10 min; (c) trifluoroacetic acid at 50 degrees C for 6 min. 131I-labeled glycogenpeptide was isolated after 131I-labeled protein-bound glycogen had been subjected to digestion with papain/pronase and passed through a Sepharose column. The results suggest that at least part of glycogen in bovine retina is firmly combined to protein as a single proteoglycogen molecule. Furthermore some of the proteoglycogen might be present as a trichloroacetic-acid-precipitable proteoglucan owing to its lower glucose content.