Asparagine-linked glycoprotein biosynthesis in rat brain: identification of glucosidase I, glucosidase II, and and endomannosidase (glucosyl mannosidase)

Expression of α-subunit of α-glucosidase II in adult mouse brain regions and selected organs

α-Glucosidase II (GII), a resident of endoplasmic reticulum (ER) and an important enzyme in the folding of nascent glycoproteins, is heterodimeric, consisting of α (GIIα) and β (GIIβ) subunits. The catalytic GIIα subunit, with the help of mannose 6-phosphate receptor homology domain of GIIβ, sequentially hydrolyzes two α1-3-linked glucose residues in the second step of N-linked oligosaccharide-mediated protein folding. The soluble GIIα subunit is retained in the ER through its interaction with the HDEL-containing GIIβ subunit. N-glycosylation and correct protein folding are crucial for protein stability and trafficking and cell surface expression of several proteins in the brain. Alterations in N-glycosylation lead to abnormalities in neuronal migration and mental retardation, various neurodegenerative diseases, and invasion of malignant gliomas. Inhibitors of GII are used to inhibit cell proliferation and migration in a variety of different pathologies, such as viral infection, cancer, and diabetes. Despite the widespread use of GIIα inhibitory drugs and the role of GIIα in brain function, little is known about its expression in brain and other tissues. Here, we report generation of a highly specific chicken antibody to the GIIα subunit and its characterization by Western blotting and immunoprecipitation using cerebral cortical extracts. By using this antibody, we showed that the GIIα protein is highly expressed in testis, kidney, and lung, with the lowest amount in heart. GIIα polypeptide levels in whole brain were comparable to those in spleen. However, a higher expression of GIIα protein was detected in the cerebral cortex, reflecting its continuous requirement in correct folding of cell surface proteins.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

Purification and enzymatic properties of endo-alpha 1, 2-mannosidase from pig liver involved in oligosaccharide processing

An endo-alpha 1,2-mannosidase, which is involved in N-linked oligosaccharide processing, has been purified to homogeneity from crude pig liver microsomes using conventional techniques. Two catalytically active polypeptides, of 48 kDa, have been isolated which degrade [14C]Glc3-1-Man9,-GlcNAc2 to [14C]Glc3-1-Man and a specific Man8-GlcNAc2 isomer. They are not, however, active on synthetic alpha-mannosides. [14C]Glc1-Man9-GlcNAc2 was found to be approximately sevenfold more rapidly hydrolyzed than the [14C]Glc2- and [14C]Glc3-homologues. The 48 kDa and 50 kDa proteins are not N-glycosylated and ran on Superdex 75 as monomers. Kinetic studies showed that these proteins had similar catalytic properties: (i) the pH optima were found to be close to 6.5; (ii) neither activity was metal ion dependent; (iii) hydrolysis of [14C]Glc3-Man9-GlcNAc2 was inhibited strongly by Glc-alpha 1,3-Man (app. Ki approximately 120 microM), but not by 1-deoxymannojirimycin or swainsonine. Other evidence, including immunological data, strongly suggests that the 48 kDa and 50 kDa polypeptides are proteolytic degradation products of a single endo-alpha 1,2-mannosidase, rather than distinct subunits of an oligomeric complex. Possible functions of the endo-alpha 1,2-mannosidase in N-linked oligosaccharide processing are discussed.

Rat epididymal luminal fluid acid beta-D-galactosidase optimally hydrolyses glycoprotein substrate at neutral pH

Several glycosidases, purified and characterized from mammalian tissues, have been shown to be optimally active under acidic conditions when p-nitrophenyl (PNP) or 4-methylumbelliferyl glycosides are used as substrates. Although high levels of the glycosidases are present in the epididymal lumen, their physiological role remains uncertain. To be functional, the glycosidases are expected to be enzymatically active at or near the physiological pH of luminal fluid. In this report, we demonstrate that the rat epididymal luminal fluid beta-D-galactosidase, optimally active toward PNP beta-D-galactoside at pH 3.5, shows maximum activity towards a glycoprotein substrate ([Gal-3H]fetuin) at neutral pH. Several lines of evidence, including immunoprecipitation studies using antibody to the acid beta-D-galactosidase, and substrate competition studies, indicate that PNP galactosidase and [3H]Gal galactosidase activities are caused by a single enzyme, and that the two substrates are probably cleaved by the same catalytic site(s). Competition studies with various disaccharides indicate that this enzyme is capable of cleaving a variety of galactose linkages found in both O- and N-linked oligosaccharides. Molecular-sieve column chromatography of the beta-D-galactosidase of luminal fluid under several conditions of buffer and pH show that, whereas the enzyme eluted as a tetramer (apparent M(r) 320,000) under acidic conditions (pH 3.5-4.3), only dimers and monomers (apparent M(r) 180,000 and 92,000 respectively) were observed in neutral conditions (pH 6.8). This aggregation/dissociation phenomenon is reversible. These studies indicate that beta-D-galactosidase is present in the luminal fluid in dissociated forms, and is therefore optimally active towards glycoprotein substrates at physiological pH. The potential role of the enzyme in modification of sperm surface glycoproteins is discussed.

Asparagine-linked glycoprotein biosynthesis in rat epididymis. Presence of a mannosidase II-like enzyme

Previous studies from this laboratory using p-nitrophenyl alpha-D-mannoside (p-NPM) as substrate provided no evidence for the presence of mannosidase II in the rat epididymis [Skudlarek & Orgebin-Crist (1988) J. Reprod. Fertil. 84, 611-617]. However, rat epididymal epithelial cells cultured in the presence of swainsonine, an inhibitor of mannosidase II, produce abnormally processed N-linked glycoproteins containing hybrid-type oligosaccharides instead of complex-type [Tulsiani, Skudlarek & Orgebin-Crist (1990) Biol. Reprod. 43, 130-138], a result providing indirect evidence for the presence of mannosidase II-like enzyme in rat epididymis. In the studies described here, we present evidence for the occurrence of this processing enzyme in rat epididymal Golgi membranes. This enzyme is an integral Golgi membrane component. Like liver mannosidase II, the epididymal enzyme cleaves alpha 1,3- and alpha 1,6-linked mannosyl residues from GlcNAcMan5GlcNAc. However, unlike liver mannosidase II, the epididymal enzyme shows no activity towards the synthetic substrate, p-NPM. The epididymal mannosidase cross-reacts with liver anti-(mannosidase II) antibody, a result suggesting that the two enzymes share a common antigenic site(s). Immunoblotting studies following resolution of liver and epididymal Golgi membranes on SDS/PAGE show that, whereas the liver mannosidase II was resolved as a doublet of Mr 120,000 and 122,000, only the Mr 120,000 band was observed in the epididymal Golgi membranes. Immunoblotting of the Golgi-rich fractions, resolved under non-denaturing conditions, showed different patterns of charge and/or size isomers from the two tissues. These studies demonstrate tissue-specific differences in processing enzymes with similar function.