Endoplasmic reticulum kifunensine-resistant alpha-mannosidase is enzymatically and immunologically related to the cytosolic alpha-mannosidase

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.

Demonstration of a peptide:N-glycosidase in the endoplasmic reticulum of rat liver

Prompted by previous observations that polymannose oligosaccharides are released from newly synthesized glycoproteins [Anumula and Spiro (1983) J. Biol. Chem. 258, 15274-15282], we examined rat liver endoplasmic reticulum (ER) for the presence of endoglycosidases that could be involved in an event presumed to be a function of the protein quality control machinery. Our investigations indicated that a peptide:N-glycanase (PNGase) is present in ER membranes that has the capacity to release from radiolabelled glycopeptides glucosylated as well as non-glucosylated polymannose oligosaccharides terminating at their reducing end in a di-N-acetylchitobiose sequence (OS-GlcNAc2). This enzyme, which was found to be luminal in orientation, was most active in the pH range 5.5-7.0 and although it had no exogenous bivalent-cation requirements it was inhibited by EDTA. Detailed studies with Man9GlcNAc2-peptides demonstrated that in addition to the free oligosaccharide (Man9GlcNAc2) an additional neutral product characterized as Man9GlcNAc2 linked to an as yet unidentified aglycone was released in a manner that suggests its role as an intermediate. Our observation that ER, in contrast with cytosol, had no endo-beta-N-acetylglucosaminidase activity would indicate that oligosaccharides terminating in a single GlcNAc residue (OS-GlcNAc1), which have been noted to appear in the extravesicular compartment shortly after N-glycosylation [Moore and Spiro (1994) J. Biol. Chem. 269, 12715-12721] are released from the protein as OS-GlcNAc2 and undergo an ER-to-cytosol translocation in that form before undergoing cleavage of their chitobiose core.

Transfer of free polymannose-type oligosaccharides from the cytosol to lysosomes in cultured human hepatocellular carcinoma HepG2 cells

Large, free polymannose oligosaccharides generated during glycoprotein biosynthesis rapidly appear in the cytosol of HepG2 cells where they undergo processing by a cytosolic endo H-like enzyme and a mannosidase to yield the linear isomer of Man5GlcNAc (Man[alpha 1-2]Man[alpha 1-2]Man[alpha 1-3][Man alpha 1-6]Man[beta 1-4] GlcNAc). Here we have examined the fate of these partially trimmed oligosaccharides in intact HepG2 cells. Subsequent to pulse-chase incubations with D-[2-3H]mannose followed by permeabilization of cells with streptolysin O free oligosaccharides were isolated from the resulting cytosolic and membrane-bound compartments. Control pulse-chase experiments revealed that total cellular free oligosaccharides are lost from HepG2 cells with a half-life of 3-4 h. In contrast use of the vacuolar H+/ATPase inhibitor, concanamycin A, stabilized total cellular free oligosaccharides and enabled us to demonstrate a translocation of partially trimmed oligosaccharides from the cytosol into a membrane-bound compartment. This translocation process was unaffected by inhibitors of autophagy but inhibited if cells were treated with either 100 microM swainsonine, which provokes a cytosolic accumulation of large free oligosaccharides bearing 8-9 residues of mannose, or agents known to reduce cellular ATP levels which lead to the accumulation of the linear isomer of Man5GlcNAc in the cytosol. Subcellular fractionation studies on Percoll density gradients revealed that the cytosol-generated linear isomer of Man5GlcNAc is degraded in a membrane-bound compartment that cosediments with lysosomes.

Definition of the lectin-like properties of the molecular chaperone, calreticulin, and demonstration of its copurification with endomannosidase from rat liver Golgi

Calreticulin was identified by immunochemical and sequence analyses to be the higher molecular mass (60 kDa) component of the polypeptide doublet previously observed in a rat liver Golgi endomannosidase preparation obtained by chromatography on a Glc alpha 1 --> 3Man-containing matrix. The affinity for this saccharide ligand, which paralleled that of endomannosidase and was also observed with purified rat liver calreticulin, suggested that this chaperone has lectin-like binding properties. Studies carried out with immobilized calreticulin and a series of radiolabeled oligosaccharides derived from N-linked carbohydrate units revealed that interactions with this protein were limited to monoglucosylated polymannose components. Although optimal binding occurred with Glc1Man9GlcNAc, substantial interaction with calreticulin was retained after sequential trimming of the polymannose portion down to the Glc1Man5GlcNAc stage. The alpha 1 --> 6-mannose branch point of the oligosaccharide core, however, appeared to be essential for recognition as Glc1Man4GlcNAc did not interact with the calreticulin. The carbohydrate-peptide linkage region had no discernible influence on binding as monoglucosylated oligosaccharides in N-glycosidic linkage interacted with the chaperone to the same extent as in their unconjugated state. The immobilized calreticulin proved to be a highly effective tool for sorting out monoglucosylated polymannose oligosaccharides or glycopeptides from complex mixtures of processing intermediates. The copurification of calreticulin and endomannosidase from a Golgi fraction in comparable amounts and the strikingly similar saccharide specificities of the chaperone and the processing enzyme have suggested a tentative model for the dissociation through glucose removal of calreticulin-glycoprotein complexes in a post-endoplasmic reticulum locale; in this scheme, deglucosylation would be brought about by the action of endomannosidase rather than glucosidase II.