alpha-D-Mannosidases of rat liver Golgi membranes. Mannosidase II is the GlcNAcMAN5-cleaving enzyme in glycoprotein biosynthesis and mannosidases Ia and IB are the enzymes converting Man9 precursors to Man5 intermediates

Transcription of biochemical defenses by the harmful brown tide pelagophyte, Aureococcus anophagefferens, in response to the protozoan grazer, Oxyrrhis marina

Aureococcus anophagefferens is a small marine pelagophyte that forms recurrent harmful brown tides blooms with adverse ecological and economic impacts. During blooms, A. anophagefferens experiences lower zooplankton grazing mortality than other phytoplankton potentially due to the synthesis of anti-predator compounds including extracellular polysaccharides. This study characterized the transcriptomic response of A. anophagefferens when exposed to the protozooplankton, Oxyrrhis marina, and assessed whether this response involved chemical cues. Transcriptomes were generated from A. anophagefferens populations grown at high (1×106 cells mL-1) and low (5×105 cells mL-1) cell densities incubated directly with O. marina or receiving only filtrate from co-cultures of A. anophagefferens and O. marina to evaluate the role of chemical cues. There were a greater number of genes differentially expressed in response to grazing in the lower concentration of A. anophagefferens compared to the high concentration treatment and in response to direct grazing compared to filtrate. KEGG pathway analysis revealed that direct grazer exposure led to a significant increase in transcripts of genes encoding secondary metabolite production (p < 0.001). There was broad transcriptional evidence indicating the induction of biosynthetic pathways for polyketides and sterols in response to zooplankton grazers, compounds associated with damage to marine organisms. In addition, exposure to O. marina elicited changes in the abundance of transcripts associated with carbohydrate metabolism that could support the formation of an extracellular polysaccharide matrix including genes related to glycoprotein synthesis and carbohydrate transport. Collectively, these findings support the hypothesis that A. anophagefferens can induce biochemical pathways that reduce grazing mortality and support blooms.

Evolution and phylogenetic distribution of endo-α-mannosidase

While glycans underlie many biological processes, such as protein folding, cell adhesion, and cell-cell recognition, deep evolution of glycosylation machinery remains an understudied topic. N-linked glycosylation is a conserved process in which mannosidases are key trimming enzymes. One of them is the glycoprotein endo-α-1,2-mannosidase which participates in the initial trimming of mannose moieties from an N-linked glycan inside the cis-Golgi. It is unique as the only endo-acting mannosidase found in this organelle. Relatively little is known about its origins and evolutionary history; so far it was reported to occur only in vertebrates. In this work, a taxon-rich bioinformatic survey to unravel the evolutionary history of this enzyme, including all major eukaryotic clades and a wide representation of animals, is presented. The endomannosidase was found to be more widely distributed in animals and other eukaryotes. The protein motif changes in context of the canonical animal enzyme were tracked. Additionally, the data show the two canonical vertebrate endomannosidase genes, MANEA and MANEAL, arose at the second round of the two vertebrate genome duplications and one more vertebrate paralog, CMANEAL, is uncovered. Finally, a framework where N-glycosylation co-evolved with complex multicellularity is described. A better understanding of the evolution of core glycosylation pathways is pivotal to understanding biology of eukaryotes in general, and the Golgi apparatus in particular. This systematic analysis of the endomannosidase evolution is one step toward this goal.

N-linked glycans: an underappreciated key determinant of T cell development, activation, and function

N-linked glycosylation is a post-translational modification that results in the decoration of newly synthesized proteins with diverse types of oligosaccharides that originate from the amide group of the amino acid asparagine. The sequential and collective action of multiple glycosidases and glycosyltransferases are responsible for determining the overall size, composition, and location of N-linked glycans that become covalently linked to an asparagine during and after protein translation. A growing body of evidence supports the critical role of N-linked glycan synthesis in regulating many features of T cell biology, including thymocyte development and tolerance, as well as T cell activation and differentiation. Here, we provide an overview of how specific glycosidases and glycosyltransferases contribute to the generation of different types of N-linked glycans and how these post-translational modifications ultimately regulate multiple facets of T cell biology.

In Situ Analysis of N-Linked Glycans as Potential Biomarkers of Clinical Course in Human Prostate Cancer

Prostate cancer is the most common cancer in men worldwide. Despite its prevalence, there is a critical knowledge gap in understanding factors driving disparities in survival among different cohorts of patients with prostate cancer. Identifying molecular features separating disparate populations is an important first step in prostate cancer research that could lead to fundamental hypotheses in prostate biology, predictive biomarker discovery, and personalized therapy. N-linked glycosylation is a cotranslational event during protein folding that modulates a myriad of cellular processes. Recently, aberrant N-linked glycosylation has been reported in prostate cancers. However, the full clinical implications of dysregulated glycosylation in prostate cancer has yet to be explored. Herein, we performed direct on-tissue analysis of N-linked glycans using matrix-assisted laser desorption ionization-mass spectrometry imaging (MALDI-MSI) from tissue microarrays of over 100 patient tumors with over 10 years of follow-up metadata. We successfully identified a panel of N-glycans that are unique between benign and prostate tumor tissue. Specifically, high-mannose as well as tri-and tetra-antennary N-glycans were more abundant in tumor tissue and increase proportionally with tumor grade. Further, we expanded our analyses to examine the N-glycan profiles of Black and Appalachian patients and have identified unique glycan signatures that correlate with recurrence in each population. Our study highlights the potential applications of MALDI-MSI for digital pathology and biomarker discovery for prostate cancer. IMPLICATIONS: MALDI-MSI identifies N-glycan perturbations in prostate tumors compared with benign tissue. This method can be utilized to predict prostate cancer recurrence and study prostate cancer disparities.

Glycan-protein interactions determine kinetics of N-glycan remodeling

A hallmark of N-linked glycosylation in the secretory compartments of eukaryotic cells is the sequential remodeling of an initially uniform oligosaccharide to a site-specific, heterogeneous ensemble of glycostructures on mature proteins. To understand site-specific processing, we used protein disulfide isomerase (PDI), a model protein with five glycosylation sites, for molecular dynamics (MD) simulations and compared the result to a biochemical in vitro analysis with four different glycan processing enzymes. As predicted by an analysis of the accessibility of the N-glycans for their processing enzymes derived from the MD simulations, N-glycans at different glycosylation sites showed different kinetic properties for the processing enzymes. In addition, altering the tertiary structure of the glycoprotein PDI affected its N-glycan remodeling in a site-specific way. We propose that the observed differential N-glycan reactivities depend on the surrounding protein tertiary structure and lead to different glycan structures in the same protein through kinetically controlled processing pathways.

Atomistic glycoprotein simulations reveal a site-specific availability of glycan substrates in time-resolved mass spectrometry of maturating enzyme kinetics.

N-Glycosylation

N-glycosylation is a highly conserved glycan modification, and more than 7000 proteins are N-glycosylated in humans. N-glycosylation has many biological functions such as protein folding, trafficking, and signal transduction. Thus, glycan modification to proteins is profoundly involved in numerous physiological and pathological processes. The N-glycan precursor is biosynthesized in the endoplasmic reticulum (ER) from dolichol phosphate by sequential enzymatic reactions to generate the dolichol-linked oligosaccharide composed of 14 sugar residues, Glc₃Man₉GlcNAc₂. The oligosaccharide is then en bloc transferred to the consensus sequence N-X-S/T (X represents any amino acid except proline) of nascent proteins. Subsequently, the N-glycosylated nascent proteins enter the folding step, in which N-glycans contribute largely to attaining the correct protein fold by recruiting the lectin-like chaperones, calnexin, and calreticulin. Despite the N-glycan-dependent folding process, some glycoproteins do not fold correctly, and these misfolded glycoproteins are destined to degradation by proteasomes in the cytosol. Properly folded proteins are transported to the Golgi, and N-glycans undergo maturation by the sequential reactions of glycosidases and glycosyltransferases, generating complex-type N-glycans. N-Acetylglucosaminyltransferases (GnT-III, GnT-IV, and GnT-V) produce branched N-glycan structures, affording a higher complexity to N-glycans. In this chapter, we provide an overview of the biosynthetic pathway of N-glycans in the ER and Golgi.

A human case of SLC35A3-related skeletal dysplasia

Researchers have identified a subset of Holstein having a range of skeletal deformities, including vertebral anomalies, referred to as complex vertebral malformation due to mutations in the SLC35A3 gene. Here, we report the first case in humans of SLC35A3-related vertebral anomalies. Our patient had prenatally diagnosed anomalous vertebrae, including butterfly, and hemivertebrae throughout the spine, as well as cleft palate, micrognathia, patent foramen ovale, patent ductus arteriosus, posterior embryotoxon, short limbs, camptodactyly, talipes valgus, rocker bottom feet, and facial dysmorphism including proptosis, nevus flammeus, and a cupped left ear. Clinical exome sequencing revealed a novel missense homozygous mutation in SLC35A3. Follow-up biochemical analysis confirmed abnormal protein glycosylation, consistent with a defective Golgi UDP-GlcNAc transporter, validating the mutations. Congenital disorders of glycosylation, including SLC35A3-CDG, can present as a wide phenotypic spectrum, including skeletal dysplasia. Previously reported patients with SLC35A3-CDG have been described with syndromic autism, epilepsy, and arthrogryposis.

Sequential processing of mannose-containing glycans by two α-mannosidases from Solitalea canadensis

Two putative α-mannosidase genes isolated from the rather unexplored soil bacterium Solitalea canadensis were cloned and biochemically characterised. Both recombinant enzymes were highly selective in releasing α-linked mannose but no other sugars. The α-mannosidases were designated Sca2/3Man2693 and Sca6Man4191, and showed the following biochemical properties: the temperature optimum for both enzymes was 37 °C, and their pH optima lay at 5.0 and 5.5, respectively. The activity of Sca2/3Man2693 was found to be dependent on Ca(2+) ions, whereas Cu(2+) and Zn(2+) ions almost completely inhibited both α-mannosidases. Specificity screens with various substrates revealed that Sca2/3Man2693 could release both α1-2- and α1-3-linked mannose, whereas Sca6Man4191 only released α1-6-linked mannose. The combined enzymatic action of both recombinant α-mannosidases allowed the sequential degradation of high-mannose-type N-glycans. The facile expression and purification procedures in combination with strict substrate specificities make α-mannosidases from S. canadensis promising candidates for bioanalytical applications.

Galactosyltransferase 4 is a major control point for glycan branching in N-linked glycosylation

Protein N-glycosylation is a common post-translational modification that produces a complex array of branched glycan structures. The levels of branching, or antennarity, give rise to differential biological activities for single glycoproteins. However, the precise mechanism controlling the glycan branching and glycosylation network is unknown. Here, we constructed quantitative mathematical models of N-linked glycosylation that predicted new control points for glycan branching. Galactosyltransferase, which acts on N-acetylglucosamine residues, was unexpectedly found to control metabolic flux through the glycosylation pathway and the level of final antennarity of nascent protein produced in the Golgi network. To further investigate the biological consequences of glycan branching in nascent proteins, we glycoengineered a series of mammalian cells overexpressing human chorionic gonadotropin (hCG). We identified a mechanism in which galactosyltransferase 4 isoform regulated N-glycan branching on the nascent protein, subsequently controlling biological activity in an in vivo model of hCG activity. We found that galactosyltransferase 4 is a major control point for glycan branching decisions taken in the Golgi of the cell, which might ultimately control the biological activity of nascent glycoprotein.

The class I α1,2-mannosidases of Caenorhabditis elegans

During the biosynthesis of N-glycans in multicellular eukaryotes, glycans with the compositions Man(5)GlcNAc(2-3) are key intermediates. However, to reach this 'decision point', these N-glycans are first processed from Glc(3)Man(9)GlcNAc(2) through to Man(5)GlcNAc(2) by a number of glycosidases, whereby up to four α1-2-linked mannose residues are removed by class I mannosidases (glycohydrolase family 47). Whereas in the yeast Saccharomyces cerevisiae there are maximally three members of this protein family, in higher organisms there are multiple class I mannosidases residing in the endoplasmic reticulum and Golgi apparatus. The genome of the model nematode Caenorhabditis elegans encodes seven members of this protein family, whereby four are predicted to be classical processing mannosidases and three are related proteins with roles in quality control. In this study, cDNAs encoding the four predicted mannosidases were cloned and expressed in Pichia pastoris and the activity of these enzymes, designated MANS-1, MANS-2, MANS-3 and MANS-4, was verified. The first two can, dependent on the incubation time, remove three to four residues from Man(9)GlcNAc(2), whereas the action of the other two results in the appearance of the B isomer of Man(8)GlcNAc(2); together the complementary activities of these enzymes result in processing to Man(5)GlcNAc(2). With these data, another gap is closed in our understanding of the N-glycan biosynthesis pathway of the nematode worm.

Free oligosaccharides in the cytosol of Caenorhabditis elegans are generated through endoplasmic reticulum-golgi trafficking

Free oligosaccharides (FOSs) in the cytosol of eukaryotic cells are mainly generated during endoplasmic reticulum (ER)-associated degradation (ERAD) of misfolded glycoproteins. We analyzed FOS of the nematode Caenorhabditis elegans to elucidate its detailed degradation pathway. The major FOSs were high mannose-type ones bearing 3-9 Man residues. About 94% of the total FOSs had one GlcNAc at their reducing end (FOS-GN1), and the remaining 6% had two GlcNAc (FOS-GN2). A cytosolic endo-beta-N-acetylglucosaminidase mutant (tm1208) accumulated FOS-GN2, indicating involvement of the enzyme in conversion of FOS-GN2 into FOS-GN1. The most abundant FOS in the wild type was Man(5)GlcNAc(1), the M5A' isomer (Manalpha1-3(Manalpha1-6)Manalpha1-6(Manalpha1-3)Manbeta1-4GlcNAc), which is different from the corresponding M5B' (Manalpha1-2Manalpha1-2Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAc) in mammals. Analyses of FOS in worms treated with Golgi alpha-mannosidase I inhibitors revealed decreases in Man(5)GlcNAc(1) and increases in Man(7)GlcNAc(1). These results suggested that Golgi alpha-mannosidase I-like enzyme is involved in the production of Man(5-6)-GlcNAc(1), which is unlike in mammals, in which cytosolic alpha-mannosidase is involved. Thus, we assumed that major FOSs in C. elegans were generated through Golgi trafficking. Analysis of FOSs from a Golgi alpha-mannosidase II mutant (tm1078) supported this idea, because GlcNAc(1)Man(5)GlcNAc(1), which is formed by the Golgi-resident GlcNAc-transferase I, was found as a FOS in the mutant. We concluded that significant amounts of misfolded glycoproteins in C. elegans are trafficked to the Golgi and are directly or indirectly retro-translocated into the cytosol to be degraded.

Altered protein expression at early-stage rat hepatic neoplasia

Protein expression patterns were analyzed in a rat model of hepatic neoplasia to detect changes reflecting biological mechanism or potential therapeutic targets. The rat resistant hepatocyte model of carcinogenesis was studied, with a focus on the earliest preneoplastic lesion visible in the liver, the preneoplastic hyperplastic nodule. Expression differences were shown by two-dimensional polyacrylamide gel electrophoresis and image analysis. Polypeptide masses were measured by peptide mass fingerprinting using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) and their sequences were obtained by tandem mass spectrometry. Alterations in expression of cytoskeletal and functional proteins were demonstrated, consistent with biological changes known to occur in the preneoplastic cells. Of particular interest was the differential expression of a serine protease inhibitor (serpin) with a role implicated in angiogenesis. Serpin, implicated in the inhibition of angiogenesis, is present in normal liver but has greatly reduced expression at the preneoplastic stage of liver cancer development. Immunofluorescence microscopy with antibodies to this serpin, kallistatin, supports the proteomic identification. Immunofluorescence microscopy with antibodies to the blood vessel marker von Willebrand factor provides evidence for neovascularization in the liver containing multiple preneoplastic nodules. These observations suggest that at an early stage of liver carcinogenesis reduction or loss of angiogenesis inhibitors may contribute to initiation of neoangiogenesis. A number of other identified proteins known to be associated with hepatomas are also present at early-stage neoplasia.

A deletion in the golgi alpha-mannosidase II gene of Caenorhabditis elegans results in unexpected non-wild-type N-glycan structures

The processing of N-linked oligosaccharides by alpha-mannosidases in the endoplasmic reticulum and Golgi is a process conserved in plants and animals. After the transfer of a GlcNAc residue to Asn-bound Man(5)GlcNAc(2) by N-acetylglucosaminyltransferase I, an alpha-mannosidase (EC 3.2.1.114) removes one alpha1,3-linked and one alpha1,6-linked mannose residue. In this study, we have identified the relevant alpha-mannosidase II gene (aman-2; F58H1.1) from Caenorhabditis elegans and have detected its activity in both native and recombinant forms. For comparative studies, the two other cDNAs encoding class II mannosidases aman-1 (F55D10.1) and aman-3 (F48C1.1) were cloned; the corresponding enzymes are, respectively, a putative lysosomal alpha-mannosidase and a Co(II)-activated alpha-mannosidase. The analysis of the N-glycan structures of an aman-2 mutant strain demonstrates that the absence of alpha-mannosidase II activity results in a shift to structures not seen in wild-type worms (e.g. N-glycans with the composition Hex(5-7)HexNAc(2-3)Fuc(2)Me) and an accumulation of hybrid oligosaccharides. Paucimannosidic glycans are almost absent from aman-2 worms, indicative also of a general lack of alpha-mannosidase III activity. We hypothesize that there is a tremendous flexibility in the glycosylation pathway of C. elegans that does not impinge, under standard laboratory conditions, on the viability of worms with glycotypes very unlike the wild-type pattern.

Essential and mutually compensatory roles of {alpha}-mannosidase II and {alpha}-mannosidase IIx in N-glycan processing in vivo in mice

Many proteins synthesized through the secretory pathway receive posttranslational modifications, including N-glycosylation. alpha-Mannosidase II (MII) is a key enzyme converting precursor high-mannose-type N-glycans to matured complex-type structures. Previous studies showed that MII-null mice synthesize complex-type N-glycans, indicating the presence of an alternative pathway. Because alpha-mannosidase IIx (MX) is a candidate enzyme for this pathway, we asked whether MX functions in N-glycan processing by generating MII/MX double-null mice. Some double-nulls died between embryonic days 15.5 and 18.5, but most survived until shortly after birth and died of respiratory failure, which represents a more severe phenotype than that seen in single-nulls for either gene. Structural analysis of N-glycans revealed that double-nulls completely lack complex-type N-glycans, demonstrating a critical role for at least one of these enzymes for effective N-glycan processing. Recombinant mouse MX and MII showed identical substrate specificities toward N-glycan substrates, suggesting that MX is an isozyme of MII. Thus, either MII or MX can biochemically compensate for the deficiency of the other in vivo, and either of two is required for late embryonic and early postnatal development.

An antibody produced in tobacco expressing a hybrid beta-1,4-galactosyltransferase is essentially devoid of plant carbohydrate epitopes

N-glycosylation of a mAb may have a major impact on its therapeutic merits. Here, we demonstrate that expression of a hybrid enzyme (called xylGalT), consisting of the N-terminal domain of Arabidopsis thaliana xylosyltransferase and the catalytic domain of human beta-1,4-galactosyltransferase I (GalT), in tobacco causes a sharp reduction of N-glycans with potentially immunogenic core-bound xylose (Xyl) and fucose (Fuc) residues as shown by Western blot and MALDI-TOF MS analysis. A radioallergosorbent test inhibition assay with proteins purified from leaves of WT and these transgenic tobacco plants using sera from allergic patients suggests a significant reduction of potential immunogenicity of xylGalT proteins. A mAb purified from leaves of plants expressing xylGalT displayed an N-glycan profile that featured high levels of galactose, undetectable xylose, and a trace of fucose. Hence, a transgenic plant expressing the hybrid GalT might yield more effective and safer monoclonals for therapeutic purposes than WT plants and even transgenic plants expressing the unchanged GalT.

Cation-independent mannose 6-phosphate and 78 kDa receptors for lysosomal enzyme targeting are located in different cell compartments

The distribution of the cation-independent mannose 6-phosphate and 78 kDa receptors was studied in postnuclear subcellular fractions from two rat liver cell lines. ELISA assays revealed that the mannose 6-phosphate receptor is enriched in the light buoyant Percoll fractions that contain Golgi structures and early endosomes. Most of the 78 kDa receptor is localized in a heavy fraction at the bottom of the Percoll gradient and smaller amounts in the endosomal fractions. The high-density compartment is denser than lysosomes, contains LAMP2 but not LIMPII or acid hydrolases, and is not disrupted with glycyl-l-phenylalanine 2-naphthylamide, a substrate for cathepsin C that selectively disrupts lysosomes. Immunofluorescence microscopy studies indicate no colocalization of the 78 kDa receptor with the mannose 6-phosphate receptor or LIMPII. Mannose 6-phosphate-independent endocytosed beta-glucuronidase was found in the lysosomal, the early and late endosomal fractions. These fractions were immunoadsorbed in columns containing antibodies against the 78 kDa receptor. Only the endocytosed beta-glucuronidase present in the early and late endosomal fractions is associated to immunoadsorbed vesicles. In these vesicles, LAMP2 was detected but no LIMPII or the mannose 6-phosphate receptor. Results obtained suggest that the 78 kDa receptor is found along the endocytic pathway, but in vesicles different from the cation-independent mannose 6-phosphate receptor.

The humanization of N-glycosylation pathways in yeast

Yeast and other fungal protein-expression hosts have been extensively used to produce industrial enzymes, and are often the expression system of choice when manufacturing costs are of primary concern. However, for the production of therapeutic glycoproteins intended for use in humans, yeast have been less useful owing to their inability to modify proteins with human glycosylation structures. Yeast N-glycosylation is of the high-mannose type, which confers a short half-life in vivo and thereby compromises the efficacy of most therapeutic glycoproteins. Several approaches to humanizing yeast N-glycosylation pathways have been attempted over the past decade with limited success. Recently however, advances in the glycoengineering of yeast and the expression of therapeutic glycoproteins with humanized N-glycosylation structures have shown significant promise - this review summarizes the most important developments in the field.

Advances in the production of human therapeutic proteins in yeasts and filamentous fungi

Yeast and fungal protein expression systems are used for the production of many industrially relevant enzymes, and are widely used by the research community to produce proteins that cannot be actively expressed in Escherichia coli or require glycosylation for proper folding and biological activity. However, for the production of therapeutic glycoproteins intended for use in humans, yeasts have been less useful because of their inability to modify proteins with human glycosylation structures. Yeast glycosylation is of the high-mannose type, which confers a short in vivo half-life to the protein and may render it less efficacious or even immunogenic. Several ways of humanizing yeast-derived glycoproteins have been tried, including enzymatically modifying proteins in vitro and modulating host glycosylation pathways in vivo. Recent advances in the glycoengineering of yeasts and the expression of therapeutic glycoproteins in humanized yeasts have shown significant promise, and are challenging the current dominance of therapeutic protein production based on mammalian cell culture.

Inhibition of Golgi Mannosidase II with Mannostatin A Analogues: Synthesis, Biological Evaluation, and Structure-Activity Relationship Studies

Mannostatin and aminocyclopentitetrol analogues with various substitutions at the amino function were synthesized. These compounds were tested as inhibitors of human Golgi and lysosomal alpha-mannosidases. Modification of the amine of mannostatin had only marginal effects, whereas similar modifications of aminocyclopentitetrol led to significantly improved inhibitors. Ab initio calculations and molecular docking studies were employed to rationalize the results. It was found that mannostatin and aminocyclopentitretrol could bind to Golgi alpha-mannosidase II in a similar mode to that of the known inhibitor swainsonine. However, due to the flexibility of the five-membered rings of these compounds, additional low-energy binding modes could be adopted. These binding modes may be relevant for the improved activities of the benzyl-substituted compounds. The thiomethyl moiety of mannostatin was predicted to make favorable hydrophobic interactions with Arg228 and Tyr727 that would possibly account for its greater inhibitory activity.

Golgi alpha-mannosidase II deficiency in vertebrate systems: implications for asparagine-linked oligosaccharide processing in mammals

The maturation of N-glycans to complex type structures on cellular and secreted proteins is essential for the roles that these structures play in cell adhesion and recognition events in metazoan organisms. Critical steps in the biosynthetic pathway leading from high mannose to complex structures include the trimming of mannose residues by processing mannosidases in the endoplasmic reticulum (ER) and Golgi complex. These exo-mannosidases comprise two separate families of enzymes that are distinguished by enzymatic characteristics and sequence similarity. Members of the Class 2 mannosidase family (glycosylhydrolase family 38) include enzymes involved in trimming reactions in N-glycan maturation in the Golgi complex (Golgi mannosidase II) as well as catabolic enzymes in lysosomes and cytosol. Studies on the biological roles of complex type N-glycans have employed a variety of strategies including the treatment of cells with glycosidase inhibitors, characterization of human patients with enzymatic defects in processing enzymes, and generation of mouse models for the enzyme deficiency by selective gene disruption approaches. Corresponding studies on Golgi mannosidase II have employed swainsonine, an alkaloid natural plant product that causes "locoism", a phenocopy of the lysosomal storage disease, alpha-mannosidosis, as a result of the additional targeting of the broad-specificity lysosomal mannosidase by this compound. The human deficiency in Golgi mannosidase II is characterized by congenital dyserythropoietic anemia with splenomegaly and various additional abnormalities and complications. Mouse models for Golgi mannosidase II deficiency recapitulate many of the pathological features of the human disease and confirm that the unexpectedly mild effects of the enzyme deficiency result from a tissue-specific and glycoprotein substrate-specific alternate pathway for synthesis of complex N-glycans. In addition, the mutant mice develop symptoms of a systemic autoimmune disorder as a consequence of the altered glycosylation. This review will discuss the biochemical features of Golgi mannosidase II and the consequences of its deficiency in mammalian systems as a model for the effects of alterations in vertebrate N-glycan maturation during development.

Overexpression of the Golgi-localized enzyme alpha-mannosidase IIx in Chinese hamster ovary cells results in the conversion of hexamannosyl-N-acetylchitobiose to tetramannosyl-N-acetylchitobiose in the N-glycan-processing pathway

Golgi alpha-mannosidase II is an enzyme that processes the intermediate oligosaccharide Gn(1)M(5)Gn(2) to Gn(1)M(3)Gn(2) during biosynthesis of N-glycans. Previously, we isolated a cDNA encoding a protein homologous to alpha-mannosidase II and designated it alpha-mannosidase IIx. Here, we show by immunocytochemistry that alpha-mannosidase IIx resides in the Golgi in HeLa cells. When coexpressed with alpha-mannosidase II, alpha-mannosidase IIx colocalizes with alpha-mannosidase II in COS cells. A protein A fusion of the catalytic domain of alpha-mannosidase IIx hydrolyzes a synthetic substrate, 4-umbelliferyl-alpha-D-mannoside, and this activity is inhibited by swainsonine. [(3)H]glucosamine-labeled Chinese hamster ovary cells overexpressing alpha-mannosidase IIx show a reduction of M(6)Gn(2) and an accumulation of M(4)Gn(2). Structural analysis identified M(4)Gn(2) to be Man alpha 1-->6(Man alpha 1-->2Man alpha 1-->3)Man beta 1-->4GlcNAc beta 1-->4GlcNAc. The results suggest that alpha-mannosidase IIx hydrolyzes two peripheral Man alpha 1-->6 and Man alpha 1-->3 residues from [(Man alpha 1-->6)(Man alpha 1-->3)Man alpha 1-->6](Man alpha 1-->2Man alpha 1-->3)Man beta 1-->4GlcNAc beta 1-->4GlcNAc, during N-glycan processing.

Structural analysis of the asparagine-linked glycan units of the ZP2 and ZP3 glycoproteins from mouse zona pellucida

Zona pellucida (ZP), the extracellular glycocalyx that surrounds the mammalian egg plasma membrane, is a relatively simple structure consisting of three to four glycoproteins. In the mouse, the ZP is composed of three glycoproteins, namely ZP1 (200 kDa), ZP2 (120 kDa), and ZP3 (83 kDa). Extensive studies in this species have resulted in the identification of primary (mZP3) and secondary (mZP2) binding sites for spermatozoa. The two zona components are highly glycosylated containing N-linked and O-linked glycan units. In an attempt to characterize N-linked glycan units, mZP2 and mZP3 were purified and the N-linked carbohydrate chains were released by exhaustive digestion with N-glycanase. The released oligosaccharides (OSs) were radiolabeled by reduction with NaB3H4 and resolved by gel filtration on a column of Bio-Gel P-4. The OSs separated into several peaks indicating the presence of a variety of N-linked glycans. Interestingly, the radioactive peaks resolved from mZP2 and mZP3 were quite different, a result suggesting qualitative and quantitative differences in the glycans. The [SH]-labeled glycans present in mZP2 and mZP3 were pooled separately and fractionated by serial lectin chromatography. Experimental evidence included in this report strongly suggests that mZP3 (but not mZP2) contains polylactosaminyl glycan with terminal, nonreducing alpha-galactosyl residues. The mZP3 glycans eluted from the immobilized lectin columns were further characterized by lectin and sizing column chromatography before or after digestion with endo-/ exo-glycohydrolases. Data revealed the presence of a variety of OSs, including poly-N-acetyllactosaminyl, bi-, tri-, and tetraantennary complex-type, and high-mannose-type glycans. Taken together, these results provide additional evidence on the complex nature of the glycan chains present on mZP glycoconjugates.

Trafficking and release of mycobacterial lipids from infected macrophages

Analysis of infected macrophages revealed that lipid-containing moieties of the mycobacterial cell wall are actively trafficked out of the mycobacterial vacuole. To facilitate the analysis of vesicular trafficking from mycobacteria-containing phagosomes, surface-exposed carbohydrates were labeled with hydrazide-tagged markers. The distribution of labeled carbohydrate/lipid moieties and subsequent interaction with cellular compartments were analyzed by immunoelectron microscopy and by fluorescence microscopy of live cells. The released mycobacterial constituents were associated with several intracellular organelles and were enriched strikingly in tubular endocytic compartments. Subcellular fractionation of infected macrophages by density gradient electrophoresis showed temporal movement of labeled bacterial constituents through early and late endosomes. Thin layer chromatography analysis of these subcellular fractions confirmed their lipid nature and revealed five dominant bacteria-derived species. These mycobacterial lipids were also found in extracellular vesicles isolated from the medium and could be observed in un-infected 'bystander' cells. Their transfer to bystander cells could expand the bacteria's sphere of influence beyond the immediate confines of the host cell.

The alpha-mannosidases: phylogeny and adaptive diversification

alpha-Mannosidase enzymes comprise a class of gylcoside hydrolases involved in the maturation and degradaton of glycoprotein-linked oligosaccharides. Various alpha-mannosidase enzymatic activities are encoded by an ancient and ubiquitous gene superfamily. A comparative sequence analysis was employed here to characterize the evolutionary relationships and dynamics of the alpha-mannosidase superfamily. A series of lineage-specific BLAST searches recovered the first ever recognized archaean and eubacterial alpha-mannosidase sequences, in addition to numerous eukaryotic sequences. Motif-based alignment and subsequent phylogenetic analysis of the entire superfamily revealed the presence of three well-supported monophyletic clades that represent discrete alpha-mannosidase families. The comparative method was used to evaluate the phylogenetic distribution of alpha-mannosidase functional variants within families. Results of this analysis demonstrate a pattern of functional diversification of alpha-mannosidase paralogs followed by conservation of function among orthologs. Nucleotide polymorphism among the most closely related pair of duplicated genes was analyzed to evaluate the role of natural selection in the functional diversification of alpha-mannosidase paralogs. Ratios of nonsynonymous and synonymous variation show an increase in the rate of nonsynonymous change after duplication and a relative excess of fixed nonsynonymous changes between the two groups of paralogs. These data point to a possible role for positive Darwinian selection in the evolution of alpha-mannosidase functional diversification following gene duplication.

Fucosylated hybrid-type N-glycans on the secreted human epidermal growth factor receptor from swainsonine-treated A431 cells

N-Glycans linked to the human secreted form of epidermal growth factor receptor were isolated from A431 cells after swainsonine treatment. Analysis of the oligosaccharides by (1)H NMR spectroscopy and mass spectrometry shows the presence of oligomannose- and (alpha2-3)-sialylated hybrid-type glycans. The major hybrid-type oligosaccharide chains are fucosylated at the Asn-bound GlcNAc residue. Smaller amounts of the hybrid-type structures are also fucosylated at peripheral GlcNAc residues, constituting the sialyl-Le(x) antigen. No complex-type glycans are found, suggesting the absence of alpha-mannosidase III. An assay for alpha-mannosidase III on the A431 cells in the absence and presence of 6 microM swainsonine shows that Man(5)GlcNAc(2) is not converted into Man(3)GlcNAc(2), thereby confirming that these cells do not contain alpha-mannosidase III activity.

Cloning and expression of a specific human alpha 1,2-mannosidase that trims Man9GlcNAc2 to Man8GlcNAc2 isomer B during N-glycan biosynthesis

We report the isolation of a novel human cDNA encoding a type II membrane protein of 79.5 kDa with amino acid sequence similarity to Class I alpha 1,2-mannosidases. The catalytic domain of the enzyme was expressed as a secreted protein in Pichia pastoris. The recombinant enzyme removes a single mannose residue from Man9GlcNAc and [1H]-NMR analysis indicates that the only product is Man8GlcNAc isomer B, the form lacking the middle-arm terminal alpha 1,2-mannose. Calcium is required for enzyme activity and both 1-deoxymannojirimycin and kifunensine inhibit the human alpha 1,2-mannosidase. The properties and specificity of this human alpha 1,2-mannosidase are identical to the endoplasmic reticulum alpha 1,2-mannosidase from Saccharomyces cerevisiae and differ from those of previously cloned Golgi alpha 1,2-mannosidases that remove up to four mannose residues from Man9GlcNAc2 during N-glycan maturation. Northern blot analysis showed that all human tissues examined express variable amounts of a 3 kb transcript. This highly specific alpha 1,2-mannosidase is likely to be involved in glycoprotein quality control since there is increasing evidence that trimming of Man9GlcNAc2 to Man8GlcNAc2 isomer B in yeast cells is important to target misfolded glycoproteins for degradation.

Characterization of the pharmacological-sensitivity profile of neoglycoprotein-induced acrosome reaction in mouse spermatozoa

Mammalian spermatozoa undergo the acrosome reaction (AR) in response to the interaction of a carbohydrate-recognizing molecule(s) on the sperm plasma membrane (sperm surface receptor) and its complementary glycan (ligand) moiety(ies) on the zona pellucida (ZP). Previously, we demonstrated that a hexose (mannose) or two amino sugars (glucosaminyl or galactosaminyl residues) when covalently conjugated to a protein backbone (neoglycoproteins) mimicked the mouse ZP3 glycoprotein and induced the AR in capacitated mouse spermatozoa (Loeser and Tulsiani, Biol Reprod 1999; 60:94-101). To elucidate the mechanism underlying sperm-neoglycoprotein interaction and the induction of the AR, we have examined the effect of several AR blockers on neoglycoprotein-induced AR. Our data demonstrate that two known L-type Ca(2+) channel blockers prevented the induction of the AR by three neoglycoproteins (mannose-BSA, N-acetylglucosamine-BSA, and N-acetylgalactosamine-BSA). The fact that the L-type Ca(2+) channel blockers (verapamil, diltiazem) had no inhibitory effect on sperm surface galactosyltransferase or alpha-D-mannosidase, two carbohydrate-recognizing enzymes thought to be sperm surface receptors, suggests that the reagents block the AR by a mechanism other than binding to the active site of the enzymes.

Alpha-mannosidases involved in N-glycan processing show cell specificity and distinct subcompartmentalization within the Golgi apparatus of cells in the testis and epididymis

The Golgi apparatus is enriched in specific enzymes involved in the maturation of carbohydrates of glycoproteins. Among them, alpha-mannosidases IA, IB and II are type II transmembrane Golgi-resident enzymes that remove mannose residues at different stages of N-glycan maturation. alpha-Mannosidases IA and IB trim Man9GlcNAc2 to Man5GlcNAc2, while alpha-mannosidase II acts after GlcNAc transferase I to remove two mannose residues from GlcNAcMan5GlcNAc2 to form GlcNAcMan3GlcNAc2 prior to extension into complex N-glycans by Golgi glycosyltransferases. The objective of this study is to examine the expression as well as the subcellular localization of these Golgi enzymes in the various cells of the male rat reproductive system. Our results show distinct cell-and region-specific expression of the three mannosidases examined. In the testis, only alpha-mannosidase IA and II were detectable in the Golgi apparatus of Sertoli and Leydig cells, and while alpha-mannosidase IB was present in the Golgi apparatus of all germ cells, only the Golgi apparatus of steps 1-7 spermatids was reactive for alpha-mannosidase IA. In the epididymis, principal cells were unreactive for alpha-mannosidase II, but they expressed alpha-mannosidase IB in the initial segment and caput regions, and alpha-mannosidase IA in the corpus and cauda regions. Clear cells expressed alpha-mannosidase II in all epididymal regions, and alpha-mannosidase IB only in the caput and corpus regions. Ultrastructurally, alpha-mannosidase IB was localized mainly over cis saccules, alpha-mannosidase IA was distributed mainly over trans saccules, and alpha-mannosidase II was localized mainly over medial saccules of the Golgi stack. Thus, the cell-specific expression and distinct Golgi subcompartmental localization suggest that these three alpha-mannosidases play different roles during N-glycan maturation.

Rat sperm surface mannosidase is first expressed on the plasma membrane of testicular germ cells

In previous publications (Tulsiani et al., Biochem J 1993; 290:427-436 and Tulsiani et al., Dev Biol 1995; 167:584-595), we reported that sperm surface mannosidase is present in rat testis and is modified during spermatogenesis and sperm maturation. The present studies were directed towards examining the origin of alpha-D-mannosidase activity present on fertile spermatozoa. Mixed germ cells prepared after sequential enzymatic digestions of rat testis were separated by unit gravity sedimentation using 2-4% linear bovine serum albumin gradient. Fractions enriched in spermatocytes, round spermatids, and condensed/elongated spermatids (> 95% pure cells) were separately pooled and assayed for [3H]Man9-mannosidase activity before (intact) and after lysis with Triton X-100. Interestingly, the cells contained a significant level of alpha-D-mannosidase activity. Approximately 70% of the total [3H]Man9-mannosidase activity present in the detergent-solubilized germ cell extract cross-reacted with anti-rat sperm mannosidase, and 25% of the activity cross-reacted with anti-Golgi mannosidase I. This result indicates that most of the mannosidase activity present in the germ cell extract is antigenically similar to the enzyme present on the cauda spermatozoa. Using cell fractionation techniques, we obtained evidence suggesting that the germ cell-associated mannosidase activity is an integral component of the plasma membranes. Taken together, these results indicate that sperm surface mannosidase is first expressed on the testicular germ cells.

Substrate specificities of recombinant murine Golgi alpha1, 2-mannosidases IA and IB and comparison with endoplasmic reticulum and Golgi processing alpha1,2-mannosidases

The catalytic domains of murine Golgi alpha1,2-mannosidases IA and IB that are involved in N-glycan processing were expressed as secreted proteins in P.pastoris . Recombinant mannosidases IA and IB both required divalent cations for activity, were inhibited by deoxymannojirimycin and kifunensine, and exhibited similar catalytic constants using Manalpha1,2Manalpha-O-CH3as substrate. Mannosidase IA was purified as a 50 kDa catalytically active soluble fragment and shown to be an inverting glycosidase. Recombinant mannosidases IA and IB were used to cleave Man9GlcNAc and the isomers produced were identified by high performance liquid chromatography and proton-nuclear magnetic resonance spectroscopy. Man9GlcNAc was rapidly cleaved by both enzymes to Man6GlcNAc, followed by a much slower conversion to Man5GlcNAc. The same isomers of Man7GlcNAc and Man6GlcNAc were produced by both enzymes but different isomers of Man8GlcNAc were formed. When Man8GlcNAc (Man8B isomer) was used as substrate, rapid conversion to Man5GlcNAc was observed, and the same oligosaccharide isomer intermediates were formed by both enzymes. These results combined with proton-nuclear magnetic resonance spectroscopy data demonstrate that it is the terminal alpha1, 2-mannose residue missing in the Man8B isomer that is cleaved from Man9GlcNAc at a much slower rate. When rat liver endoplasmic reticulum membrane extracts were incubated with Man9GlcNAc2, Man8GlcNAc2was the major product and Man8B was the major isomer. In contrast, rat liver Golgi membranes rapidly cleaved Man9GlcNAc2to Man6GlcNAc2and more slowly to Man5GlcNAc2. In this case all three isomers of Man8GlcNAc2were formed as intermediates, but a distinctive isomer, Man8A, was predominant. Antiserum to recombinant mannosidase IA immunoprecipitated an enzyme from Golgi extracts with the same specificity as recombinant mannosidase IA. These immunodepleted membranes were enriched in a Man9GlcNAc2to Man8GlcNAc2-cleaving activity forming predominantly the Man8B isomer. These results suggest that mannosidases IA and IB in Golgi membranes prefer the Man8B isomer generated by a complementary mannosidase that removes a single mannose from Man9GlcNAc2.

Molecular cloning, chromosomal mapping and tissue-specific expression of a novel human alpha1,2-mannosidase gene involved in N-glycan maturation

Class I alpha1,2-mannosidases play an essential role in the elaboration of complex and hybrid N -glycans in mammalian cells. Using degenerate primers based on amino acid sequences conserved in all members of this enzyme family for RT-PCR, two distinct PCR products were obtained from placenta and lymphocyte cDNAs. One of these was related to the previously cloned human and murine alpha1, 2-mannosidase IA whereas the other was very similar to murine alpha1, 2-mannosidase IB. Northern blot analysis of human tissues with these two alpha1,2-mannosidase probes revealed very different patterns of tissue-specific expression. Similar tissue-specific expression of alpha1,2-mannosidase IA and IB was also observed on Northern blots of adult mouse tissues. A human placenta cDNA library was screened and PCR of brain, placenta, and lymphocyte cDNAs was performed in order to isolate the human alpha1,2-mannosidase IB cDNA. This cDNA encodes a type II membrane protein of 73 kDa that is 94% identical in amino acid sequence to the murine alpha1,2-mannosidase IB (Herscovics et al., 1994, J. Biol. Chem., 269, 9864-9871). A truncated soluble form of the human alpha1,2-mannosidase IB lacking its N -terminal transmembrane domain was expressed as a secreted protein in Pichia pastoris . The recombinant enzyme was incubated with [3H]Man9GlcNAc and [3H]Man8GlcNAc (isomer B), and high performance liquid chromatography analysis of the products showed that [3H]Man9GlcNAc was readily converted to [3H]Man6GlcNAc and much more slowly to [3H]Man5GlcNAc, whereas [3H]Man8GlcNAc was rapidly trimmed to [3H]Man5GlcNAc. The human alpha1,2-mannosidase IB gene was isolated from a P1 human genomic library and shown to be at least 60 kb in size and to contain at least 13 exons. The gene was localized by fluorescence in situ hybridization to human chromosome 1p13, a region that undergoes many aberrations in various types of human cancers. These results show that there are at least two Class I alpha1,2-mannosidases in the human and murine genomes with very distinct transcriptional regulation in different tissues.

The biological and functional significance of the sperm acrosome and acrosomal enzymes in mammalian fertilization

The mammalian spermatozoon undergoes continuous modifications during spermatogenesis, maturation in the epididymis, and capacitation in the female reproductive tract. Only the capacitated spermatozoa are capable of binding the zona-intact egg and undergoing the acrosome reaction. The fertilization process is a net result of multiple molecular events which enable ejaculated spermatozoa to recognize and bind to the egg's extracellular coat, the zona pellucida (ZP). Sperm-egg interaction is a species-specific event which is initiated by the recognition and binding of complementary molecule(s) present on sperm plasma membrane (receptor) and the surface of the ZP (ligand). This is a carbohydrate-mediated event which initiates a signal transduction cascade resulting in the exocytosis of acrosomal contents. This step is believed to be a prerequisite which enables the acrosome reacted spermatozoa to penetrate the ZP and fertilize the egg. This review focuses on the formation and contents of the sperm acrosome as well as the mechanisms underlying the induction of the acrosome reaction. Special emphasis has been laid on the synthesis, processing, substrate specificity, and mechanism of action of the acid glycohydrolases present within the acrosome. The hydrolytic action of glycohydrolases and proteases released at the site of sperm-zona binding, along with the enhanced thrust generated by the hyperactivated beat pattern of the bound spermatozoon, are important factors regulating the penetration of ZP. We have discussed the most recent studies which have attempted to explain signal transduction pathways leading to the acrosomal exocytosis.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Mutant Rab7 causes the accumulation of cathepsin D and cation-independent mannose 6-phosphate receptor in an early endocytic compartment

Stable BHK cell lines inducibly expressing wild-type or dominant negative mutant forms of the rab7 GTPase were isolated and used to analyze the role of a rab7-regulated pathway in lysosome biogenesis. Expression of mutant rab7N125I protein induced a dramatic redistribution of cation-independent mannose 6-phosphate receptor (CI-MPR) from its normal perinuclear localization to large peripheral endosomes. Under these circumstances approximately 50% of the total receptor and several lysosomal hydrolases cofractionated with light membranes containing early endosome and Golgi markers. Late endosomes and lysosomes were contained exclusively in well-separated, denser gradient fractions. Newly synthesized CI-MPR and cathepsin D were shown to traverse through an early endocytic compartment, and functional rab7 was crucial for delivery to later compartments. This observation was evidenced by the fact that 2 h after synthesis, both markers were more prevalent in fractions containing light membranes. In addition, both were sensitive to HRP-DAB- mediated cross-linking of early endosomal proteins, and the late endosomal processing of cathepsin D was impaired. Using similar criteria, the lysosomal membrane glycoprotein 120 was not found accumulated in an early endocytic compartment. The data are indicative of a post-Golgi divergence in the routes followed by different lysosome-directed molecules.

Alpha-mannosidase-II deficiency results in dyserythropoiesis and unveils an alternate pathway in oligosaccharide biosynthesis

Alpha-mannosidase-II (alphaM-II) catalyzes the first committed step in the biosynthesis of complex asparagine-linked (N-linked) oligosaccharides (N-glycans). Genetic deficiency of alphaM-II should abolish complex N-glycan production as reportedly does inhibition of alphaM-II by swainsonine. We find that mice lacking a functional alphaM-II gene develop a dyserythropoietic anemia concurrent with loss of erythrocyte complex N-glycans. Unexpectedly, nonerythroid cell types continued to produce complex N-glycans by an alternate pathway comprising a distinct alpha-mannosidase. These studies reveal cell-type-specific variations in N-linked oligosaccharide biosynthesis and an essential role for alphaM-II in the formation of erythroid complex N-glycans. alphaM-II deficiency elicits a phenotype in mice that correlates with human congenital dyserythropoietic anemia type II.

Man9-mannosidase from pig liver is a type-II membrane protein that resides in the endoplasmic reticulum. cDNA cloning and expression of the enzyme in COS 1 cells

Man9-mannosidase, one of three different alpha 1,2-exo-mannosidases known to be involved in N-linked oligosaccharide processing, has been cloned in lambda gt10, using a mixed-primed pig liver cDNA library. Three clones were isolated which allowed the reconstruction of a 2731-bp full-length cDNA. The cDNA construct contained a single open reading frame of 1977 bp, encoding a 659-residue polypeptide with a molecular mass of approximately 73 kDa. The Man9-mannosidase specificity of the cDNA construct was verified by the observation that all peptide sequences derived from a previously purified, catalytically active 49-kDa fragment were found within the coding region. The N-terminus of the 49-kDa fragment aligns with amino acid 175 of the translated cDNA, indicating that the catalytic activity is associated with the C-terminus. Transfection of COS 1 cells with the Man9-mannosidase cDNA gave rise to a > 30-fold over-expression of a 73-kDa protein whose catalytic properties, including substrate specificity, susceptibility towards alpha-mannosidase inhibitors and metal ion requirements, were similar to those of the 49-kDa enzyme fragment. Thus deletion of 174 N-terminal amino acids in the 73-kDa protein appears to have only marginal influence on the catalytic properties. Structural and hydrophobicity analysis of the coding region, as well as the results from tryptic degradation studies, point to pig liver Man9-mannosidase being a non-glycosylated type-II transmembrane protein. This protein contains a 48-residue cytosolic tail followed by a 22-residue membrane anchor (which probably functions as internal and non-cleavable signal sequence), a lumenal approximately 100-residue-stem region and a large 49-kDa C-terminal catalytic domain. As shown by immuno-fluorescence microscopy, the pig liver enzyme expressed in COS 1 cells, is resident in the endoplasmic reticulum, in contrast to COS 1 Man9-mannosidase from human kidney which is Golgi-located [Bieberich, E. & Bause, E. (1995) Eur. J. Biochem. 233, 644-649]. Localization of the porcine enzyme in the endoplasmic reticulum is consistent with immuno-electron-microscopic studies using pig hepatocytes. The different intracellular distribution of pig liver and human kidney Man9-mannosidase is, therefore, enzyme-specific rather than a COS-1-cell-typical phenomenon. Since we observe approximately 81% sequence similarity between the two alpha-mannosidases, we deduce that the localization in either endoplasmic reticulum or Golgi is likely to be sequence-dependent.

Preparation and biological activity of manno- and galacto-validamines, new 5a-carba-glycosylamines as alpha-glycosidase inhibitors

Manno- and galacto-validamines, which are epimers of validamine, were semi-synthesized by the configurational inversion of validamine, a pseudo-sugar analogue of alpha-D-gluco-pyranose that has inhibitory activity for alpha-glucosidases. The inhibitory activities of these analogues were determined against several mannosidases and galactosidase. Manno-validamine shows potent inhibition for the alpha-mannosidases (competitive. K(i) = 4.6 x 10(-5) M for jack beans, and competitive, Ki = 2.8 x 10(-5) M for almonds), and galacto-validamine shows weak inhibition for the alpha-galactosidases (coffee bean and E. coli). The inhibitory effect of the epimers on the N-linked oligosaccharide-processing mannosidases involved in glycoprotein biosynthesis and lysosomal mannosidase from rat liver were also examined. Mannovalidamine shows potent inhibition on the endoplasmic reticulumal alpha-mannosidase (competitive, K(i) = 1.2 x 10(-6) M), Golgi mannosidases IA, II (competitive, K(i) = 2.8 x 10(-5) M), and lysosomal alpha-mannosidase (competitive, Ki = 1.7 x 10(-5) M).

Distinct egg membrane vesicles differing in binding and fusion properties contribute to sea urchin male pronuclear envelopes formed in vitro

We have identified three distinct membrane vesicle populations from sea urchin egg cytoplasm that cooperate in assembling the male pronuclear envelope in vitro. Membranes from sea urchin egg homogenates were separated by buoyant density into five vesicle fractions, three of which bind to demembranated sperm nuclei. Each requires a membranous element (lipophilic structure) derived from the sperm nuclear envelope at the tip and base (poles) of the nucleus in order to bind. Binding is differentially sensitive to protease, high salt and N-ethyl maleimide treatment of the membrane vesicles. MV1 binds at the poles and is required for fusion of the membrane vesicle fractions to each other and to the lipophilic structures. MV2 beta binds over the entire chromatin surface and is enriched in an endoplasmic reticulum marker enzyme. MV2 alpha binds at the nuclear poles, is enriched in a Golgi enzyme marker and is required for fusion of MV2 beta. All three fractions are required for nuclear envelope formation in vitro. The results suggest a multistep process for nuclear envelope formation involving contributions from both sperm and egg, roles for both endoplasmic reticulum and non-endoplasmic reticulum-derived vesicles, and the localization of a critical element of the fusion machinery in MV1.

Two naturally occurring mouse alpha-1,2-mannosidase IB cDNA clones differ in three point mutations. Mutation of Phe592 to Ser592 is sufficient to abolish enzyme activity

In mammalian cells, alpha-1,2-mannosidases play an essential role in the early steps of N-linked oligosaccharide maturation. We previously reported (Herscovics, A., Schneikert, J., Athanassiadis, A., and Moremen, K. W. (1994) J. Biol. Chem. 269, 9864-9871) the isolation of mouse alpha-mannosidase IB cDNA clones from a Balb/c 3T3 cDNA library. Clone 4 encodes a type II membrane protein of 641 amino acids with a cytoplasmic tail of 35 amino acids, followed by a transmembrane domain and a large C-terminal catalytic domain, whereas clone 16 encodes only the last 471 amino acids. Their overlapping sequences (from amino acid 152) are identical, except for three point mutations that result in three amino acid differences in the catalytic domain of the enzyme (Thr411, Leu468, and Ser592 in clone 4 to Met411, Phe468, and Phe592 in clone 16, respectively). Both sequences could be amplified by polymerase chain reaction using templates of cDNAs derived from colon and brain of CD1 mice and from L cells derived from the C3H/An mouse, indicating that both are natural isoforms found in two inbred and one outbred mouse strains. When expressed in COS7 cells as a secreted protein A fusion protein, the catalytic domain of clone 16 displays alpha-1,2-mannosidase activity using [3H]mannose-labeled Man9GlcNAc as substrate, but the corresponding region of clone 4 is poorly secreted under identical conditions. The contribution of each point mutation to this differential secretion and enzyme activity of the two fusion proteins was assessed by testing the six recombinants corresponding to all the possible sequence permutations. Mutation of Phe592 to Ser592, as found in clone 4, is sufficient to abolish alpha-1,2-mannosidase activity, whereas mutation of Met411 to Thr411 or of Phe468 to Leu468 affects secretion with relatively little effect on enzyme activity. Phe592 is part of a highly conserved region that seems important for enzyme activity of class 1 alpha-1,2-mannosidases.

Insect cells contain an unusual, membrane-bound beta-N-acetylglucosaminidase probably involved in the processing of protein N-glycans

The beta-N-acetylglucosaminidase activity in the lepidopteran insect cell line Sf21 has been studied using pyridylaminated oligosaccharides and chromogenic synthetic glycosides as substrates. Ultracentrifugation experiments indicated that the insect cell beta-N-acetylglucosminidase exists in a soluble and a membrane-bound form. This latter form accounted for two-thirds of the total activity and was associated with vesicles of the same density as those containing GlcNAc-transferase I. Partial membrane association of the enzyme was observed with all substrates tested, i.e. 4-nitrophenyl beta-N-acetylglucosaminide, tri-N-acetylchitotriose, and an N-linked biantennary agalactooligosaccharide. Inhibition studies indicted a single enzyme to be responsible for the hydrolysis of all these substrates. With the biantennary substrate, the beta-N-acetylglucosaminidase exclusively removed beta-N-acetylglucosamine from the alpha 1,3-antenna. GlcNAcMan5GlcNAc2, the primary product of GlcNAc-transferase I, was not perceptibly hydrolyzed. beta-N-Acetylglucosaminidases with the same branch specificity were also found in the lepidopteran cell lines Bm-N and Mb-0503. In contrast, beta-N-acetylglucosaminidase activities from rat or frog (Xenopus laevis) liver and from mung bean seedlings were not membrane-bound, and they did not exhibit a strict branch specificity. An involvement of this unusual beta-N-acetylglucosaminidase in the processing of asparagine-linked oligosaccharides in insects is suggested.

Processing of asparagine-linked oligosaccharides in insect cells: evidence for alpha-mannosidase II

The occurrence of alpha-D-mannosidase II activity in insect cells was studied using pyridylaminated oligosaccharides as substrates and two-dimensional HPLC and glycosidase digestion for the analysis of products. GlcNAcMan5GlcNAc2 was converted to GlcNAcMan3GlcNAc2 by each of the three cell lines investigated (Bm-N, Sf-21, and Mb-0503). The respective activity was highest in Bm-N cells which were used for further experiments. Man5GlcNAc2 was not degraded by the Bm-N cell homogenate. Thus, this alpha-mannosidase essentially exhibits the same substrate specificity as mammalian and plant Golgi alpha-mannosidase II. The alpha-mannosidase II-like activity from Bm-N cells exhibits a pH optimum of 6.0-6.5, has no requirement for divalent metal ions, and is highly sensitive to swainsonine. The alpha 1,6-linked mannosyl residue is removed first as deduced from the elution time on reversed phase HPLC of the intermediate product. The same branch preference was found with alpha-mannosidase II from mung bean seedlings and Xenopus liver. Upon ultracentrifugation of Bm-N cell homogenate, 72% of the mannosidase acting on the GlcNAcMan5GlcNAc2 substrate was found in the microsomal pellet indicating the enzyme to be membrane-bound.