Characterization and distribution of multiple antigens on N-linked oligosaccharides of Dictyostelium discoideum proteins

Mass spectrometric analysis of neutral and anionic N-glycans from a Dictyostelium discoideum model for human congenital disorder of glycosylation CDG IL

The HL241 mutant strain of the cellular slime mold Dictyostelium discoideum is a potential model for human congenital disorder of glycosylation type IL (ALG9-CDG) and has been previously predicted to possess a lower degree of modification of its N-glycans with anionic moieties than the parental wild-type. In this study, we first showed that this strain has a premature stop codon in its alg9 mannosyltransferase gene compatible with the occurrence of truncated N-glycans. These were subject to an optimized analytical workflow, considering that the mass spectrometry of acidic glycans often presents challenges due to neutral loss and suppression effects. Therefore, the protein-bound N-glycans were first fractionated, after serial enzymatic release, by solid phase extraction. Then primarily single glycan species were isolated by mixed hydrophilic-interaction/anion-exchange or reversed-phase HPLC and analyzed using chemical and enzymatic treatments and MS/MS. We show that protein-linked N-glycans of the mutant are of reduced size as compared to those of wild-type AX3, but still contain core α1,3-fucose, intersecting N-acetylglucosamine, bisecting N-acetylglucosamine, methylphosphate, phosphate, and sulfate residues. We observe that a single N-glycan can carry up to four of these six possible modifications. Due to the improved analytical procedures, we reveal fuller details regarding the N-glycomic potential of this fascinating model organism.

The Dictyostelium discoideum beta-1,4-mannosyltransferase gene, mntA, has two periods of developmental expression

The precise roles of protein glycosylation in multicellular development are poorly understood. We have characterized the mntA gene from Dictyostelium discoideum which encodes the beta-1,4-mannosyltransferase enzyme that catalyzes the reaction: GDP-Man + dolichol-PP-GlcNAc2 --> dolichol-PP-GlcNAc2-Man + GDP. This gene has a central role in the synthesis of the lipid-linked oligosaccharide precursor which becomes the core of all asparagine-linked (N-linked) glycans. The mntA gene contains a single small intron and encodes a 493 aa protein with a predicted molecular size of 56 kDa. It is located 5' to the repE gene on chromosome IV and is transcribed in the opposite orientation to repE with which it shares a 585 bp of upstream intergenic region. The predicted mntA gene product shares 38% homology with the S. cerevisiae ALG1 gene product. The MntA protein has a region homologous to the putative dolichol-binding region in the yeast ALG1 protein, but it is located in a different part of the molecule. Northern analysis revealed that the expression of the mntA gene is regulated during multicellular development with two periods of mRNA accumulation. The mntA gene product has a classical endoplasmic reticulum retention motif, and is the first Dictyostelium gene encoding a protein that is active in this organelle. The identification of this gene will allow expanded studies of the role of N-linked glycans in multicellular development.

A lysosomal cysteine proteinase from Dictyostelium discoideum contains N-acetylglucosamine-1-phosphate bound to serine but not mannose-6-phosphate on N-linked oligosaccharides

Previous studies showed that vegetative Dictyostelium discoideum cells make a lysosomal proteinase, proteinase-1, that contains multiple GlcNAc-alpha-1-P residues in phosphodiester linkage to serine. We extended these studies and, in contrast to earlier reports, found that proteinase-1 contains 7.5 mol of Fuc, 8 mol of Man, 2 mol of Xyl, and 30 mol of GlcNAc per calculated mol of protein but no Man-6-P residues. The protein binds to concanavalin A and wheat germ agglutinin lectin affinity columns, and PNGase-F digestion released most of the mannose and xylose but little of the GlcNAc. beta-Elimination under reducing conditions released only GlcNAc-alpha-1-P. There was no evidence for the release of disaccharides or of fucitol. A rabbit antiserum and monoclonal antibodies prepared against proteinase-1 recognize GlcNAc-alpha-1-P residues in immunoblots and are specifically competed by UDP-GlcNAc or GlcNAc-alpha-1-P. Use of other monoclonal antibodies showed the presence of mannose-6-sulfate on N-linked sugar chains, and alpha-fucose residues on the protein. Thus, proteinase-1 has at least two types of modifications: Glc NAc-alpha-1-P-Ser, which we call phosphoglycosylation, and N-linked oligosaccharides. This is the first purified lysosomal enzyme in Dictyostelium that does not contain Man-6-P residues. The GlcNAc-alpha-1-P-specific antibodies also recognize a group of developmentally regulated proteins, especially enriched in vegetative cells. Some of them are also lysosomal cysteine proteinases, and all bind to the GlcNAc-alpha-1-P-specific monoclonal antibody but not to the mammalian CI-Man-6-P receptor. Conversely, lysosomal enzymes that have Man-6-P do not bind to the GlcNAc-alpha-1-P-specific antibody. An exception to this is beta-N-acetylglucosaminidase, where 15% of the activity binds to this antibody. Thus, there appear to be two sets of lysosomal enzymes with distinct post-translational modifications.

Immunogenicity of the N-glycans of peanut peroxidase

The three tryptic glycopeptides of cationic peanut peroxidase (C. PRX) and the sole one of anionic peanut peroxidase (A. PRX) were individually coupled to bovine serum albumin to raise antisera. The three categories of antibodies directed towards three N-glycans of C. PRX (anti-GLa, anti-GLb and anti-GLc) were isolated from antisera with glycan-conjugated ECH Sepharose 4B affinity columns and the distribution of epitopes on the N-glycans was investigated. The reactivity of anti-GLa, anti-GLb and anti-GLc is inhibited 25-40% by 1 M fucose, compared with a slight inhibition by N-acetylglycosamine and xylose. Mannose and galactose showed no inhibition to anti-GLa and only a slight inhibition to anti-GLb and anti-GLc. All of anti-GLa, anti-GLb and anti-GLc recognize A. PRX and horseradish peroxidase but do not recognize fetuin. Also, their reactivity is inhibited by bromelain by more than 70%. The three categories of antibodies present high homogeneity and appear to be directed mainly towards the core structure [Xyl] (Man)3 [Fuc] (GlcNAc)2. An effective and simple method to screen antibodies with carbohydrate specificities is described herein.

Biochemical and genetic analysis of an antigenic determinant found on N-linked oligosaccharides in Dictyostelium

Dictyostelium discoideum synthesizes many highly immunogenic carbohydrates of unknown structure and function. We have used monoclonal antibodies prepared against one of these called CA1 to investigate its structure and the consequences of its loss. CA1 is preferentially expressed on lysosomal enzymes as a specific arrangement of mannose-6-SO4 residues on N-linked oligosaccharides. Mutant strains HL241 and HL243 do not express CA1, and synthesize a truncated lipid-linked oligosaccharide (LLO) precursor that lacks the critical mannose residues needed for expression. The lesion appears to result from the loss of mannosyl transferase activity involved in LLO biosynthesis. The truncated LLO is poorly transferred to an artificial peptide acceptor in a cell-free N-glycosylation assay, and this appears to result from improper topological localization of the LLO or to a lower affinity of the LLO for the oligosaccharyl transferase. Although both mutants share these lesions, they are biochemically and genetically distinct. Only HL243 is lower in N-glycosylation in intact cells, and this is not a result of an altered structure of the LLO. There are other differences between the strains. HL241 can form fruiting bodies at a slower rate than normal while HL243 cannot aggregate. Genetic analysis of defects shows that the CA1 lesion in HL241 is recessive, while the lesion in both CA1 and in development are dominant and co-segregate in HL243 and are, therefore, likely to be in the same gene. Lysosomal enzyme targeting is normal but enzyme processing proceeds at a 2-3 fold slower rate in HL241 and HL243 compared to wild-type. Strain HL244 does not express CA1 since it completely lacks protein sulfation, but lysosomal enzyme targeting and processing proceeds at a normal rate, showing that sulfate is not essential for these processes. Alterations in oligosaccharide structure can have individualized effects on the biosynthesis of lysosomal enzymes. The results presented here illustrate how this approach can be used to study both the structure and function of carbohydrate epitopes.

Maturation of asparagine-linked oligosaccharides in Dictyostelium discoideum analyzed with modification-specific probes

Lysosomal enzymes in Dictyostelium discoideum contain high mannose oligosaccharides that contain mannose 6-phosphate and several unusual structures. The synthesis and distribution of these post-translational modifications were studied using probes for different carbohydrate groups. These probes include lectin-like antibodies directed to two distinct sulfated and one nonsulfated N-linked determinants, the lectin Con A, and the mammalian 215-kDa phosphomannosyl receptor. Only Con A binds to newly synthesized alpha-mannosidase present in the rough endoplasmic reticulum. The other modifications are acquired at different rates and are first detected on protein in light density Golgi-like membranes. Mutations which prevent protein transport to Golgi membranes block synthesis of these moieties, but inhibitors which prevent later transport steps have no effect. The majority of modified proteins are in lysosomes but significant amounts are delivered to nonlysosomal destinations. Different lysosomal proteins contain unequal amounts of each modification.