A new Saccharomyces cerevisiae mnn mutant N-linked oligosaccharide structure

Structural classification of carbohydrates in glycoproteins by mass spectrometry and high-performance anion-exchange chromatography

A general strategy has been developed for determining the structural class (oligomannose, hybrid, complex), branching types (biantennary, triantennary, etc.), and molecular microheterogeneity of N-linked oligosaccharides at specific attachment sites in glycoproteins. This methodology combines mass spectrometry and high-performance anion-exchange chromatography with pulsed amperometric detection to take advantage of their high sensitivity and the capability for analysis of complex mixtures of oligosaccharides. Glycopeptides are identified and isolated by comparative HPLC mapping of proteolytic digests of the protein prior to, and after, enzymatic release of carbohydrates. Oligosaccharides are enzymatically released from each isolated glycopeptide, and the attachment site peptide is identified by fast atom bombardment mass spectrometry (FAB-MS) of the mixture. Part of each reaction mixture is then permethylated and analyzed by FAB-MS to identify the composition and molecular heterogeneity of the carbohydrate moiety. Fragment ions in the FAB mass spectra are useful for detecting specific structural features such as polylactosamine units and bisecting N-acetylhexosamine residues, and for locating inner-core deoxyhexose residues. Methylation analysis of these fractions provides the linkages of monomers. Based on the FAB-MS and methylation analysis data, the structural classes of carbohydrates at each attachment site can be proposed. The remaining portions of released carbohydrates from specific attachment sites are preoperatively fractionated by high-performance anion-exchange chromatography, permethylated, and analyzed by FAB-MS. These analyses yield the charge state and composition of each peak in the chromatographic map, and provide semiquantitative information regarding the relative amounts of each molecular species. Analytically useful data may be obtained with as little as 10 pmol of derivatized carbohydrate, and fmol sensitivity has been achieved. The combined carbohydrate mapping and structural fingerprinting procedures are illustrated for a recombinant form of the CD4 receptor glycoprotein.

Identification and characterization of a gene and protein required for glycosylation in the yeast Golgi

The MNN2 gene of Saccharomyces cerevisiae has been cloned by complementation of the mnn2 mutant phenotype scored by a change in cell surface carbohydrate structure resulting from a lack of alpha 1----2-mannose branching in the outer chain. The gene was subcloned as a 3 kb DNA fragment that integrated at the MNN2 locus, and a gene disruption yielded the mnn2 phenotype. A lacZ-MNN2 gene fusion protein, produced in Escherichia coli, was used to raise a specific antiserum that recognized a 65 kD wild-type yeast protein. This MNN2 gene product lacks N-linked carbohydrate but appears to be an integral membrane protein. Overproduction of MNN2p does not enhance the alpha 1----2-mannosyltransferase activity of yeast cells. The results suggest that MNN2p is a Golgi-associated protein that is involved in mannoprotein sorting rather than glycosylation.

Characterization of O-glycosylation sites in recombinant B-chain of platelet-derived growth factor expressed in yeast using liquid secondary ion mass spectrometry, tandem mass spectrometry and Edman sequence analysis

High-performance tandem mass spectrometry has been employed to structurally characterize intact O-linked glycopeptides and establish the complexity and extent of glycosylation for recombinant human platelet-derived growth factor B chain (rhPDGF-B) expressed in yeast. In addition, liquid secondary ion mass spectrometry (LSIMS) and Edman degradation have been employed to verify the protein sequence. LSIMS of high-performance liquid chromatographically fractionated proteolytic digests confirmed the complete amino acid sequence predicted by the human PDGF-B gene structure. Potential glycopeptides (as indicated by a mass shift of 162 or 324 Da from the mass of a predicted cleavage product) were sequenced using tandem mass spectrometry and Edman degradation. Ultraviolet matrix laser desorption mass spectrometry of rhPDGF-B dimer was used to determine the molecular weight distribution for the intact recombinant glycoprotein. In addition to the presence of unmodified peptides, corresponding peptides bearing monomannosyl moieties were found on serine 26 and threonines 20, 63, 88, 90 and 101. Further, dimannosyl moieties were found on threonines 6 and 63. These data reveal the presence of O-linked glycosylation at sites which do not fortify the concept of a consensus sequence involving proline residues, but which strengthen the concept of secondary and tertiary structure requirements. The advantages of high-energy collisionally induced dissociation analysis of O-linked glycopeptides over conventional base elimination and borohydride reduction and other mass spectrometric techniques are presented for the first time.

Two glycosylation patterns for a single protein (exoglucanase) in Saccharomyces cerevisiae

Exoglucanases (beta-glucosidases) I and II secreted into the culture medium by Saccharomyces cerevisiae were purified from cell cultures harvested at the early exponential phase of growth in order to avoid contamination of the second by a new immunologically-related material. The amino acid composition of the purified enzymes was roughly the same. In addition, both exoglucanases exhibited an identical NH2-terminal sequence (50 residues). These results confirm our previous results about the identity of the protein moieties of both enzymes. Exoglucanase I appears to arise by elongation of one or both short oligosaccharides present in enzyme II.

Separation of yeast asparagine-linked oligosaccharides by high-performance anion-exchange chromatography

Oligosaccharides obtained from Saccharomyces cerevisiae mannoproteins by digestion with endo-N-acetyl-beta-D-glucosaminidase H were fractionated by anion-exchange chromatography, by elution with 50-100mM NaOH without or with a sodium-acetate gradient, and detected with a pulsed amperometric detector (PAD). The elution times of homologous oligosaccharides fell on a straight line having a slope characteristic of the structural type. The response of the PAD detector per mole of oligosaccharide increased about 2-fold going from Man3GlcNAc to Man13GlcNAc, and appeared to depend primarily on the oxidation of the reducing-end N-acetylglucosamine unit common to all the oligosaccharides. The digestion of a Man10GlcNAc with jack-bean alpha-mannosidase was monitored by injecting portions of the crude reaction mixture, and the intermediates were characterized by their elution positions and n.m.r. spectra in the anomeric proton region. One commercial jack-bean alpha-mannosidase preparation contained a novel endolytic activity that released N-acetylglucosamine from the reducing ends of the oligosaccharides and was shown to convert P----6 alpha Man----6 alpha Man----6 beta Man----4 alpha beta GlcNAc to P----6 alpha Man----6 alpha Man----6 alpha beta Man plus free N-acetylglucosamine. Another commercial jack-bean alpha-mannosidase converted the Man10GlcNAc to a Man3GlcNAc having the structure alpha Man----6 beta Man----4 alpha beta GlcNAc, [formula: see text] whereas the Oerskovia sp. alpha-mannosidase converted the same oligosaccharide to a Man4GlcNAc having the structure alpha Man----6 alpha Man----6 beta Man----4 alpha beta GlcNAc. [formula: see text]

Protein glycosylation defects in the Saccharomyces cerevisiae mnn7 mutant class. Support for the stop signal proposed for regulation of outer chain elongation

Total cell mannoprotein was isolated from Saccharomyces cerevisiae X2180 mutants that have defects in elongation of the outer chain attached to the N-linked core oligosaccharides (mnn7, mnn8, mnn9, and mnn10) (Ballou, L., Cohen, R. E., and Ballou, C. E. (1980) J. Biol. Chem. 255, 5986-5991). Comparison of the oligosaccharides released by endoglucosaminidase H digestion confirmed that the mnn9 mutation eliminates all but two mannoses of the outer chain, whereas the mnn8 and mnn10 strains produce outer chains of variable but similar lengths. The isolate designated mnn7 was found to be allelic with mnn8. Haploid mutants of the type mnn8 mnn9 or mnn9 mnn10 had the mnn9 phenotype, which established that the mnn9 defect is dominant and presumably acts at a processing step prior to the steps affected by mnn8 and mnn10. Analysis of the mnn1 mnn2 mnn10 oligosaccharides revealed that the heterogeneous outer chain contained 6-16 alpha 1----6-linked mannose units and each was terminated by a single alpha 1----2-linked mannose unit, whereas the core lacked one such unit that was present in the mnn9 oligosaccharide. The results are consistent with and support the hypothesis (Gopal, P. K., and Ballou, C. E. (1988) Proc. Natl. Acad. Sci. U.S.A. 84, 8824-8828) that addition of such a side-chain mannose unit is associated with termination of outer chain elongation in these mutants and may serve as a stop signal that regulates outer chain synthesis in the parent wild-type strain.