Mary C. Beranek

Evolutionary Conservation of the Sulfated Oligosaccharides on Vertebrate Glycoprotein Hormones That Control Circulatory Half-life

The circulatory half-life of the mammalian glycoprotein hormone lutropin is controlled by its unique Asn-linked oligosaccharides, which terminate with the sequence SO-4-GalNAcβ1,4GlcNAc. A cluster of basic amino acids essential for recognition of the α subunit by the glycoprotein hormone:N-acetylgalactosaminyltransferase is located within two turns of an α helix (Mengeling, B. J., Manzella, S. M., and Baenziger, J. U.(1995) Proc. Natl. Acad. Sci. U. S. A. 92, 502-506). The amino acids within this region are virtually invariant in the α subunits of all vertebrates, indicating that the recognition determinant utilized by the N-acetylgalactosaminyltransferase has been conserved in species ranging from teleost fish to mammals. We demonstrate that the glycoprotein hormone:N-acetylgalactosaminyltransferase and the N-acetylgalactosamine-4-sulfotransferase responsible for the synthesis of these unique sulfated oligosaccharides are expressed in the pituitaries of vertebrates ranging from teleost fish to mammals. Furthermore, we show that Asn-linked oligosaccharides terminating with SO-4-GalNAcβ1,4GlcNAc are present on the α and β subunits of the salmon glycoprotein hormone GTH II. Asn-linked oligosaccharides terminating with SO-4-GalNAcβ1,4GlcNAc are unique structural features of the glycoprotein hormones that have been conserved during vertebrate evolution, suggesting they are critical for the expression of hormone biologic activity.

N-Linked Oligosaccharides on the Low Density Lipoprotein Receptor Homolog SorLA/LR11 Are Modified with Terminal GalNAc-4-SO4 in Kidney and Brain

Sorting protein-related receptor (SorLA/LR11) is a highly conserved mosaic receptor that is expressed by cells in a number of different tissues including principal cells of the collecting ducts in the kidney and neurons in the central and peripheral nervous systems. SorLA/LR11 has features that indicate it serves as a sorting receptor shuttling between the plasma membrane, endosomes, and the Golgi. We have found that a fraction of SorLA/LR11 that is synthesized in the kidney and the brain bears N-linked oligosaccharides that are modified with terminal β1,4-linked GalNAc-4-SO4. Oligosaccharides located in the vacuolar sorting (Vps) 10p domain (Vps10p domain) are modified with β1,4-linked GalNAc when the Vps10p domain is expressed in cells along with either of two recently cloned protein-specific β1,4GalNAc-transferases, GalNAcTIII and GalNAcTIV. Either of two sequences with basic amino acids located within the Vps10p domain is able to mediate recognition by these β1,4GalNAc-transferases. The highly specific modification of oligosaccharides in the Vps10p domain of SorLA/LR11 with terminal GalNAc-4-SO4 suggests that this unusual modification may modulate the interaction of SorLA/LR11 with proteins and influence their trafficking.

A necessary and sufficient determinant for protein-selective glycosylation in vivo.

A limited number of glycoproteins including luteinizing hormone and carbonic anhydrase-VI (CA6) bear N-linked oligosaccharides that are modified with β1,4-linked N-acetylgalactosamine (GalNAc). The selective addition of GalNAc to these glycoproteins requires that the β1,4-N-acetylgalactosaminyltransferase (βGT) recognize both the oligosaccharide acceptor and a peptide recognition determinant on the substrate glycoprotein. We report here that two recently cloned βGTs, βGT3 and βGT4, that are able to transfer GalNAc to GlcNAc in β1,4-linkage display the necessary glycoprotein specificity in vivo. Both βGTs transfer GalNAc to N-linked oligosaccharides on the luteinizing hormone α subunit and CA6 but not to those on transferrin (Trf). A single peptide recognition determinant encoded in the carboxyl-terminal 19-amino acid sequence of bovine CA6 mediates transfer of GalNAc to each of its two N-linked oligosaccharides. The addition of this 19-amino acid sequence to the carboxyl terminus of Trf confers full acceptor activity onto Trf for both βGT3 and βGT4 in vivo. The complete 19-amino acid sequence is required for optimal GalNAc addition in vivo, indicating that the peptide sequence is both necessary and sufficient for recognition by βGT3 and βGT4.

Molecular Basis for Protein-specific Transfer of N-Acetylgalactosamine to N-Linked Glycans by the Glycosyltransferases β1,4-N-Acetylgalactosaminyl Transferase 3 (β4GalNAc-T3) and β4GalNAc-T4

Background: Luteinizing hormone and carbonic anhydrase-6 bear unique N-linked oligosaccharides terminating with LacdiNAc (GalNAcβ1,4GlcNAc). Results: Two related β1,4-N-acetylgalactosaminyltransferases have distinct specificities for peptide motifs on luteinizing hormone and carbonic anhydrase-6. Conclusion: Affinity for the peptide motif determines the efficiency of GalNAc transfer to oligosaccharide acceptors. Significance: Kinetic parameters define the basis for protein-specific synthesis of carbohydrate structures in vivo. Two closely related β1,4-N-acetylgalactosaminyltransferases, β4GalNAc-T3 and β4GalNAc-T4, are thought to account for the protein-specific addition of β1,4-linked GalNAc to Asn-linked oligosaccharides on a number of glycoproteins including the glycoprotein hormone luteinizing hormone and carbonic anhydrase-6 (CA6). We have utilized soluble, secreted forms of β4GalNAc-T3 and β4GalNAc-T4 to define the basis for protein-specific GalNAc transfer in vitro to chimeric substrates consisting of Gaussia luciferase followed by a glycoprotein substrate. Transfer of GalNAc by β4GalNAc-T3 and β4GalNAc-T4 to terminal GlcNAc is divalent cation-dependent. Transfer of GalNAc to glycoprotein acceptors that contain a peptide recognition determinant is maximal between 0.5 and 1.0 mm MnCl2; however, transfer is increasingly inhibited by concentrations of MnCl2 above 1 mm and by anion concentrations above 15 mm. In contrast, transfer of GalNAc to the simple sugar acceptor N-acetylglucosamine-β-p-nitrophenol (GlcNAcβ-pNP) is not inhibited by concentrations of MnCl2 or anions that would inhibit transfer to glycoprotein acceptors by >90%. This finding indicates that interaction with the peptide recognition determinant in the substrate is sensitive to the anion concentration. β4GalNAc-T3 and β4GalNAc-T4 have similar but distinct specificities, resulting in a 42-fold difference in the IC50 for transfer of GalNAc to chimeric glycoprotein substrates by agalacto human chorionic gonadotropin, comprising 29 nm for β4GalNAc-T3 and 1.2 μm for β4GalNAc-T4. Our in vitro analysis indicates that enzymatic recognition of the peptide determinant and the oligosaccharide acceptor are independent events.

Peptide-specific Transfer of N-Acetylgalactosamine to O-Linked Glycans by the Glycosyltransferases β1,4-N-Acetylgalactosaminyl Transferase 3 (β4GalNAc-T3) and β4GalNAc-T4

Background: LacdiNAc (GalNAcβ1,4GlcNAc) is present on O- and N-linked carbohydrate moieties of pro-opiomelanocortin. Results: β1,4-N-acetylgalactosaminyltransferases β4GalNAc-T3 and β4GalNAc-T4 mediate peptide-specific transfer of GalNAc to O-linked structures in vivo and in vitro. Conclusion: β4GalNAc-T3 and β4GalNAc-T4 can account for LacdiNAc sequences on O-linked structures on specific glycoproteins. Significance: The protein-specific addition of LacdiNAc to O-linked carbohydrates generates a family of unique structures recognized by carbohydrate-specific receptors. N- and O-linked oligosaccharides on pro-opiomelanocortin both bear the unique terminal sequence SO4-4-GalNAcβ1,4GlcNAcβ. We previously demonstrated that protein-specific transfer of GalNAc to N-linked oligosaccharides on glycoprotein substrates is dependent on the presence of both an oligosaccharide acceptor and a peptide recognition motif consisting of a cluster of basic amino acids. We characterized how two β1,4-N-acetylgalactosaminyltransferases, β4GalNAc-T3 and β4GalNAc-T4, require the presence of both the peptide recognition motif and the N-linked oligosaccharide acceptors to transfer GalNAc in β1,4-linkage to GlcNAc in vivo and in vitro. We now show that β4GalNAc-T3 and β4GalNAc-T4 are able to utilize the same peptide motif to selectively add GalNAc to β1,6-linked GlcNAc in core 2 O-linked oligosaccharide structures to form Galβ1,3(GalNAcβ1,4GlcNAcβ1,6)GalNAcαSer/Thr. The β1,4-linked GalNAc can be further modified with 4-linked sulfate by either GalNAc-4-sulfotransferase 1 (GalNAc-4-ST1) (CHST8) or GalNAc-4-ST2 (CHST9) or with α2,6-linked N-acetylneuraminic acid by α2,6-sialyltransferase 1 (ST6Gal1), thus generating a family of unique GalNAcβ1,4GlcNAcβ (LacdiNAc)-containing structures on specific glycoproteins.