Site-specific processing of the N-linked oligosaccharides of the human chorionic gonadotropin α subunit

M. Bielinska, M. M. Matzuk, I. Boime

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31 Scopus citations

Abstract

Two forms of the gonadotropin α subunit are synthesized in placenta and in human chorionic gonadotropin (hCG)-producing tumors: an uncombined (monomer) form and a combined (dimer) form. These forms show differences in their migration on sodium dodecyl-sulfate-polyacrylamide gel electrophoresis. The slower migration of the monomeric form on sodium dodecyl sulfate-polyacrylamide gel electrophoresis has been attributed to a different glycosylation pattern. Previous studies demonstrated different roles of each of the two α N-linked glycosylation sites (Asn-52 and Asn-78) in secretion of the uncombined subunit and the biologic activity of hCG dimer. To assess the influence of formation of dimer on the processing pattern at the individual sites, we characterized the N-linked oligosaccharides of monomer and dimer forms of recombinant human choriogonadotropin α subunit. Two approaches were employed. First, site-directed mutagenesis was used to alter the two N-linked oligosaccharide attachment sites, thus allowing the expression of α subunits containing only one glycosylation site. Second, tryptic glycopeptides of the wild-type subunits were examined. Concanavalin A (ConA) binding and sialic acid content indicated that the oligosaccharides at each glycosylation site of the uncombined α subunit are processed differently. Oligosaccharides present at Asn-52 are almost exclusively ConA-unbound and contain three sialic acid residues. The majority of Asn-78-linked oligosaccharides are ConA-bound and disialylated. Both sites are processed independently because no significant differences were observed between the oligosaccharides at the same sites in wild-type and mutant monomeric α subunits. By contrast, the majority of the oligosaccharides at both glycosylation sites of the dimer α are bound to ConA. Thus, combination primarily affects the processing pattern of the Asn-52-linked species. Because glycosylation at this site is essential for hCG assembly and signal transduction, these data imply a critical link between the site-specific processing and hormone function.

Original languageEnglish
Pages (from-to)17113-17118
Number of pages6
JournalJournal of Biological Chemistry
Volume264
Issue number29
StatePublished - 1989

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