The role of the asparagine-linked oligosaccharides of the α subunit in the secretion and assembly of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) is a member of a family of heterodimeric glycoprotein hormones that have a common α subunit but differ in their hormone-specific β subunit. Site-directed mutagenesis of the two asparagine-linked glycosylation sites of hCGα was used to study the function of the individual oligosaccharide chains in secretion and subunit assembly. Expression vectors for the α genes (wild-type and mutant) and the hCGβ gene were constructed and transfected into Chinese hamster ovary cells. Loss of the oligosaccharide at position 78 causes the mutant subunit to be degraded quickly and <20% is secreted. However, the presence of hCGβ stabilizes this mutant and allows ~45% of the subunit in the form of a dimer to exit the cell. Absence of carbohydrate at asparagine 52 does not perturb the stability or transport of the α subunit but does affect dimer secretion; under conditions where this mutant or hCGβ was in excess, <30% is secreted in the form of a dimer. Mutagenesis of both glycosylation sites affects monomer and dimer secretion but at levels intermediate between the single-site mutants. We conclude that there are site-specific functions of the hCGα asparagine-linked oligosaccharides with respect to the stability and assembly of hCG.