The Carboxyl-Terminal Region Is a Determinant for the Intracellular Behavior of the Chorionic Gonadotropin β Subunit: Effects on the Processing of the Asn-Linked Oligosaccharides

Abstract

Abstract The placental hormone human CG (hCG) consists of two noncovalently linked α- and β-subunits similar to the other glycoprotein hormones LH, FSH, and TSH. These heterodimers share a common α subunit but differ in their structurally distinct β subunits. The CGβ subunit is distinguished among the β subunits by the presence of a C-terminal extension with four serine-linked oligosaccharides (carboxyl terminal peptide or CTP). In previous studies we observed that deleting this sequence decreased assembly of the truncated CGβ subunit (CGβ114) with the α-subunit and increased the heterogeneity of the secreted forms of the uncombined subunit synthesized in transfected Chinese hamster ovary (CHO) cells. The latter result was attributed to alterations in the processing of the two N-linked oligosaccharides. To examine at what step this heterogeneity occurs, the CGβ and CGβ114 genes were transfected into wild-type and mutant CHO cell lines that are defective in the late steps of the N-linked carbohydrate-processing pathway. We show here that removal of the CTP alters the processing of the core mannosyl unit of the subunit to complex forms at both glycosylation sites and that the oligosaccharides contain polylactosamine. Although it has been presumed that there is little intramolecular interaction between the CTP and the proximal domains of the subunit, our data suggest that the CTP sequence participates in the folding of the newly synthesized subunit, which is manifest by the posttranslational changes observed here.