Gonadotropin beta subunits determine the rate of assembly and the oligosaccharide processing of hormone dimer in transfected cells.

Abstract

The glycoprotein hormones lutropin (LH) and chorionic gonadotropin (CG) share a common structure consisting of an identical alpha subunit noncovalently linked to a hormone-specific beta subunit. While LH is produced in the anterior pituitary, CG is synthesized in placenta. To compare the assembly, processing, and secretion of human LH and CG in the same cell type, we have expressed their subunits, individually and together, in mouse C-127 mammary tumor cells. Analysis of transfected clones revealed an unexpected difference in the secretion of individually expressed subunits. Whereas alpha and CG beta subunits were rapidly and quantitatively secreted, only 10% of newly synthesized LH beta subunit reached the medium. The remaining subunit was found in an intracellular, endoglycosidase H (endo H)-sensitive pool that had a turnover rate of approximately 8 h. Coexpression with alpha subunit resulted in "rescue" of LH beta subunit by formation of LH dimer, which was efficiently secreted. However, combination of LH beta with alpha was slow, with an overall efficiency of only 50% despite the presence of excess alpha. In contrast, CG beta was rapidly assembled with the alpha subunit after synthesis. The two beta subunits also differed in their influence on the N-linked oligosaccharide processing of combined alpha. The oligosaccharides of LH dimer were endo H resistant, while those of CG dimer remained partially endo H sensitive. Thus, despite a high degree of homology between LH beta and CG beta, the two subunits differ in their secretion as free subunits, their rate of assembly with alpha subunit, and in their effect on the N-linked oligosaccharide processing of combined alpha.