PARENT SESSION

(T758) THE CARBOXYL TERMINAL SEQUENCE IN THE CG SUBUNIT IS A DETERMINANT FOR POST-TRANSLATIONAL PROCESSING OF THE ASPARAGINE-LINKED OLIGOSACCHARIDES IN GH₃ CELLS.

Jablonka-Shariff, Albina¹, Boime, Irving¹, ¹ Washington University, St. Louis, MO

ABSTRACT- Lutropin (LH), follitropin (FSH) and chorionic gonadotropin (CG) are heterodimers comprised of a common subunit and a hormone-specific subunit. A critical feature of these proteins is their unique secretory patterns. LH is released through a regulated secretory pathway, while FSH is primarily secreted constitutively and CG displays features of both pathways. The asparagine-linked (N) oligosaccharides on these glycoproteins are hormone-specific and are key functional components. The N-linked carbohydrate on LH terminates with sulfate, whereas the mature form of placental CG contains sialic acid. Previously, we reported that this secretion could be recapitulated in GH₃ cells, which are derived from pituitary somatotropes and secrete proteins through the constitutive and regulated pathways. The GH₃ line synthesizes sulfated N-linked carbohydrates on the LH subunit. These data suggested that the GH₃ line could be used to study the determinants that govern sulfate addition to the N-linked oligosaccharides. Earlier we also demonstrated that although majority of CG was released constitutively, a large fraction of the CG accumulated in secretory vesicles. The LH and CG subunits share 85% amino acid identity in the first 114 amino acids. A major difference between them is the presence in the CG of a 31-amino acid carboxyl-terminal extension (CTP) compared with hydrophobic heptapeptide in LH subunit. To examine sorting of LH and CG, we constructed mutants and study their secretion patterns from GH₃ cells. Deleting the heptapeptide from LH subunit increased secretion and assembly with subunit but reduced the extent of sulfation compared to the wild type subunit. By contrast, deleting the CTP from the CG subunit enhanced sulfation on the N-linked oligosaccharides, which, reflects an increased similarity to the LH subunit. These data suggest that the carboxyl terminus of the CG and LH subunits influence the post-translation of the heterodimers.

KEY WORDS: gonadotropins, sulfation, pituitary, sorting