A pituitary N-acetylgalactosamine transferase that specifically recognizes glycoprotein hormones

Abstract

The glycoprotein hormones lutropin (LH) and follitropin (FSH), which have common alpha-subunits but hormone-specific beta-subunits, are both synthesized in the gonadotroph. However, they bear Asn-linked oligosaccharides that differ in structure. Those on LH terminate with the sequence SO4-4GalNAc beta 1----4GlcNAc beta 1----2Man alpha, whereas those on FSH terminate with the sequence sialic acid alpha-Gal beta 1----4GlcNAc beta 1----2Man alpha. A GalNAc-transferase was identified in bovine pituitary membranes that recognizes features of the alpha-subunit peptide and adds GalNAc to its oligosaccharides with an apparent Michaelis constant of 25 micromolar. The different patterns of glycosylation for LH and FSH indicate that access to the protein recognition marker on the alpha-subunit is modulated by the associated beta-subunit. The tightly regulated synthesis of sulfated and sialylated oligosaccharides on the pituitary glycoprotein hormones suggests these oligosaccharides have an important biological role.