Role of the Pro-Leu-Arg motif in glycosylation of human gonadotropin alpha-subunit.

CG, LH, FSH, and TSH are a family of heterodimeric glycoprotein hormones that contain a common alpha-subunit, but differ in their hormone-specific beta-subunit. Processing of the N-linked oligosaccharide of the glycoprotein family is both tissue and dimer specific. LH, TSH, and free alpha synthesized in pituitary bear oligosaccharide terminating with sulfate (SO4) and N-acetylgalactosamine (GalNAc), whereas the termination of oligosaccharide in CG synthesized in placenta and FSH is sialic acid and galactose (Gal). Using site-directed mutagenesis and gene transfer, we studied the role of the Pro-Leu-Arg motif, which has been shown to be a recognition marker of glycoprotein hormone-specific GalNAc transferase, in sulfation of N-linked oligosaccharide in alpha-subunit. The wild-type or mutated alpha gene was transfected into GH3 cells. Our data revealed that substitution of the Pro-Leu-Arg motif by Ala-Leu-Ala did not affect the sulfation of N-linked oligosaccharide, but generated the attachment of O-linked oligosaccharide. alpha-Subunit containing either of the two N-linked glycosylations is also sulfated. We conclude that in GH3 cells, the Pro-Leu-Arg motif plays no role in the sulfation of oligosaccharide in alpha-subunit, and both N-glycosylations are terminated with SO4.