Carbohydrate moiety of follitropin receptor is not required for high affinity hormone-binding or for functional coupling between receptor and guanine nucleotide-binding protein in bovine calf testis membranes

Abstract

Previously, we have extensively studied FSH-receptor interactions using bovine calf testis membranes, and demonstrated that the high-affinity FSH binding to receptors and coupling of FSH receptors with guanine nucleotide-binding protein (Gs protein) in a GTP-sensitive state are important initial events in FSH action. In this study, using the same plasma membrane system, we examined the glycoprotein nature of the FSH receptor and determined the contribution of carbohydrate moieties to these functions of the FSH receptor. Our approach involved enzymic deglycosylation of FSH receptors present in calf testis plasma membranes and then removal of incompletely deglycosylated FSH receptors by lectin affinity chromatography. Following treatment of testis membranes with peptide N-glycosidase, the receptor, as identified by ligand-blot analysis, had a higher electrophoretic mobility indicating a decrease in M(r) from 240-200K. Treatment of testis membranes with neuraminidase caused a reduction (to approximately 225K) in the size of the receptor consistent with desialylation. However, digestion with O-glycosidase (endo-alpha-N-acetylgalactosaminidase) did not affect the mobility of the FSH receptor. These results suggest that bovine testis FSH receptor contains predominantly N-linked oligosaccharide chains, a finding which is consistent with recent predictions that N-linked glycosylation, but not O-linked glycosylation sites are present in cloned FSH receptor from rat testis. Moreover, calf testis membranes after treatment with peptide N-glycosidase F, were solubilized with Triton X-100 under optimum conditions that preserve the physical and functional coupling of FSH receptors with guanine nucleotide-binding protein, and then subjected to lectin affinity chromatography. Scatchard analysis indicated that intact and deglycosylated FSH receptors bound 125I-human FSH with similar affinities. In the presence of GTP, the binding of 125I-human FSH to intact and deglycosylated receptors decreased similarly and in a noncompetitive manner. Treatment of testis membranes with NAD plus cholera toxin, but not NAD plus pertussis toxin, eliminated the GTP effect on FSH binding to enzymic deglycosylated as well as intact receptors, suggesting that the guanine nucleotide binding protein mediating GTP regulation of FSH binding in these membranes is probably Gs protein. Our results suggest that the bovine testis FSH receptor contains predominantly N-linked oligosaccharide chains consistent with recently predicted N-linked glycosylation sites of cloned FSH receptor of rat testis. The bovine testis FSH receptor does not require N-linked carbohydrate for high-affinity hormone binding.