title: Posttranslational modifications of the lutropin receptor: mass spectrometric analysis.
abstract: Our present knowledge of the lutropin (LH/hCG) receptor structure derives from deductions made from its amino acid sequence as established by studying the cDNA. To obtain direct experimental information, luteinizing hormone (LH) receptor expressed in L cells was immunopurified in sufficient amounts to warrant analysis by mass spectrometry and microsequencing. The mature receptor, complexed to human chorionic gonadotropin (hCG), was purified by using monoclonal antibodies recognizing the hormone, whereas the mannose-rich non-hormone-binding precursor was purified by use of antireceptor antibodies. Determination of the N-terminus showed that (2)/(3) of protein molecules started at Thr24 whereas (1)/(3) started at Ala28. All these molecules bound hCG, suggesting that the most N-terminal region of the receptor does not participate in hormone binding. Six N-glycosylation sites have been predicted from the amino acid sequence. One of them (Asn299) was found to be nonglycosylated in both the precursor and the mature protein. The most heavily glycosylated residue was Asn291, followed by Asn195 and Asn99. These three sites accounted for 82% and 97% of carbohydrate moieties in the mature receptor and in the mannose-rich precursor, respectively. The presence of some receptor molecules nonglycosylated at sites 99, 174, and 195 in hormone-receptor complexes dismisses a direct role of these glycosylation sites in hormone binding or in the correct folding of the protein. The mature carbohydrate chains were homogeneous at position 174, 195, and 313 (absence of Golgi mannosidase II activity at positions 174 and 313, absence of GlcNAc tranferases III and IV activity at position 195). Heterologous carbohydrates were present at sites 99 and 291. The latter, which is highly variable in carbohydrate chains, is unlikely to participate in a direct interaction with hormone. Site 313 thus remains as the main candidate for a role in hormone binding.
authors.affiliation: Unité de Recherche INSERM Unité 135, Hormones, Gènes et Reproduction, Hôpital Bicêtre, 94275 Le Kremlin Bicêtre Cedex, France.
authors.forename: M T
authors.initials: MT
authors.lastname: Vu-Hai
authors.forename: J C
authors.initials: JC
authors.lastname: Huet
authors.forename: K
authors.initials: K
authors.lastname: Echasserieau
authors.forename: J M
authors.initials: JM
authors.lastname: Bidart
authors.forename: C
authors.initials: C
authors.lastname: Floiras
authors.forename: J C
authors.initials: JC
authors.lastname: Pernollet
authors.forename: E
authors.initials: E
authors.lastname: Milgrom
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002240:Carbohydrate Sequence; D006063:Chorionic Gonadotropin / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D006031:Glycosylation; D007739:L Cells; D013058:Mass Spectrometry; D051379:Mice; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000096:biosynthesis; D009943:Organophosphorus Compounds; D010446:Peptide Fragments / Q000737:chemistry; D011485:Protein Binding; D011498:Protein Precursors / Q000737:chemistry* / Q000302:isolation & purification; D011499:Protein Processing, Post-Translational; D011974:Receptors, LH / Q000737:chemistry* / Q000302:isolation & purification; D020539:Sequence Analysis, Protein
chemical_list: C043517:2-(4-isothiocyanatophenoxy)-1,3,2-dioxaphosphinene 2-oxide; D006063:Chorionic Gonadotropin; D009844:Oligosaccharides; D009943:Organophosphorus Compounds; D010446:Peptide Fragments; D011498:Protein Precursors; D011974:Receptors, LH