Structure and expression of a membrane component of the inhibin receptor system

The purification and cloning of a membrane-anchored proteoglycan with affin ity for inhibin A are described. Bovine pituitary membranes were isolated, and membrane-anchored proteins were solubilized and used as an enriched sou rce of inhibin binding protein. The extract was passed over an inhibin A af finity column, and a protein, designated p120, was identified as an inhibin -binding moiety. A partial amino acid sequence was determined for the prote in, which matched two human complementary DNAs (cDNAs) in the database. The full-length cDNA predicts a 1336-amino acid glycoprotein. Full-length p120 -encoding cDNAs were isolated from human testis RNA and cloned into express ion vectors. Two p120 messenger RNA transcripts of 4.6 kb and 2 kb are dete cted in rat pituitary by RNA blot analysis. Similar analysis of rat testis RNA revealed transcripts of identical molecular mass, albeit at lower abund ance. To determine the cellular localization of p120 in pituitary and testi s, an antibody directed against the predicted extracellular domain of the p rotein was generated and used in an immunohistochemical analysis of thin ti ssue sections. p120 immunostaining is coincident with FSH beta immunopositi ve gonadotrope cells in rat pituitary, p120 staining is intense in the test icular Leydig cells, which bind iodinated inhibin but not iodinated activin . In summary, an inhibin-binding protein has been isolated that is produced in tissues that are targets of inhibin action.