Thyrotropin cross-links to the thyrotropin receptor through both the alpha and beta subunits

We have recently shown that the beta subunit of thyrotropin (TSH) can be cross-linked to the TSH receptor [Buckland, Strickland, Pierce & Rees Smith (1985) Endocrinology (Baltimore) 116, 2122-2124; Buckland, Strickland & Rees Smith (1985) Biochem. Soc. Trans. 13, 942-943]. We failed, however, to cross-link the alpha subunit to the receptor, leaving the role of this subunit in the TSH-TSH-receptor interaction uncertain. We now report the successful cross-linking of the TSH alpha subunit to the receptor by the use of two different cross-linking reagents. Our studies suggest therefore that both subunits of TSH form part of the hormone's receptor-binding site.