An investigation of the properties of TSH receptors on FRTL5 cells using affinity labelling with a 125 I-labelled photoactive derivative of TSH is described. Our studies suggest that FRTL5 cells contain 2 principal types of cell surface TSH receptors. One form, probably a precursor, consists of a single polypeptide chain (M r 120 000) with an intrachain loop of amino acids formed by a disulphide bridge. The other type of receptor consists of a water-soluble A chain (M r 55000) linked to an amphiphilic B chain (M r 35000) by a disulphide bridge. The 2 chain structure is probably derived from the single chain 120 000 protein by enzymatic cleavage of peptide sequences within the loop of amino acids formed by the intrachain disulphide bridge.
