The thyroid-stimulating hormone receptor (TSHR) is an integral membrane glycoprotein which forms a binding site for TSH on the outside surface of thyroid follicular cells. The level of expression of the TSHR on the cell surface is very low (in the region of 1000-10000 sites per cell) and consequently special techniques are required to study the receptor .
Before the receptor had been cloned, most data regarding its structure were provided by studies with affinity-labelled receptors . In these studies, stable complexes of TSH and TSHR were formed, thus allowing analysis by a variety of techniques such as polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate (SDS) gel filtration and density gradient centrifugation. This type of analysis has indicated that the porcine TSHR consists of two subunits linked by a disulphide bridge . One of the subunits (A, mol. mass 50 kDa) is water soluble and forms the binding site for TSH on the outside surface of the membrane. The other subunit (B, mol. mass 30 kDa) penetrates lipid bilayer. The noncovalent bonds between the A and B subunits of the receptor are readily dissociated at neutral pH in the presence of 50 mM NaCI . Consequently, when thyroid membranes are reduced under these conditions, the TSH receptor A subunit is released. The release is much less in the absence of NaCI, indicating that noncovalent interactions between the A and B subunits are markedly dependent on the ionic environment.
A similar two-subunit structure was also found for the human TSHR. In the case of FRTL s cells (an immortalised rat thyroid cell line which expressed functional TSHR), two principal types of cell surface receptors were found . One form, probably a precursor, consists of a single polypeptide chain (mol. mass 120 kDa) with an intrachain loop of amino-acids formed by a disulphide bridge. The other type of receptor consists of a water-soluble A chain (mol. mass 55 kDa) linked to an amphiphilic B chain (mol. mass 35 kDa) by a disulphide bridge. The