Point mutations of the thyrotropin receptor determining structural requirements for its ability to bind thyrotropin and to stimulate adenylate cyclase activity.

The two cysteines C494 and C569, located in the first and second extracellular loop, respectively, of the thyrotropin (TSH) receptor, were mutated to serines to test the functional significance of the putative disulfide bond between these two cysteines. Single (C494S and C569S) and double (C494/569S) mutant receptors were generated, transiently expressed in COS cells, and compared with regard to the ability to bind ligand and to mediate stimulation of adenylate cyclase activity. The double mutant retained ligand binding capacity, in contrast to the single cysteine mutants that were essentially devoid of binding capacity. The ability of the mutated receptor variants to stimulate adenylate cyclase activity was lost or greatly reduced.