Is receptor cleavage into two subunits necessary for thyrotropin action?

Unlike the wild-type thyrotropin (TSH) receptor, the chimeric TSH-LH/CG receptor TSH-LHR-14 does not cleave into two subunits when cross-linked to [125I]TSH on the surface of intact cells. Immunoblotting of TSH-LHR-14 in whole cell homogenates demonstrated that only a single chain receptor was detected under reducing conditions. TSH-LHR-14, like the A subunit of another chimeric receptor (TSH-LHR-10) that does cleave into two subunits, was almost entirely resistant to endoglycosidase H, indicating that it contains predominantly complex carbohydrate. The fact that TSH-LHR-14 reaches the cell surface where it binds TSH with high affinity and transduces a signal indicates that receptor cleavage into two subunits is not a prerequisite for TSH action.