Covalent crosslinking of thyrotropin to thyroid plasma membrane receptors: Subunit composition of the thyrotropin receptor

Abstract

The subunit composition of the thyrotropin (TSH) receptor has been characterized using the bifunctional crosslinking agent, disuccinimidyl suberate (DSS), to covalently link [ 125I]TSH to its receptor. Purified thyroid membranes were labeled with [ 125I]TSH, and the hormone-receptor complex was crosslinked by incubation with 0.1 m m DSS. Analysis of this crosslinked complex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions indicated the presence of a specifically labeled hormone-receptor complex, corresponding to a M r of 68,000 ± 3000 before correction for the relative molecular mass of TSH. When reducing agents were absent during SDS solubilization, the mobility of the band increased slightly, suggesting the presence of intramolecular disulfide bonds. The labeling of the 68,000 band was specifically inhibited by TSH, but not by other glycoprotein hormones. Specific labeling occurred only in thyroid, and not in liver or muscle plasma membranes. Protease-free immunoglobulin G, isolated from sera of patients with Graves' disease and capable of competing with TSH for binding to its receptor, inhibited the labeling of the 68,000 complex. When the hormone-receptor complex was crosslinked with higher concentrations of DSS (>0.3 m m), a second specifically labeled band was observed, with a M r of 80,000 ± 5000. This complex exhibited hormone, tissue, and immunologic specificities similar to those of the 68,000 band. Continuous sucrose density gradient analysis indicated that the intact solubilized receptor possessed a sedimentation coefficient of 10.5 S prior to correction for detergent binding. However, this value increased to 16 S when determined under conditions which took into account the change in hydrodynamic properties attributable to bound Triton X-100. These data suggest that the 80,000 and 68,000 bands represent binding components of the TSH receptor and that the receptor molecule most likely contains multiple subunits, linked by noncovalent forces.