Structure of the lutropin receptor on granulosa cells. Photoaffinity labeling with the alpha subunit in human choriogonadotropin.

Ovarian granulosa cells have specific receptors for lutropin and human choriogonadotropin (hCG). To investigate the nature of the receptor, several different photo-cross-linkable derivatives of radioiodinated hCG were incubated with porcine granulosa cells and hormone derivative-receptor complexes extracted with 0.5% Triton X-100. The extracted complexes appeared in a single peak on gel permeations and in rat zonal sedimentation studies. The estimated molecular weight was found to be 250,000-370,000 with an associated s value of 7.7 +/- 0.3. When Triton X-100 extracts were examined by gel electrophoresis, the autoradiograph revealed four bands. Three were conspicuous, corresponding for molecular weights of 96,000 +/- 5,000, 76,000 +/- 4,000, and 73,000 +/- 4,000, but a fourth band, corresponding to a molecular weight of 120,000 +/- 6,000, was less well defined. Reducing agents, 20 mM dithiothreitol and 2% beta-mercaptoethanol, acid, and 3 M urea did not affect the production of the bands. Furthermore, the bands once formed were resistant to these agents, indicating 1) covalent nondisulfide bond formation in photoaffinity labeling and 2) noncovalent association of photoaffinity-labeled membrane components with the natural hormone receptor.