Hormone-binding properties and glycosylation pattern of a recombinant form of the extracellular domain of the luteinizing hormone/chorionic gonadotropin receptor expressed in mammalian cells.

The present studies were undertaken to characterize the properties of a recombinant truncated rat LH/CG (rLH/CG) receptor representing the extracellular domain (rLHR-t338) expressed in mammalian cells. Using detergent-soluble extracts of stably transfected cells, we show that the binding of ovine (o) LH to the extracellular domain is different from the binding of oLH to the full-length receptor. Thus, whereas the full-length receptor from either rat corpora lutea or stably transfected 293 cells binds oLH with two apparent affinities, rLHR-t338 binds oLH with only one affinity, which is intermediate between the high and low affinities of the full-length receptors. However, rLHR-t338 as well as the full-length receptor binds hCG and hLH with similar high affinities and by a model assuming the presence of only one binding site. These data are consistent with the possibility that the carboxyl-terminal half of the LH/CG receptor may contribute (either directly or indirectly) to a second low affinity hormone-binding site. The truncated receptor retains normal binding specificity, as it does not bind hFSH, demonstrating that the extracellular domain alone contains all of the necessary elements for conferring specificity among the gonadotropin hormones. Our studies also show that rLHR-t338 is glycosylated and migrates as two proteins of 48 and 44 kilodaltons (kDa) on sodium dodecyl sulfate gels (where the 44-kDa species is probably a partially degraded form of the 48-kDa protein). However, the N-linked oligosaccharides of the truncated receptor do not appear to be fully processed, as they remain in a high mannose form. rLHR-t338 is not secreted from cells, but remains trapped intracellularly. It can be detected, though, in detergent-soluble extracts of cells or in homogenates. Upon centrifugation of homogenates, it was found that about 90% of the recoverable truncated receptor can be obtained in the supernatant fraction. Thus, mammalian cells could provide a source of a water-soluble extracellular domain of the LH/CG receptor.