The genomic structure of the LH receptor is important to our understanding of its expression mechanisms, functional domains, relationships with other hormone receptors, and evolution. We have isolated four overlapping cosmid clones and six subgenomic clones of the rat LH receptor gene. They span a total of 95.6 kilobases (kb) and extend from 23 kb upstream of the translation start site to 13 kb down-stream of the stop codon. In addition, part of the human LH receptor gene has been isolated. The coding region of the rat hormone receptor gene spans over 60 kb and consists of 11 exons and 10 introns. Southern blots hybridized with exon 1 and exon 11 probes as well as gene dose analyses demonstrate that a single copy gene encodes the rat LH receptor. Sequence comparison suggests that the porcine and human LH receptor genes have similar, if not identical, exon-intron structures. There is no consensus cAMP-responsive element within 600 basepairs up-stream of the translation start site in spite of the cAMP responsiveness of the LH receptor gene. There are, however, unconventional cAMP-responsive elements in the region: one which is identical, several which are homologous to the activating protein-2-binding elements, CCCCAGGC, and several sequences which are similar to the G-rich cAMP-responsive element found in P450c21, a steroid 21-hydroxylase. The first 10 exons encode the N-terminal half of the molecule, while exon 11 encodes the C-terminal half of the molecule. This last exon is the same in the rat and human genes. The DNA and amino acid sequences of the first 10 exons show significant similarities and reveal repetitive sequence motifs. They have similar sizes which occur in the range of 69 and 183 bases; 8 of them are from 69-81 bases. Despite these remarkable similarities, structural predictions of exons 1-10 show a diversity of structures. The N-terminal half of the LH receptor appears to have a folded structure, with frequent turns and an extensive surface area. Part of the surface is predicted to be covered by amphiphilic helices and beta structures, types of secondary structure frequently found at the interfaces between subunits or between 2 interacting molecules. The introns dividing these exons also share many similarities.(ABSTRACT TRUNCATED AT 400 WORDS)
We have examined roles of carbohydrates of the lutropin receptor in a murine Leydig tumor cell line (MLTC) and primary cultures of rat granulosa cells. We approached this issue by deglycosylating mature receptors with glycosidases and by preventing glycosylation of nascent receptors with tunicamycin B2, an inhibitor of protein glycosylation but not protein synthesis. Deglycosylation of mature receptors with neuraminidase, N-glycanase or both did not affect ligand binding capacity. Regardless of glycosidase treatment, the number of hormone binding sites was similar. The Kas for native receptors, asialoreceptors and aglycoreceptors, are also comparable, being 2.0 x 10(9) M-1, 1.9 x 10(9) M-1 and 1.7 x 10(9) M-1 respectively. In contrast, cells treated with tunicamycin B2 failed to bind the hormone. These results demonstrate that N-oligosaccharides of mature lutropin receptors are not required for ligand binding. In addition, our data suggest, for the first time, that N-glycosylation of the receptor may be necessary for expressing functional receptors on the cell surface and that there exist striking similarities in roles of oligosaccharides of lutropin and its receptor.
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