Abstract
The luteinizing hormone receptor (LHR) consists of an approximately 350-amino acid-long N-terminal extracellular exodomain and a membrane-associated endodomain of similar size. Human chorionic gonadotropin (hCG) binds to the exodomain, and then hCG/exodomain complex is thought to make a secondary contact with the endodomain and generate hormone signals. The sequence alignment of the exodomain shows imperfectly matching eight to nine Leu-rich repeats (LRRs). In the preceding article (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435), we have shown that LRR2 and LRR4 are crucial for hormone binding. In this work, we have examined the residues of LRR4, in particular Leu(103) and Ile(105) in the putative beta strand. Our data show that Leu(103) and Ile(105) are involved in the specific, hydrophobic interaction of the LRR4 loop, likely to form the hydrophobic core. This loop is crucial for the structural integrity of all of the LRRs. In contrast, the downstream sequence consisting of Asn(107), Thr(108), Gly(109), and Ile(110) of LRR4 is crucial for cAMP induction but not for hormone binding, folding, and surface expression. This implicates, for the first time, its involvement in the interaction with the endodomain and signal generation. The evidence for the interaction is presented in the following article.
Highlights
The luteinizing hormone receptor (LHR) consists of an ϳ350-amino acid-long N-terminal extracellular exodomain and a membrane-associated endodomain of similar size
Chem. 276, 3426-3435), we have shown that LRR2 and LRR4 are crucial for hormone binding
We have examined the residues of LRR4, in particular Leu103 and Ile105 in the putative  strand
Summary
Mutant human LHR cDNAs were prepared, expressed in HEK 293 cells, and assayed for hormone binding and intracellular cAMP production as described previously [20, 22]. All assays were carried out in duplicate and repeated four to six times. FLAG-LHR was prepared by inserting the FLAG epitope [23], Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys, between the C terminus (Ser26) of the signal sequence and the N terminus (Arg27) of mature receptors. FLAG-LHR was prepared by inserting the FLAG epitope [23], Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys, between the C terminus (Ser26) of the signal sequence and the N terminus (Arg27) of mature receptors. [20]
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