IDENTIFICATION AND CHARACTERIZATION OF CONTACT SITES BETWEEN HUMAN CHORIONIC GONADOTROPIN AND LUTEINIZING HORMONE/CHORIOGONADOTROPIN RECEPTOR

The luteinizing hormone receptor (LHR) belongs to the G protein-coupled receptorfamily. It consists of two distinct domains; the N-terminal extracellular exodomain and themembrane associated endodomain which includes 7 transmembrane domains, 3 exoloops, 3cytoloops and a C-terminal tail. Sequence alignment and computer modeling suggest thepresence of Leu Rich Repeat (LRR) motifs in the exodomain. Although their structuralsimilarity is high, each LRR is not equally important for hormone binding. Ala-scanning andtruncation studies performed in our laboratory suggest that LRR2 and LRR4 appear to be themost crucial. The Ala-scanning data suggest that Leu103 and Ile105 in LRR4 are important forhormone binding. However, it is not clear whether these two residues make direct contact withhuman chorionic gonadotropin (hCG) or if they are necessary for the overall structural integrityof LRR4. In this work, the LHR peptide mimics of LRR4 were used for photoaffinity labeling todetermine whether Leu103 and Ile105 directly interact with hormone. Furthermore, LRR4peptides containing the photoactivable benzoylphenylalanine (Bpa) were used to determinewhether the LRR structure really exists in the LHR exodomain, whether LRR 4 interact withhCG, and which residues of LRR4 interact with hCG. Bpa was directly incorporated intodifferent positions of the LRR4 peptide sequence to examine the labeling ability of individualamino acids. The results suggest that LRR4, in particular the sequence of Lys101-Cys106,makes direct contact with hCG. However Leu103 and Ile105 do not interact with hCG but mayform the hydrophobic core of the LRR4 loop, which appears to be crucial for the LRR structure.Existing data suggest that glycoprotein hormones initially bind the exodomain. Thehormone/exodomain complex undergoes conformational adjustments and stimulates theendodomain of the receptor to generate hormone signals. The exoloops modulate hormonebinding and signaling; however, little is known about whether the hormone/exodomain complexcontacts the endodomain. To address this issue, we investigated whether the exoloops interactwith the hormone. First, we examined exoloop 3 that connects transmembrane domains 6 and 7which are important for signal generation. We present the first physical evidence that LHRexoloop 3 interacts with hCG