THE AMINO-TERMINAL REGION OF THE LUTEINIZING-HORMONE CHORIOGONADOTROPIN RECEPTOR CONTACTS BOTH SUBUNITS OF HUMAN CHORIOGONADOTROPIN-II - PHOTOAFFINITY-LABELING

The luteinizing hormone/choriogonadotropin receptor, a seven-transmemb rane receptor, is composed of two equal halves, the N-terminal extrace llular exodomain and the C-terminal membrane-associated endodomain. Un like most seven-transmembrane receptors, the exodomain alone is respon sible for high affinity hormone binding, whereas signal is generated i n the endodomain. These physical separations of hormone-binding and re ceptor activation sites are attributed to unique mechanisms for hormon e binding and receptor activation of this receptor and its subfamily m embers. However, the precise hormone contact sites in the exodomain ar e unclear. In the preceding article (Hong, S,, Phang, T,, Ji, I., and Ji, T, H, (1998) J, Biol. Chem, 273, 13835-13840), a region immediatel y downstream of the N terminus of the exodomain was shown to be crucia l for hormone binding. To test if the region interacts with the hormon e, human choriogonadotropin (hCG) was photoaffinity-labeled with a pep tide mimic corresponding to Gly(18)-Tyr(36) Of the receptor. This pept ide mimic specifically photoaffinity-labeled both the alpha- and beta- subunits of hCG:, Interestingly, hCG alpha was preferentially labeled, On the other hand, denatured hCG was not labeled, and a mutant analog of the peptide failed to label hCG, Furthermore, the affinity labelin g was UV-dependent and saturable, indicating the specificity of the ph otoaffinity labeling. Our results indicate that the region of the exod omain interacts with hCG and that the contact points are near both sub units of hCG, Particularly, the alternate residues (Leu(20), Cys(22), and Gly(24)) are crucial for hCG; binding, In addition, the results un derscore the fact that there is a crucial hormone contact site outside of the popularly believed primary hormone-binding site that is compos ed of Leu-rich repeats and is located in the middle of the exodomain, Our observations are crucial for understanding the molecular mechanism through which the initial high affinity hormone binding leads to rece ptor activation in the endodomain.