Gene, interaction, signal generation, signal divergence and signal transduction of the LH/CG receptor
Authors: Ryu K.S. , Gilchrist R.L. , Koo Y.B. , Ji I. , Ji T.H. 
Published: July 2002

Trophoblastic neoplasms and choriocarcinoma cells express high levels of the hCG receptor. The hCG receptor is encoded by a single gene in chromosome 2p21‐p16, spanning over −70 kb with 11 exons and 10 introns. Multiple mRNA species are produced from the gene utilizing two proximal promoters and several Sp‐1 elements as well as proximal and distal suppressors. In fact, regulatory proteins which bind to one of these suppressors are expressed less in choriocarcinoma cell lines than in placenta.

The LH/CG receptor is comprised of two structurally and functionally distinct domains, extracellular N‐terminal exodomain and membrane embedded endodomain. These two domains can separately be expressed and processed, including folding. The exodomain alone has the high affinity hormone binding site but is not capable of generating hormonal signal. In contrast, the endodomain alone has the site for receptor activation. These two domains contact each other in holo‐receptor and split receptor. This interaction, particularly through exoloops 2 and 3, constrains the high affinity hormone binding at the exodomain. Conversely, the exodomain could be involved in receptor activation. Therefore, these two domains are not entirely independent although they can be independently synthesized and processed. The existing envidence indicate that hCG and the receptor undergo multiple stages of interactions leading to receptor activation. Initial high affinity binding of hCG to the exodomain results into conformational adjustments of the hCG/exodomain complex. This leads to the secondary, low affinity contact of the hCG/exodomain complex with the endodomain. This secondary contact is responsible for generating signals. They are transduced through TM to the cytoplasmic portion (cytoloops and the C‐terminal tail) of the receptor and then, transferred to cytoplasmic signaling molecules, such as G protein.

Mutations in the exodomain and endodomain (N‐extension, exoloops, TM, cytoloops, and cytoplasmic tail) have the potential to interfere with receptor activation at different steps, signal generation, transduction and transfer. Binding of hCG to the LH/CG receptor are known to induce two signals, one for adenylyl cyclase/ cAMP and the other for phospholipase C/inositol phosphate/diacylglycerol. The cAMP signal and IP signal diverge at the surface of the receptor. These independent signals are separately transduced through the transmembrane domains to the cytoplasmic part of the receptor, indicating the existence of the distinct transducers for each of the signals. Furthermore, it is likely that the divergent signals are separately transferred to cytoplasmic signal molecules such as G protein. In addition, each of the cAMP signal and IP signal consists of at least three separate subsignals: affinity signal, maximal production (efficacy) signal and basal level signal. In heterodimeric hCG, there are distinct parts responsible for high affinity receptor binding and receptor activation. Particularly, the C‐terminal reduces of the α subunit play a crucial role in receptor activation. This α subunit is shared with other glycoprotein hormones, follicle stimulating hormone and thyroid stimulating hormone. Interesting, the α C‐terminal residues play distinct roles in all three hormones, despite its common nature.