COOH-TERMINAL AMINO-ACIDS OF THE ALPHA-SUBUNIT PLAY COMMON AND DIFFERENT ROLES IN HUMAN CHORIOGONADOTROPIN AND FOLLITROPIN

Human choriogonadotropin (hCG) and follitropin (FSH) belong to the gly coprotein hormone family. These hormones are heterodimers and composed of a common alpha subunit and a distinct beta subunit which confers r eceptor-binding specificities. In addition to this structural similari ty, they share a similar signal pathway involving G protein, adenylylc yclase and induction of cAMP synthesis. Therefore, a presumptive relat ionship of these common structure and function has been the subject of extensive past investigations, but a definitive clue has been elusive . As a step to address this important issue, a series of recombinant m utants of hCG and human FSH were generated in which the COOH-terminal amino acids of the alpha subunit were successively removed or substitu ted. Furthermore, a set of peptides were synthesized with sequences co rresponding to different regions of the alpha subunit. Deletion of the alpha COOH-terminal Ser92 had no effect on receptor-binding or cAMP i nduction by FSH and hCG. Truncation of alphaLys91-Ser92 or alphaHis90- Lys91-Ser92 abolished the ability of both hormones to induce cAMP synt hesis. It significantly reduced receptor binding of FSH but not hCG. T he different functions of the alpha COOH-terminal region are further n oticed with a peptide corresponding to the last 10 amino acids of alph a. It failed to bind to the FSH receptor but was capable of binding to the LH/CG receptor and stimulating cAMP synthesis. These results are the first conclusive evidence that alphaHis90-Lys91 play an essential role in cAMP induction of both hormones. In contrast to this common ro le, they are necessary for FSH binding to the FSH receptor but not for hCG binding to the LH/CG receptor. The hCGalpha COOH-terminal region makes direct contact with the LH/CG receptor, and this low affinity co ntact is necessary and sufficient to activate the receptor for signal generation. This conclusion is supported by the study using mutant hCG s in which either alphaHis90 or Lys91 was substituted.