Amino/carboxyl-terminal deletion mutants of human choriogonadotropin beta

Human choriogonadotropin (hCG) contains an alpha subunit common to other members of the glycoprotein hormone family, lutropin (LH), follitropin, and thyrotropin, and a hormone-specific beta subunit. hCG beta contains a carboxyl-terminal extension of 25-30 amino acid residues not present in the other beta subunits; also, CG beta and lutropin beta have an additional 6 or 7 amino-terminal residues that are not present in follitropin beta and thyrotropin beta. To delineate the contribution of these extensions in hCG beta, site-directed mutagenesis was used to prepare several deletion fragments. Plasmids containing cDNAs for wild-type and mutant hCG beta were transiently transfected into Chinese hamster ovary cells containing a stably integrated gene for bovine alpha. Medium from the transfected cells was used in two in vitro assays with a transformed murine Leydig cell line, MA-10. The deletion fragments, des(1-7), des(111-145), and des(1-7, 111-145), associated with alpha as well as hCG beta wild-type; moreover, the potencies of the three mutant hormones were comparable to that of control. In contrast, des(1-7, 101-145)hCG beta yielded very little heterodimer, although that which formed was partially active. These results define the shortest known core fragment of hCG beta, amino acid residues 8-110, that retains significant functionality in vitro.