科研之友
微信
新浪微博
Facebook
分享链接
摘要
The choriogonadotropin beta-subunit is unique in the human glycoprotein hormone family in containing a carboxyl-terminal extension, with four sites of O-glycosylation, that is not present in the other beta-subunits. We have used site-directed mutagenesis to define boundaries on the extent to which truncations can be made at the COOH terminus without abolishing subunit assembly and biological activity. Two COOH-terminal deletion mutant chains of human choriogonadotropin-beta, des(93-145) and des(101-145), were prepared and expressed in Chinese hamster ovary cells containing a stably integrated gene for bovine-alpha. The heterologous gonadotropin, bovine alpha-human choriogonadotropin des(101-145)beta, formed a heterodimer and, when assayed with transformed murine Leydig cells in vitro, competed with the binding of standard human choriogonadotropin and stimulated both cAMP and progesterone production, albeit with a reduced potency relative to bovine alpha-human choriogonadotropin beta-wild type. In contrast, human choriogonadotropin des(93-145)beta, expressed under identical conditions in the presence of bovine-alpha, failed to form heterodimer and thus exhibited no competitive binding and was without effect on cAMP and progesterone levels. Consequently, removal of the putative determinant loop region of the beta-subunit (residues 93-100), which is believed to be important in determining receptor specificity, abolishes association with alpha. Hence, in addition to its possible role as a receptor determinant, this region of the molecule appears to be critical for proper folding or subunit interaction. The truncated form of human choriogonadotropin-beta lacking residues 101-145 is the shortest form of the subunit yet described that retains biological activity. Moreover, these results demonstrate that the proposed disulfide between Cys-26 and Cys-110 is not requried for subunit assembly or for receptor binding and subsequent intracellular signaling.
- 出版日期1991-4-15
