Human chorionic gonadotropin (hCG) is a member of the family of glycoprotein hormones containing a common alpha-subunit and distinct beta-subunits that confer hormonal specificity. hCG binds to the relatively large ectodomain of the human luteinizing hormone receptor (hLHR), a member of the G protein-coupled receptor superfamily, leading to increased intracellular production of cAMP. Using protein engineering, two miniaturized versions of hCGbeta have been separately fused to the N-terminus of the alpha-subunit to give N-des[1-91]hCGbeta-alpha-C and N-des[1-91,110-114]hCGbeta-alpha-C, i.e. fusion proteins of the hCGbeta determinant loop (extended to include the complete seat belt and carboxy-terminal peptide) coupled to the alpha-subunit. Bioactivity of these single-chain gonadotropin analogs was assessed in two systems following transient transfections into HEK 293 cells and subsequent cAMP measurements. In one, each mini-beta-alpha cDNA was fused to that of hLHR and transfected into cells to create yoked miniaturized hCG-hLHR complexes; in the other, the cDNA of each single chain mini-beta-alpha was co-transfected with that of hLHR in an effort to produce non-covalent miniaturized hCG-hLHR complexes. Using yoked hCG-hLHR and hLHR as positive and negative controls respectively, expression of each mini-hCG-hLHR complex was confirmed using antibody and ligand binding assays. The two mini-hCGs led to minimal activation of hLHR, suggesting weak intrinsic activity of the mini-beta-alpha fusion proteins. These results suggest that potent agonists and antagonists will require the presence of other portions of hCGbeta in addition to the determinant loop/seat belt.
F. Chen and D. Puett
The heterodimer, human chorionic gonadotrophin (hCG), contains an a subunit that is common to the glycoprotein hormones and a hormone-specific β subunit. A comparison of all known β amino acid sequences shows that an aspartic acid at position 99 (with the numbering scheme for hCG-β) is one of the seven non-Cys invariant residues. Using site-directed mutagenesis we have replaced hCG-β Asp99 with Arg. Chinese hamster ovary cells, containing a stably integrated gene for bovine a subunit, were transiently transfected with plasmids containing wild-type and mutant hCG-β cDNAs. The Arg99 β mutant associated with the a subunit, but the resulting heterodimer failed to enhance intracellular cyclic AMP production in a gonadotrophin-responsive transformed murine Leydig cell line. Thus, a single amino acid residue replacement in this glycosylated heterodimer containing 237 amino acid residues is sufficient to abolish activity.
TP Meehan, D Puett and P Narayan
We have previously reported the construction of a constitutively active luteinizing hormone receptor by covalently linking a fused heterodimeric hormone to the extracellular domain of the G protein-coupled receptor. This yoked hormone-receptor complex (YHR) was found to produce high levels of cAMP in the absence of exogenous hormone. Stable lines expressing YHR were generated in HEK 293 cells to obtain lines with different expression levels; however, in a relatively short time of continued passage, it was found that YHR expression was greatly reduced. Herein, we describe the development of clonal lines of HEK 293 cells in which the expression of YHR is under the control of a tetracycline-regulated system. Characterization of clonal lines revealed tight control of YHR expression both by dose and time of incubation with doxycycline. These experiments demonstrated a good correlation between expression levels of the receptor and basal cAMP production. Moreover, the reduction in receptor expression following doxycycline removal revealed that YHR mRNA and protein decayed at similar rates, again suggesting a strong linkage between mRNA and protein levels. The controlled expression of YHR in this cell system will allow for a more detailed analysis of the signaling properties associated with constitutive receptor activation and may prove to be advantageous in developmental studies with transgenic animals.
K-C Peng, D Puett and J M Brewer
Although differing in their amino acid sequences, the folding patterns of the α and β subunits of human choriogonadotropin are similar in the crystal structure of the HF-treated glycoprotein hormone. Each subunit forms a cystine-knot motif like that found in several growth factors that form homodimers and heterodimers. In order to ascertain if the α and β subunits can self-associate, e.g. to form homodimers, sedimentation equilibrium at various glycoprotein concentrations and temperatures was used to study the subunits of bovine lutropin, which are expected to exhibit conformations like those of the choriogonadotropin subunits. Each subunit was found to form homodimers with K d values of 0·3 and 0·1 mm for α and β respectively at 37 °C. Self-association was weakly exothermic for α and endothermic for β; entropic factors made a major contribution for each. It is unlikely that homodimer formation of either subunit would be physiologically important, although homodimers may form to some extent intracellularly because of the relatively high concentrations during biosynthesis.
H Xia, J Huang, T-M Chen and D Puett
Human chorionic gonadotrophin (hCG), like other members of the glycoprotein hormone family, contains a common α subunit and a hormone-specific β subunit. The latter is a 145 amino acid residue polypeptide with six sites of glycosylation. Positions 2 and 104 are occupied by basic amino acid residues in the 12 known amino acid sequences of mammalian β subunits from CG and LH, a related gonadotrophin that acts through the same receptor. Lysine residues are found in both these positions in hCG-β. Using site-directed mutagenesis, each of these two lysines in hCG-β was replaced with glutamic acid. The mutant and wild-type cDNAs were subcloned into a eukaryotic expression vector, which was then transiently transfected into Chinese hamster ovary cells containing a stably integrated gene for the bovine α subunit. Holoprotein formation occurred with each of the two heterologous gonadotrophin mutants, i.e. the bovine α subunit bound to hCG-β (Glu2) and to hCG-β (Glu104), as well as with the control, i.e. the bovine α subunit bound to the hCG-β wild-type subunit. In two in-vitro assays, one a competitive binding assay with 125I-labelled hCG as bound ligand and the other based on stimulation of progesterone production in a transformed murine Leydig cell line, MA-10, both the heterodimers containing a mutant β subunit exhibited bioactivity, but their potencies were lower than that of the bovine α subunit bound to the hCG-β wild-type subunit. These results suggest that the basic amino acid residues at positions 2 and 104 in hCG-β participate, either directly or indirectly, in receptor binding.
L Shen, H Xia, N Bhowmick, P Narayan and D Puett
The Arg68-Leu69 sequence is invariant in the β subunits of chorionic gonadotrophin and luteinizing hormone from a variety of species. Using site-directed mutagenesis of the human chorionic gonadotrophin (hCG)-β cDNA, several replacements of Arg68, an Ala replacement of Leu69, and a multiple replacement with Ala-Ala-Ala-Ala of the tetrapeptide sequence, Arg68-Leu69-Pro70-Gly71, were prepared and characterized. The wild-type and mutant cDNAs were subcloned into a pRSV expression vector and transiently transfected into CHO cells containing a stably integrated gene for bovine a. Concentrations of secreted wild-type and mutant hCG-β subunit and holoprotein were determined using radioimmunoassays; potencies, i.e. the ratio of biologic to immunologic activity, of several of the mutant heterodimers were measured in vitro via gonadotrophin-mediated steroidogenesis in transformed murine Leydig cells (MA-10). The Leu69→Ala mutant formed a mutant holoprotein that was essentially equipotent with wild-type hormone in the steroidogenesis assay. The Arg68 replacements with Lys, Ala, and Leu were poorly secreted by the cells, e.g. <10% that of wild-type hCG; however, sufficient quantities of mutant holoproteins containing Lys68 and Ala68 were obtained for biological assays, and both exhibited greater apparent potencies than wild-type hormone. Likewise, a mutant holoprotein containing the Arg68-Leu69-Pro70-Gly71→Ala-Ala-Ala-Ala multiple replacement was apparently more potent than wild-type hormone, but it too was secreted at lower levels than wild-type. These results establish that replacements of Arg68 in hCG-β diminish secretion, but the small amount of holoprotein that is formed and secreted appears to be of somewhat greater potency than wild-type hormone.