Leslie Lobel

High-level bacterial expression of a natively folded, soluble extracellular domain fusion protein of the human luteinizing hormone/chorionic gonadotropin receptor in the cytoplasm of Escherichia coli.

We have expressed the extracellular domain of the human luteinizing hormone/chorionic gonadotropin (hLH/CG) receptor as a fusion protein with thioredoxin in the cytoplasm of an Escherichia coli strain that contains mutations in both the thioredoxin reductase and glutathione reductase genes. The chimeric protein isolated following induction of expression is purified in a soluble form and binds hCG with an affinity approximating that of native receptor. This truncated form of the receptor displays the same specificity as intact hLH/CG receptor and does not bind human follicle stimulating hormone. This cytoplasmically produced protein is expressed at levels that exceed 10 mg/L. Expression of properly folded extracellular domain of the hLH/CG receptor in the cytoplasm of E. coli allows the facile and economic purification of large quantities of material. This will facilitate the determination of the structure of the hormone-binding domain of the glycoprotein receptor as well as the production of epitope-specific antibodies.

Expression and characterization of recombinant β-subunit hCG homodimer

We have linked two human chrionic gonadotropin (hCG) β-subunit cDNAs in tandem such that the expressed fusion protein consists of two mature β-subunits joined through the carboxy terminal peptide of the first β-subunit. A single glycine residue is inserted between the two subunits in the fusion protein. Chinese hamster ovary (CHO) cells transformed with a clone that contains the fused cDNAs express and secrete a protein that is consistent with it being a β-hCG homodimer protein. These β-homodimer molecules can recombine with two free α-subunits indicating that both β-subunits within the homodimer are likely folded in their native conformation. Our data also suggest that the two β-subunits fold upon each other as a globular protein and do not appear to exist as a simple fusion of two linear β-subunits. Further-more, the two β-monomer subunits in the fusion protein form a stable homodimer that can bind and activate the hLH/CG receptor specifically. Recombination of the fusion protein with α-subunits appears to favor an arrangement where two α-subunits combine with a single molecule of the fusion protein. The recombined molecule consists of four subunits and is comparable to two tethered hCG moieties, which constitutes a hCG dimer. This hormone dimer can bind and activate the hLH/CG receptor with an activity approximating that of native hCG.

A temperature-sensitive mutation constructed by "linker insertion" mutagenesis.

SummaryThe in vitro mutagenesis of cloned DNAs allows the formation of virtually any specific mutation, but no method has been found which might routinely lead to the important phenotype of temperature sensitivity. We have studied three linker insertion mutations in the envelope gene of Moloney murine leukemia virus (M-MuLV), and found that one was exquisitely temperature-sensitive for plaque formation. We suggest that the construction of short insertion mutations may be a fruitful approach for the generation of temperature-sensitive phenotypes in cloned genes.

Three-dimensional structures of gonadotropins

Most secreted proteins are modified post-translationally with the addition of carbohydrate. It has been difficult to use crystallography to solve the structures of these proteins due to the inherent heterogeneity of the carbohydrate. The structure of the chemically deglycosylated form (hydrogen fluoride treated) of human chorionic gonadotropin (hCG) has been solved through crystallographic techniques. Unfortunately this form of hCG is not biologically active, and exhibits immunochemical differences from native hormone. In addition, subunit interactions appear altered after chemical deglycosylation as indicated by the increased thermal stability of the HF-treated hormone. The Asn 52 glycan on the alpha-subunit of hCG has been identified as being required for biological activity, it is, therefore, of physiological importance to determine the structure of the hormone with its carbohydrate intact. Also, it has not been possible to obtain crystals of the individual glycosylated subunits of hCG. Therefore an alternative method to solve the structure of the biologically active form of the hormone in solution as well as its separated subunits is necessary. Structural information utilizing NMR techniques can be obtained from native hCG subunits in solution if they can be uniformly labeled with 13C and 15N isotopes. We have developed a universal nonradioactive isotope, labeling medium enriched in 13C and 15N which can be used to express uniformly labeled hCG from Chinese hamster ovary cells suitable for solving the structure of the individual subunits and ultimately that of the native, biologically active hormone. The isotopically labeled recombinant hCG and its purified subunits are essentially identical to urinary hCG on comparison by biochemical, immunochemical, biological activity and the ability of the isolated subunits to recombine to form a biologically active dimer. Mass spectrometric analysis and preliminary structural NMR data indicate that the labeling is uniform and there is greater than 90% incorporation, sufficient for complete structural determination studies. This labeled growth medium represents a technological advance which will enable the rapid solution of the structures of the other glycoprotein hormones, as well as other glycoproteins which have proven unsuitable for crystallographic study.

Development of a Fluorescence Based High Throughput Assay for Antagonists of the Human Chorionic Gonadotropin Receptor Extracellular Domain: Analysis of Peptide Inhibitors

A simple method for prompt fluorescent detection of inhibitors of human chorionic gonadotropin (hCG) binding to the extracellular domain of the human luteinizing hormone/chorionic gonadotropin (hLHICG) receptor was developed for high throughput screening (HTS). Construction and analysis of a recombinant phage that displays the extracellular binding domain of the hLH/CG receptor on its surface and specifically binds hCG was previously described. To facilitate the identification of molecules that disrupt the interaction of hCG with its receptor, a method for prompt fluorescent detection of these phage bound to hCG was developed. This technique is extremely sensitive and employs fluorescent labels (PBXL dyes) that are derived from red and blue-green algae. Antibodies labeled with PBXL dye were able to specifically detect phage that display the extracellular domain of the hLH/CG receptor when bound to hCG immobilized in 96-well microplates. Decreases in fluorescence correlate with the concentration of exogenous hCG or hCG antagonists in the assay. This prompt fluorescence detection assay was optimized in a 96-well format as a model system for HTS applications that target the receptors for the group of hormones known as the gonadotropins. Low-affinity molecules that disrupt binding of the phage-displayed receptor extracellular domain to hCG can be rapidly identified in this high throughput screen.