Structural analysis of yoked chorionic gonadotropin-luteinizing hormone receptor ectodomain complexes by circular dichroic spectroscopy

The luteinizing hormone receptor: insights into structure-function relationships and hormone-receptor-mediated changes in gene expression in ovarian cancer cells

The luteinizing hormone receptor (LHR), one of the three glycoprotein hormone receptors, is necessary for critical reproductive processes, including gonadal steroidogenesis, oocyte maturation and ovulation, and male sex differentiation. Moreover, it has been postulated to contribute to certain neoplasms, particularly ovarian cancer. A member of the G protein-coupled receptor family, LHR contains a relatively large extracellular domain responsible for high affinity hormone binding; transmembrane activation then leads to G protein coupling and subsequent second messenger production. This review deals with recent advances in our understanding of LHR structure and structure-function relationships, as well as hormone-mediated changes in gene expression in ovarian cancer cells expressing LHR. Suggestions are also made for critical gaps that need to be filled as the field advances, including determination of the three-dimensional structure of inactive and active receptor, elucidation of the mechanism by which hormone binding to the extracellular domain triggers the activation of Gs, clarification of the putative roles of LHR in non-gonadal tissues, and the role, if any, of activated receptor in the development or progression of ovarian cancer.

Secondary structure analysis of synthetic peptides of the extracellular domain of the human follicle-stimulating hormone receptor

Follicle-stimulating hormone receptor (FSHR) is a glycoprotein hormone receptor and possesses a large extracellular domain (ECD) instrumental in hormone binding. The ECD is characterized by the presence of leucine-rich repeat (LRR) structures made up of alpha-helices flanked by beta-strands. Our previous studies with the synthetic peptides corresponding to the potentially surface-oriented regions of the ECD had led to the identification of some of these regions in either FSH-binding or FSH-induced cAMP production or both. This study was undertaken with an aim to correlate the findings made in vitro with the secondary structures of the respective peptides. Accordingly, all peptides were screened for their secondary structures in different biochemical environments. This study correlates the observed alpha-helical signature with the previously demonstrated activity in signal generation for peptides 15-31 and 216-235 hFSHR, while FSH binding is correlated with the maintenance of beta-sheet structure in peptides 285-300 and 297-310 hFSHR as observed in vitro.

Resurrecting abandoned proteins with pure water: CD and NMR studies of protein fragments solubilized in salt-free water

Many proteins expressed in Escherichia coli cells form inclusion bodies that are neither refoldable nor soluble in buffers. Very surprisingly, we recently discovered that all 11 buffer-insoluble protein fragments/domains we have, with a great diversity of cellular function, location, and molecular size, could be easily solubilized in salt-free water. The circular dichroism (CD) and NMR characterization led to classification of these proteins into three groups: group 1, with no secondary structure by CD and with narrowly-dispersed but sharp (1)H-(15)N heteronuclear single quantum correlation (HSQC) peaks; group 2, with secondary structure by CD but with HSQC peaks broadened and, consequently, only a small set of peaks detectable; and group 3, with secondary structure by CD and also well-separated HSQC peaks. Intriguingly, we failed to find any protein with a tight tertiary packing. Therefore, we propose that buffer-insoluble proteins may lack intrinsic ability to reach or/and to maintain a well-packed conformation, and thus are trapped in partially-folded states with many hydrophobic side chains exposed to the bulk solvent. As such, a very low ionic strength is sufficient to screen out intrinsic repulsive interactions and, consequently, allow the hydrophobic clustering/aggregation to occur. Marvelously enough, it appears that in pure water, proteins have the potential to manifest their full spectrum of structural states by utilizing intrinsic repulsive interactions to suppress the attractive hydrophobic clustering. Our discovery not only gives a novel insight into the properties of insoluble proteins, but also sheds the first light that we know of on previously unknown regimes associated with proteins.

Expression of a recombinant bifunctional protein from a chimera of human lutropin receptor and human chorionic gonadotropin beta-subunit

Human lutropin (hLH) and human chorionic gonadotropin (hCG) are structurally and functionally similar and play important roles in reproduction via a common gonadal receptor (LH-R). However, hormone specific hCG-beta subunit contains 24 additional amino acid carboxyl terminal peptide (CTP), which produce specific antibodies to hCG-beta with little cross-reaction with LH. A chimeric protein containing both hLH-R and hCG-beta would provide a unique bifunctional antigen for immunocontraception. In this study is described the synthesis of a chimeric DNA construct of full-length of hLH-receptor and hCG-beta and its expression in Sf9 cells to produce a bifunctional protein. Recombinant protein was recognized by antibodies to LH-R as well as anti-hCG-beta in Western Blots, thus indicating the preservation of immunological epitopes for both hLH-R and hCG-beta in the chimera. Specific ligand binding of recombinant hLH-R component was demonstrated by the displacement of bound labeled hCG at increasing concentrations of unlabeled hCG, indicating that, the presence of hCG-beta component of the chimera did not interfere with the binding of hCG to LH-R. hCG-beta was also present in the recombinant chimeric protein as shown by a specific hCG-beta chemiluminescence assay. Treatment of transfected Sf9 cells with hCG induced dose-dependent increase in the stimulation of intracellular cAMP production, which showed that the ligand binding had functional activity. These results demonstrate that the chimeric DNA construct of hLHR-hCG-beta expressed a bifunctional protein containing both hLH-R and hCG-beta activities, which could provide a unique potential antigen for immunocontraception in vertebrates.

Evolutionary divergence of thyrotropin receptor structure

The availability of 18 thyrotropin receptor (TSHR) sequences, including two recent entries for primates and seven from fish, have allowed us to investigate diversification of residues or domains during evolution. We used a likelihood ratio test for evolutionary rate shifts [Proc. Natl. Acad. Sci. 98 (2001) 14512] using LH/CGR sequences as an out-group. At each residue in the alignment, a statistical test was performed for a rate shift at the divergence between mammals and fish. Eighty-two rate shift sites were found, significantly more than was expected (p < 0.0001). The occurrence of rate shifts was highest in the intracellular tail, lowest in the transmembrane serpentine and intermediate in the ectodomain. In 52 mammalian sites, the rates were significantly faster than for the corresponding sites in fish. We have identified rate shift in sites important to TSHR function or in intimate proximity to such regions. The former category includes residues 53 and 55 (of LLR1 beta strand) and 253 and 255 (of LLR9 beta strand), crucial to TSH thyrotropic activity, residue 113, the site of N-linked glycosylation limited to humans, residue 310, an important switch in the hinge region for receptor binding and constitutive activity and residue 382 which centres a motif important for TSH-mediated receptor activation. The rate shifts positions close to functional region include a site proximal to a TSHR-specific motif on LLR3 beta strand, sites important in TM helix structure and homodimerization as well as, in the case of the third intracellular loop, to TSHR/G protein coupling. Rate shift analyses have identified residues whose manipulation in the human TSHR may lead to better understanding of receptor functions and help in the creation of designer analogues.