Structure--function relationships of the glycoprotein hormones and their receptors

No evidence of the human chorionic gonadotropin-beta gene 5 betaV79M polymorphism in Mexican women

Human chorionic gonadotropin (hCG) is a placental hormone essential for the maintenance of pregnancy. Previous studies have shown a G to A transition in exon 3 of the hCGbeta gene 5, which changes the naturally occurring valine to methionine in codon 79. The frequency of this transition varies among different ethnic groups, being high in USA women, and less common, or absent, in various European populations. The purpose of the present study was to determine the frequency of the betaV79M allelic variant of the beta-subunit of hCG in a Mexican population, and to compare this frequency with those found in other ethnic groups. Placental DNA from 161 pregnant Mexican women was genotyped for the betaV79M by polymerase chain reaction (PCR)-restriction fragments length polymorphism analysis. No polymorphic betaV79M alleles were identified in the population studied. The allele and genotypic frequencies of betaV79M polymorphism in Mexican Mestizo women were significantly different from those reported for the US population, but not from five different European populations. In contrast to what has been found in women from the USA, it seems that the hCGbeta V79M polymorphism is absent or extremely rare in Mexican Mestizo women.

Deletions of the N-terminal regions of the human melanocortin receptors

The non-homologous N-terminal regions of four human melanocortin (MC) receptors were truncated in order to investigate their putative participation in ligand binding. Eleven constructs were made, where different numbers of residues from the N terminus were deleted. These constructs were used for transient expression experiments in COS cells and analysed by ligand binding. The results show that 27, 25, 28, and 20 amino acids could be deleted from the N terminus of the human MC1, MC3, MC4 and MC5 receptors, respectively, including all potential N-terminal glycosylation sites in the MC1 and the MC4 receptors, without affecting ligand binding or expression levels. The results indicate that the N-terminal regions of the human MC1, MC3, MC4 and MC5 receptors, do not play an important role for the ligand binding properties of these receptors.

Molecular cloning of F4/80, a murine macrophage-restricted cell surface glycoprotein with homology to the G-protein-linked transmembrane 7 hormone receptor family

F4/80 is a monoclonal antibody that recognizes a murine macrophage-restricted cell surface glycoprotein and has been extensively used to characterize macrophage populations in a wide range of immunological studies. Apart from the tightly regulated pattern of expression of the F4/80 antigen, little is known about its possible role in macrophage differentiation and function. We have sought to characterize the molecule at the molecular level, through the isolation of cDNA clones, and now describe the sequence of the F4/80 protein. The primary amino acid sequence demonstrates homology to two protein superfamilies. The NH2-terminal region consists of seven epidermal growth factor-like domains, separated by approximately 300 amino acids from a COOH-terminal region that shows homology to members of the seven transmembrane-spanning family of hormone receptors. The potential role of these distinct domains is discussed with respect to the possible function of the F4/80 molecule.

EMR1, an unusual member in the family of hormone receptors with seven transmembrane segments

Proteins with seven transmembrane segments (7TM) define a superfamily of receptors (7TM receptors) sharing the same topology: an extracellular N-terminus, three extramembranous loops on either side of the plasma membrane, and a cytoplasmic C-terminal tail. Upon ligand binding, cytoplasmic portions of the activated receptor interact with heterotrimeric G-coupled proteins to induce various second messengers. A small group, recently recognized on the basis of homologous primary amino acid sequences, comprises receptors to hormones of the secretin/vasoactive intestinal peptide/glucagon family, parathyroid hormone and parathyroid hormone-related peptides, growth hormone-releasing factor, corticotropin-releasing factor, and calcitonin. A cDNA, extracted from a neuroectodermal cDNA library, was predicted to encode a new 886-amino-acid protein with three distinct domains. The C-terminal third contains the seven hydrophobic segments and characteristic residues that allow the protein to be readily aligned with the various hormone receptors in the family. Six egf-like modules, at the N-terminus of the predicted mature protein, are separated from the transmembrane segments by a serine/threonine-rich domain, a feature reminiscent of mucin-like, single-span, integral membrane glycoproteins with adhesive properties. Because of its unique characteristics, this putative egf module-containing, mucin-like hormone receptor has been named EMR1. Southern analysis of a panel of somatic cell hybrids and fluorescence in situ hybridization have assigned the EMR1 gene to human chromosome 19p13.3.

Anti-HIV effect of beta subunit of human chorionic gonadotropin (beta hCG) in vitro

Human chorionic gonadotropin (hCG)--a pregnancy-associated immunomodulating hormone--has been recently shown in vitro to suppress reverse transcriptase activity in chronically HIV-infected lymphocytes and monocytes and to block viral transmission resulting from cell-cell contact between virus-carrying lymphocytes and placental trophoblasts. In further pursuit of the query into the mechanism of action, purified alpha and beta subunits of hCG were tested for the inhibition of p24 gag protein synthesis in virus-producing ACH-2 lymphocytes and U1 monocytes. Unlike the alpha subunit, beta-hCG displayed a distinct U-shaped dose response, characteristic of the effect of dimer hCG. Maximum inhibition of viral expression has been achieved at 10-100 ng/ml, the concentration corresponding to blood levels of beta-hCG in pregnant women. The doses that were several logs higher of normal levels seemed to increase viral production in monocytes. The data presented supports our original observations regarding the effect of intact hCG on HIV replication. While the mechanism of action remains to be established, the results suggest that the virus-interfering activity of hCG is determined by hormone-specific beta chain but not by the alpha subunit--shared with the family of glycoprotein hormones from the pituitary--follicle-stimulating hormone, luteinizing hormone and thyrotropin.

Identification of regions of the follitropin (FSH) beta-subunit that interact with the N-terminus region (residues 9-30) of the FSH receptor

We have recently identified a region, N-terminus residues 9-30, in the extracellular domain of the follicle-stimulating hormone (FSH) receptor capable of binding FSH, but not luteinizing hormone (LH) or thyroid-stimulating hormone (FSH) (Dattatreyamurty and Reichert (1992) Mol. Cell. Endocrinol. 87, 9-17). The objectives of the present study were to examine the interaction between a synthetic peptide corresponding to this receptor sequence and the beta-subunit of FSH, and to identify which FSH-beta regions are involved in the interaction. FSH-beta subunit and synthetic FSH-beta peptides 1-15, 71-85 and 101-111 effectively bound 125I-labeled FSH rec-(9-30) peptide, and binding was inhibited by excess unlabeled FSH receptors. Scatchard analysis indicated that the synthetic FSH-beta peptides had affinities for FSH rec-(9-30) peptide in the order of 10(6) M-1 (Ka), with the sum of individual peptide affinities (Ka = 1.21 x 10(7) M-1) closely approximating that of the intact beta-subunit (1.02 x 10(7) M-1). Polyclonal antibodies raised against FSH rec-(9-30) peptide completely inhibited the binding of 125I-labeled receptor peptide to hFSH, hFSH-beta, and hFSH-beta peptides 1-15, 71-85 and 101-111. Our results indicate that recognition of FSH-beta by N-terminus region (9-30) of the FSH receptor involves contact with residues in three discontinuous binding regions on FSH-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of calcitonin binding by calcium: differential effects of divalent cations

Binding of salmon calcitonin to bovine hypothalamic membranes is enhanced about 25% by calcium with a half-maximal effect at 15 mM calcium. In contrast, membranes prepared from a cell line expressing a recombinant human calcitonin receptor show no effect of calcium under similar conditions. The hypothalamic calcitonin receptor solubilized with CHAPS detergent retains an apparent Kd of 0.3 nM for salmon calcitonin; however, binding of calcitonin to the detergent-solubilized receptor complex can be inhibited by divalent cations in order of potency Mn > Ca approximately Sr approximately Mg >> NaCl with Mn and Ca having apparent Ki's of 5 mM and 20 mM respectively. Dixon and Scatchard plots of Mn and Ca inhibition of binding to the soluble receptor complex suggest a noncompetitive mechanism of inhibition. Calcium also inhibits calcitonin binding to a detergent-solubilized recombinant human calcitonin receptor. Inhibition of calcitonin binding is observed using two independent methods for determining soluble receptor-hormone complex and inhibition is reversed by EDTA.

Two receptor binding regions of human FSH show sense-antisense similarity to the human FSH receptor

The sequences of two receptor binding regions of the beta-subunit of the human follicle-stimulating hormone (hFSH-beta) were compared with the DNA-derived antisense peptide sequence of the hFSH receptor. A striking sense-antisense similarity was established between these receptor binding regions and the hFSH receptor. Based on this sense-antisense similarity four putative hormone binding regions on the N-terminal extracellular region of the hFSH receptor are identified.