A critical element of lutropin bioactivity in vivo is its rapid removal from the blood by a receptor, located in hepatic endothelial cells, that recognizes the terminal sulfated carbohydrate structure SO 4 -4-GalNAc1,4Glc-NAc1,2Man␣ (S4GGnM). We have previously shown that the macrophage mannose (Man)-receptor cDNA directs the synthesis of a protein that binds oligosaccharides with either terminal S4GGnM or terminal Man, at independent sites. We now show that the cysteine-rich (Cys-Rich) domain at the N terminus of the Man͞S4GGnM receptor accounts for binding of oligosaccharides with terminal GalNAc-4-SO 4 , whereas calcium-dependent carbohydrate recognition domains (CRDs) account for binding of ligands containing terminal Man. The Cys-Rich domain is thus a previously unrecognized carbohydrate binding motif. Cys-Rich domains have been described on the three other members of the endocytic C-type lectin family of receptors. The structural relationship of these receptors to the Man͞S4GGnM receptor raises the possibility that their Cys-Rich domains also bind carbohydrate moieties and contribute to their function.Glycoproteins such as lutropin (LH) and thyrotropin that bear multiple Asn-linked oligosaccharides terminating with the sequence SO 4 -4-GalNAc1,4GlcNAc1,2Man␣ (S4GGnM) are rapidly removed from the circulation by an S4GGnM-specific receptor found at the surface of hepatic endothelial cells (1, 2). Precise control of the circulatory half-life of LH is thought to be critical for attaining maximal stimulation of the LH receptor located in the ovary during the preovulatory surge in LH levels. We recently reported (3) that the S4GGnM receptor (S4GGnM-R) isolated from rat liver is closely related to the macrophage mannose receptor (Man-R) isolated from rat lung. However, the liver S4GGnM-R and lung Man-R display marked differences in ligand specificity and binding properties.
Lutropin (LH) and other glycoproteins bearing oligosaccharides with the terminal sequence SO 4 -4-GalNAc1,4GlcNAc1,4Man-(S4GGnM) are rapidly removed from the circulation by an S4GGnM-specific receptor (S4GGnM-R) expressed at the surface of hepatic endothelial cells. The S4GGnM-R isolated from rat liver is closely related to the macrophage mannose-specific receptor (Man-R) isolated from rat lung both antigenically and structurally. The S4GGnM-R and Man-R isolated from these tissues nonetheless differ in their ability to bind ligands bearing terminal GalNAc-4-SO 4 or Man. In this paper, we have explored the structural relationship between the Man-R and the S4GGnM-R by examining the properties of the recombinant Man-R in the form of a transmembrane protein and a soluble chimeric fusion protein in which the transmembrane and cytosolic domains have been replaced by the Fc region of human IgG1. Like the S4GGnM-R isolated from liver, the chimeric fusion protein is able to bind ligands terminating with GalNAc-4-SO 4 and Man at independent sites. When expressed in CHO cells the recombinant Man-R is able to mediate the uptake of ligands bearing either terminal GalNAc-4-SO 4 or terminal Man. We propose that the Man-R be renamed the Man͞S4GGnM receptor on the basis of its multiple and independent specificities.Glycoproteins bearing Asn-linked oligosaccharides terminating with the sequence SO 4 -4-GalNAc1,4GlcNAc1,2Man (S4GGnM) are bound by a receptor found at the surface of hepatic endothelial cells that mediates their internalization and transport to lysosomes, where they are degraded (1). The S4GGnM-specific receptor (S4GGnM-R) accounts for the rapid removal of the glycoprotein hormone lutropin (LH) from the circulation and thereby the episodic rise and fall in circulating LH after release from gonadotrophs into the blood (2). The episodic rise and fall in circulating LH levels has been proposed to be essential for attaining maximal biologic activity in vivo (3).We recently described the isolation of a receptor from rat liver that can account for the binding and internalization of LH by hepatic endothelial cells (4). The receptor is closely related to the macrophage mannose receptor (Man-R) (5, 6) on the basis of antigenicity and peptide mapping studies. Yet the S4GGnM-R isolated from liver and the Man-R isolated from lung display markedly different binding properties, suggesting that the two receptors are not identical (4). We have examined this issue by expressing a cDNA, isolated from mouse lung, which encodes the Man-R (6) in Chinese hamster ovary (CHO) cells. We have also examined the properties of a secreted, chimeric fusion protein derived from this cDNA in which the transmembrane and cytosolic domains of the receptor have been replaced by the Fc region of human IgG1 (7). Our results indicate that the Man-R cDNA directs synthesis of a receptor that displays properties characteristic of both the S4GGnM-R and the Man-R when expressed in CHO cells. The properties of the secreted chimeric fusion protein ind...
Glycoproteins, such as the glycoprotein hormone lutropin (LH), bear oligosaccharides terminating with the sequence SO 4 -4GalNAc1,4GlcNAc1,2Man␣ (S4GGnM) and are rapidly removed from the circulation by a receptor present in hepatic endothelial cells and Kupffer cells. Rapid removal from the circulation is essential for attaining maximal hormone activity in vivo. We have isolated a protein from rat liver which has the properties expected for the S4GGnM-specific receptor (S4GGnM-R). The S4GGnM-R is closely related to the macrophage mannose receptor (Man-R) both antigenically and structurally. At least 12 peptides prepared from the S4GGnM-R have amino acid sequences that are identical to those of the Man-R. Nonetheless, the ligand binding properties of the S4GGnM-R and the Man-R differ in a number of respects. The S4GGnM-R binds to immobilized LH but not to immobilized mannose, whereas the Man-R binds to immobilized mannose but not to immobilized LH. When analyzed using a binding assay that precipitates receptor ligand complexes with polyethylene glycol, the S4GGnM-R is able to bind S4GGnM-bovine serum albumin (S4GGnM-BSA) conjugates whereas the Man-R is not. In contrast both the S4GGnM-R and the Man-R are able to bind Man-BSA. Monosaccharides that inhibit binding of Man-BSA by the Man-R enhance binding by the S4GGnM-R. Oligosaccharides terminating with S4GGnM and those terminating with Man are bound at independent sites on the S4GGnM-R. The S4GGnM-R present in hepatic endothelial cells may account for clearance of glycoproteins bearing oligosaccharides terminating with S4GGnM and glycoproteins bearing oligosaccharides terminating with either mannose, fucose, or N-acetylglucosamine.Asn-linked oligosaccharides present on the glycoprotein hormones lutropin (LH) 1 and thyrotropin (TSH) terminate with the sequence SO 4 -4GalNAc1,4GlcNAc1,2Man␣ (S4GGnM), whereas those on follitropin and chorionic gonadotropin (CG) terminate with the sequence Sia␣2,3/6Gal1,4GlcNAc1, 2Man␣ (1-5). We have proposed that the sulfated oligosaccharides present on LH and TSH are critical for the expression of full biologic function by these hormones (6 -9). Terminal GalNAc-4-SO 4 does not influence binding to or activation of the LH/CG receptor itself (10) but does have a marked impact on the circulatory half-life of LH following secretion (7, 11) due to recognition of the sulfated oligosaccharides by a receptor expressed at the surface of hepatic endothelial cells and Kupffer cells (11,12). The rapid removal of LH from the circulation in conjunction with its release from granules in response to gonadotropin releasing hormone accounts for the episodic rise and fall in hormone levels seen in the circulation. Since the LH/CG receptor is a G-protein-coupled receptor, which rapidly becomes refractory to further stimulation following ligand binding (13-15), episodic stimulation may provide for maximal activation during the preovulatory surge in circulating LH levels. TSH shows similar properties with respect to half-life and receptor activatio...
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