sepharose and mannose-6-phosphate

The insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/MPR) is a type I glycoprotein that mediates both the intracellular sorting of lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) residues to the lysosome and the bioavailability of IGF-II. The extracytoplasmic region of the IGF-II/MPR contains 15 repeating domains; the two carbohydrate recognition domains (CRDs) have been localized to domains 1-3 and 7-9, and the high-affinity IGF-II binding site maps to domain 11. To characterize the carbohydrate binding properties of the IGF-II/MPR, regions of the receptor encompassing the individual CRDs were produced in a baculovirus expression system. Characterization of the recombinant proteins revealed that the pH optimum for carbohydrate binding is significantly more acidic for the carboxyl-terminal CRD than for the amino-terminal CRD (i.e., pH 6.4-6.5 vs 6.9). Equilibrium binding studies demonstrated that the two CRDs exhibit a similar affinity for Man-6-P. Furthermore, substitution of the conserved arginine residue in domain 3 (R435) or in domain 9 (R1334) with alanine resulted in a similar >1000-fold decrease in the affinity for the lysosomal enzyme, beta-glucuronidase. In contrast, the two CRDs differ dramatically in their ability to recognize the distinctive modifications (i.e., mannose 6-sulfate and Man-6-P methyl ester) found on Dictyostelium discoideum lysosomal enzymes: the amino-terminal CRD binds mannose 6-sulfate and Man-6-P methyl ester with a 14-55-fold higher affinity than the carboxyl-terminal CRD. Taken together, these results demonstrate that the IGF-II/MPR contains two functionally distinct CRDs.

Topics: Animals; Binding Sites; Chromatography, Affinity; Dictyostelium; Esters; Genetic Vectors; Glucuronidase; Humans; Lysosomes; Mannosephosphates; Mutagenesis, Insertional; Oligosaccharides; Peptide Fragments; Protein Binding; Protein Structure, Tertiary; Receptor, IGF Type 2; Recombinant Proteins; Sepharose; Sequence Deletion

A soluble truncated form of the cation-dependent mannose 6-phosphate receptor (CD-MPR) encoding only the extracytoplasmic region, Stop155, and a truncated glycosylation-deficient form of the CD-MPR, Asn81/Stop155, which has been modified to contain only one N-linked glycosylation site at position 81 instead of five, were purified from baculovirus-infected High Five insect cells. The glycosylated recombinant proteins were functional in ligand binding and acid-dependent dissociation as assessed by pentamannosyl phosphate-agarose affinity chromatography. Gel filtration, sucrose gradients, and cross-linking experiments revealed that both Stop155 and Asn81/Stop155 are dimeric, demonstrating that the transmembrane and cytoplasmic region of the receptor as well as N-linked oligosaccharides at positions 31, 57, and 87 are not required for dimerization. The Kd of Stop155 and Asn81/Stop155 for the lysosomal enzyme, beta-glucuronidase, was 0.2 and 0.3 nM, respectively. These values are very similar to those reported for the full-length CD-MPR, demonstrating that the extracellular region of the CD-MPR is sufficient for high-affinity binding and that oligosaccharides at positions 31, 57, and 87 do not influence ligand binding.

Topics: Acids; Animals; Asparagine; Binding Sites; Cations; Cattle; Cell Line; Chromatography, Affinity; Dimerization; Genetic Vectors; Glycosylation; Hydrogen-Ion Concentration; Mannosephosphates; Nucleopolyhedroviruses; Oligosaccharides; Peptide Fragments; Protein Isoforms; Receptor, IGF Type 2; Recombinant Proteins; Sepharose; Spodoptera

Envelope glycoproteins of human immunodeficiency viruses (HIV-1 and HIV-2) can interact with high-mannose glycans and with the mannosyl or N-acetylglucosaminyl core of complex-type oligosaccharidic structures. HIV-1 glycoproteins also specifically bind sulphated polysaccharides such as dextran sulphate (DS) and heparin. Here, we show that the latter property is shared by HIV-2 recombinant gp140 (rgp140) precursor glycoprotein. Binding of rgp140 and of corresponding rgp160 of HIV-1 to heparin- and DS-substituted (sulphated dextran beads; SDB) affinity matrices was inhibited by the soluble specific ligand and also by fetuin, asialofetuin or the anionic simple carbohydrate derivative mannose-6-phosphate (M6P). Interaction of HIV-1 rgp120 subunit with the two affinity matrices was also inhibited by M6P, but only rgp120 binding to heparin-agarose, and not that to SDB, was affected by fetuin and asialofetuin. These results suggest that HIV-1 and HIV-2 envelope glycoproteins presumably display different sulphated polysaccharide and carbohydrate recognition sites. Some of these may be common or in close proximity: with respect to rgp160, for example, the sites may be common on the gp41 moiety and/or in a region of gp120 which would be more accessible when expressed on rgp160 than on processed gp120, while they may be distinct on the cleaved gp120 subunit. Finally, because M6P is a marker of lysosomal enzymes, we verified that HIV-1 and HIV-2 envelope glycoproteins could specifically bind in a M6P-inhibitable manner to a representative lysosomal enzyme, bovine liver beta-glucuronidase coupled to agarose, suggesting that they may possibly interfere with lysosomal enzyme sorting in HIV-infected cells.

Topics: Animals; Binding Sites; Cattle; Dextran Sulfate; Gene Products, env; Glucuronidase; Heparin; HIV-1; HIV-2; Liver; Mannosephosphates; Protein Binding; Sepharose