Defective binding of apolipoprotein E (apoE) to heparan sulfate proteoglycans (HSPGs) is associated with increased risk of atherosclerosis due to inefficient clearance of lipoprotein remnants by the liver. The interaction of apoE with HSPGs has also been implicated in the pathogenesis of Alzheimer's disease and may play a role in neuronal repair. To identify which residues in the heparin-binding site of apoE and which structural elements of heparan sulfate interact, we used a variety of approaches, including glycosaminoglycan specificity assays, 13C nuclear magnetic resonance, and heparin affinity chromatography. The formation of the high affinity complex required Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147 from apoE and N-and 6-O-sulfo groups of the glucosamine units from the heparin fragment. As shown by molecular modeling, using a high affinity binding octasaccharide fragment of heparin, these findings are consistent with a binding mode in which five saccharide residues of fully sulfated heparan sulfate lie in a shallow groove of the ␣-helix that contains the HSPG-binding site (helix 4 of the four-helix bundle of the 22-kDa fragment). This groove is lined with residues Arg-136, Ser-139, His-140, Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147. In the model, all of these residues make direct contact with either the 2-O-sulfo groups of the iduronic acid monosaccharides or the N-and 6-O-sulfo groups of the glucosamine sulfate monosaccharides. This model indicates that apoE has an HSPG-binding site highly complementary to heparan sulfate rich in N-and O-sulfo groups such as that found in the liver and the brain. Human apolipoprotein E (apoE)1 is a 299-residue polymorphic protein that facilitates the transport and metabolism of lipids (1). ApoE is a ligand for members of the low density lipoprotein (LDL) receptor family, heparin, and heparan sulfate proteoglycans (HSPGs) (2, 3). It is composed of two domains: a 22-kDa NH 2 -terminal domain (residues 1-191) and a 10-kDa COOH-terminal domain (residues 216 -299) (4). The 22-kDa NH 2 -terminal domain contains the primary HSPGbinding site (residues 140 -150) (5) colocalized with the LDL receptor binding site (6 -8).Binding of apoE to HSPG is an initial step in the localization of apoE-containing lipoproteins to the surface of several different types of cells (9). After localization, the apoE-containing lipoproteins are transported into the cell by pathways dependent on either the LDL receptor or the LDL receptor-related protein (LRP) or by direct uptake of an apoE-containing lipoprotein-HSPG complex (10). Binding of apoE to HSPG affects neurite extension in neurons (11, 12) and localizes secreted apoE to the surface of macrophages (13). Binding of apoE to HSPG may also play a role in Alzheimer's disease through either competition between apoE and the amyloid precursor protein (APP) for HSPG-binding sites or by modulation of the HSPG/LRP uptake pathway (14 -17).The best understood physiological role of the binding of apoE to HSPG is in lipoprotein remnant clearance....
We have previously identified and characterized a heparin-binding cell surface protein (heparin͞heparan sulfate-interacting protein, or HIP) present on epithelial and endothelial cells. A synthetic peptide mimicking a heparinbinding domain of HIP is now shown to bind a small subset of heparin molecules with high affinity and, therefore, presumably recognizes a specific structural motif in the heparin molecule. Further analyses revealed that the heparin molecules exhibiting a high affinity for the HIP peptide also show an extremely high affinity for antithrombin III (AT-III), a cofactor required for heparin's anticoagulant activity. The HIP peptide was shown to compete with AT-III for binding to heparin and to neutralize the anticoagulant activity of heparin in blood plasma assays. Furthermore, the heparin subfraction that binds to the HIP peptide with high affinity exhibits an extremely high anticoagulant activity. We conclude that although the HIP peptide shows no sequence similarity with AT-III, the two proteins recognize the same or similar structural motifs in heparin.
No abstract
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.