Hepatic and extrahepatic scavenger receptors: function in relation to disease

Requirements of basic amino acid residues within the lectin-like domain of LOX-1 for the binding of oxidized low-density lipoprotein

Lectin-like OxLDL receptor-1 (LOX-1) was identified as the major receptor for oxidized low-density lipoprotein (OxLDL) in aortic endothelial cells. LOX-1 is a type II membrane protein that structurally belongs to the C-type lectin family. Here, we found that the lectin-like domain of LOX-1 is essential for ligand binding, but the neck domain is not. In particular, the large loop between the third and fourth cysteine of the lectin-like domain plays a critical role for OxLDL binding as well as C-terminal end residues. Alanine-directed mutagenesis of the basic amino acid residues around this region revealed that all of the basic residues are involved in OxLDL binding. Simultaneous mutations of these basic residues almost abolished the OxLDL-binding activity of LOX-1. Electrostatic interaction between basic residues in the lectin-like domain of LOX-1 and negatively charged OxLDL is critical for the binding activity of LOX-1.

Conserved C-terminal residues within the lectin-like domain of LOX-1 are essential for oxidized low-density-lipoprotein binding

Lectin-like oxidized low-density-lipoprotein (oxLDL) receptor-1 (LOX-1) is a cell-surface endocytosis receptor for atherogenic oxLDL, which is highly expressed in endothelial cells. Recent studies suggest that it may play significant roles in atherogenesis. LOX-1 is a type-II membrane protein that structurally belongs to the C-type lectin family molecules. This study was designed to characterize the specific domain on LOX-1 that recognizes oxLDL. Truncation of the lectin domain of LOX-1 abrogated oxLDL-binding activity. Deletion of the utmost C-terminal ten amino acid residues (261-270) was enough to disrupt the oxLDL-binding activity. Substitutions of Lys-262 and/or Lys-263 with Ala additively attenuated the activity. Serial-deletion analysis showed that residues up to 265 are required for the expression of minimal binding activity, although deletion of the C-terminal three residues (268-270) still retained full binding activity. Consistently, these alterations in LOX-1 impaired the recognition by a functionally blocking monoclonal antibody for LOX-1. These data demonstrated the distinct role of the lectin domain as the functional domain recognizing LOX-1 ligand. The conserved C-terminal residues of lectin-like domain are essential for binding oxLDL. Particularly, the basic amino acid pair is important for the binding.

Identification of binding proteins for cholesterol absorption inhibitors as components of the intestinal cholesterol transporter

To identify protein components of the intestinal cholesterol transporter, rabbit small intestinal brush border membrane vesicles were submitted to photoaffinity labeling using photoreactive derivatives of 2-azetidinone cholesterol absorption inhibitors. An integral membrane protein of M(r) 145.3+/-7.5 kDa was specifically labeled in brush border membrane vesicles from rabbit jejunum and ileum. Its labeling was concentration-dependently inhibited by the presence of cholesterol absorption inhibitors whereas bile acids, D-glucose, fatty acids or cephalexin had no effect. The inhibitory potency of 2-azetidinones to inhibit photolabeling of the 145 kDa protein correlated with their in vivo activity to inhibit intestinal cholesterol absorption. These results suggest that an integral membrane protein of M(r) 145 kDa is (a component of) the cholesterol absorption system in the brush border membrane of small intestinal enterocytes.