Characterization of residues and sequences of the carbohydrate recognition domain required for cell surface localization and ligand binding of human lectin-like oxidized LDL receptor.

Lectin-like oxidized low-density lipoprotein receptor (LOX-1) has been cloned from human aortic endothelial cells, and has a sequence identical to that from human lung. Previous studies showed that human LOX-1 can recognize modified LDL, apoptotic cells and bacteria. To further explore the relationship between the structure and function of LOX-1, a mutagenesis study was carried out. Our results showed that the carbohydrate recognition domain (CRD) was the ligand-binding domain of human LOX-1. We also investigated the sequences and residues in CRD that were essential for protein cell surface localization and ligand binding. LOX-1s carrying a mutation on each of six Cys in CRD resulted in a variety of N-glycosylation and failed to be transported to the cell surface. This was strong evidence for the involvement of all six Cys in the intrachain disulfide bonds required for proper folding, processing and transport of LOX-1. The C-terminal sequence (KANLRAQ) was also essential for protein folding and transport, while the four final residues (LRAQ) were involved in maintaining receptor function. Both positive charged (R208, R209, H226, R229 and R231) and non-charged hydrophilic (Q193, S198, S199 and N210) residues were involved in ligand binding, suggesting that ligand recognition of LOX-1 is not merely dependent on the interaction of positively charged residues with negatively charged ligands.