Low-density lipoprotein (LDL) receptor (LDLR) is a mosaic protein of 839 amino acids which belongs to the LDLR gene family. It is a cell-surface receptor that recognizes the Apo B100, which is embedded in the outer phospholipid layer of LDL particles. The receptor also recognizes the Apo E protein found in chylomicron remnants and VLDL remnants.
LDLR mediates the endocytosis of cholesterol-rich LDL. LDLR complexes are present in clathrin-coated pits (or buds) on the cell surface, which when bound to LDL-cholesterol via adaptin, are pinched off to form clathrin-coated vesicles inside the cell. This allows LDL-cholesterol to be internalized in a process known as endocytosis and prevents the LDL just diffusing around the membrane surface. This occurs in all nucleated cells, but mainly in the liver which removes ~70% of LDL from the circulation. Hyperthyroidism may be associated with hypocholesterolaemia via upregulation of the LDLR, and hypothyroidism with the converse.