Low-density lipoprotein receptor--its structure, function, and mutations.

Uptake of cholesterol, mediated by the low-density lipoprotein (LDL)-receptor, plays a crucial role in lipoprotein metabolism. The LDL-receptor is responsible for the binding and subsequent cellular uptake of apolipoprotein B- and E-containing lipoproteins. To accomplish this, the receptor has to be transported from the site of synthesis, the membranes of the rough endoplasmatic reticulum (ER), through the Golgi apparatus, to its position on the surface of the cellular membrane. The translation of LDL-receptor messenger RNA into the polypeptide chain for the receptor protein takes place on the surface-bound ribosomes of the rough ER. Immature O-linked carbohydrate chains are attached to this integral precursor membrane protein. The molecular weight of the receptor at this stage is 120.000 d. The precursor-protein is transported from the rough ER to the Golgi apparatus, where the O-linked sugar chains are elongated until their final size is reached. The molecular weight has then increased to 160.000 d. The mature LDL-receptor is subsequently guided to the "coated pits" on the cell surface. These specialized areas of the cell membrane are rich in clathrin and interact with the LDL-receptor protein. Only here can the LDL-receptor bind LDL-particles. Within 3 to 5 minutes of its formation, the LDL-particle-receptor complex is internalized through endocytosis and is further metabolized through the receptor-mediated endocytosis pathway. Mutations in the gene coding for the LDL-receptor can interfere to a varying extent with all the different stages of the posttranslational processing, binding, uptake, and subsequent dissociation of the LDL-particle-LDL-receptor complex, but invariably the mutations lead to familial hypercholesterolemia. Thus, mutations in the LDL-receptor gene give rise to a substantially varying clinical expression of familial hypercholesterolemia.