LDL receptor

The LDL Receptor is a mosaic protein that mediates the endocytosis of cholesterol-rich LDL. It is a cell-surface receptor that recognises the apoprotein B100 which is embedded in the phospholipid outer layer of LDL particles. The receptor also recognizes the apoE protein found in chylomicron remnants and VLDL remnants (IDL). Brown and Goldstein won a Nobel Prize for their identification of the Low Density Lipoprotein (LDL) receptor 30 years ago whilst they were studying familial hypercholesterolemia.

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Function of the Receptor

LDL receptor 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 bound and internalised in a process known as endocytosis and prevents the LDL just diffusing around the membrane surface. This occurs in all nucleated cells (not erythrocytes), but mainly in the liver which removes ~70% of LDL from the circulation. LDL is directly involved in the development of atherosclerosis, due to accumulation of LDL-cholesterol in the blood. Atherosclerosis is the process responsible for the majority of cardiovascular diseases.

Once the coated vesicle is internalized it will shed its clathrin coat and will fuse with an acidic late endosome. The change in pH causes a conformational change in the receptor that releases the bound LDL particle. The receptors are then either destroyed or they can be recycled via the endocytic cycle back to the surface of the cell where the neutral pH will cause the receptor to revert to its native conformation ready to receive another LDL particle.

Synthesis of receptors in the cell is regulated by the level of free intracellular cholesterol; if it is in excess for the needs of the cell then the transcription of the receptor gene will be inhibited. LDL receptors are translated by ribosomes on the endoplasmic reticulum and are modified by the golgi apparatus before travelling in vesicles to the cell surface.

Structure of the Receptor

The LDL receptor can be described as a chimeric protein. It is made up of a number of functionally distinct domains that can function independently of each other.

The N-terminus of the LDL receptor contains seven sequence repeats (~50% identical) each ~40 amino acids long, with 6 cysteine residues. These ligand binding (LB) regions fold autonomously when synthesised as individual peptides. The cysteine residues form disulphide bonds forming an octahedral lattice, coordinated to a calcium ion, in each repeat. The exact mechanism of interaction between the LB repeats and ligand (LDL) is unknown, but it is thought that the repeats act as "grabbers" to hold the LDL. Binding of ApoB requires repeats 2-7 while binding ApoE requires only repeat 5 (thought to be the ancestral repeat).

Next to the ligand binding domain is an epidermal growth factor (EGF) precursor homology domain (EGFP domain). This shows approximately 30% homology with the EGF precursor gene. There are three "growth factor" repeats; A, B and C. A and B are closely linked while C is separated by a beta-properller motif. The EGFP domain has been implicated in release of ligands bound to the receptor. It is thought that a conformational change occurs in the acidic (pH5.0) conditions of the endosome bringing the beta-propeller into contact with ligand-binding repeats 4 and 5.

A third domain of the protein is rich in O-linked oligosaccharides but appears to show little function. Knockout experiments have confirmed that no significant loss of activity occurs without this domain. It has been speculated that the domain may have ancestrally acted as a spacer to push the receptor beyond the extracellular matrix.

A membrane spanning domain containing a chain of hydrophobic amino acid residues crosses the plasma membrane of the cell. Inside the cell the C-terminus domain contains a signal sequence that is needed for receptor internalisation.

Structure of the Gene

The gene coding the LDL receptor is split into 18 exons. Exon 1 contains a signal sequence that localises the receptor to the endoplasmic reticulum for transport to the cell surface. Beyond this, exons 2-6 code the ligand binding region; 7-14 code the EGFP domain; 15 codes the oligosaccharide rich region; 16 (and some of 17) code the membrane spanning region; and 18 (with the rest of 17) code the cytosolic domain.

Mutations of the LDL receptor

There are 5 broad classes of mutation of the LDL receptor.

• Class 1 mutations affect the synthesis of the receptor in the endoplasmic reticulum (ER).
• Class 2 mutations prevent proper transport to the Golgi body needed for modifications to the receptor.
  • eg. a truncation of the receptor protein at residue number 660 leads to domains 3,4 and 5 of the EGF precursor domain being missing. This precludes the movement of the receptor from the ER to the Golgi, and leads to degradation of the receptor protein.
• Class 3 mutations stop the binding of LDL to the receptor.
  • eg. repeat 6 of the ligand binding domain (N-terminal, extracellular fluid) is deleted.
• Class 4 mutations inhibit the internalisation of the receptor-ligand complex.
  • eg. "JD" mutant results from a single point mutation in the NPVY domain (C-terminal, cytosolic; Y residue converted to a C, residue number 807). This domain recruits clathrin and other proteins responsible for the endocytosis of LDL, therefore this mutation inhibits LDL internalization.
• Class 5 mutations give rise to receptors that cannot recycle properly. This leads to a relatively mild phenotype as receptors are still present on the cell surface (but all must be newly synthesised).

Further reading