Low density lipoprotein receptor gene family

The Low density lipoprotein receptor gene family constitutes a class of structurally closely related cell surface receptors that fulfill diverse biological functions in different organs, tissues, and cell types. The role that is most commonly associated with this evolutionarily ancient gene family is the endocytosis and removal of low-density lipoprotein(LDL), the main carrier of cholesterol in the circulation of humans, by the liver. Indeed, the regulation of cholesterol homeostasis through this mechanism remains the sole known function of the family’s namesake, the LDL receptor. Yet, over the last few years, considerable evidence has accumulated at an increasingly rapid pace that shows that the members of the LDL receptor gene family also have fundamental functions in transmitting or modulating signals between specialized cells in many, if not all, multicellular organisms.

There are seven members of the LDLR family in mammals, namely:
 * LDLR
 * VLDL receptor (VLDLR)
 * ApoER2, or LRP8
 * Multiple epidermal growth factor (EGF) repeat-containing protein (MEGF7)
 * LDLR-related protein 1 (LRP1)
 * LDLR-related protein 1b (LRP1b)
 * Megalin.


 * See also: Soluble low density lipoprotein receptor-related protein (sLRP), whose impaired function is related to Alzheimer's Disease.

Structure
The members of the LDLR family are characterized by distinct functional domains present in characteristic numbers. These modules are:
 * the type A binding repeats of 40 residues each, displaying a triple-disulfide-bond-stabilized negatively charged surface; certain head-to-tail combinations of these repeats are believed to specify ligand interactions;
 * type B repeats, also containing six cysteines each;
 * EGF repeats with YWTD beta propeller domain;
 * a transmembrane domain, and
 * the cytoplasmic region with (a) signal(s) for receptor internalization via coated pits, containing the consensus tetrapeptide Asn-Pro-Xaa-Tyr (NPxY). This cytoplasmic tail controls both endocytosis and signaling by interacting with the phosphotyrosine binding (PTB) domain-containing proteins.