Low-density lipoprotein receptor gene family

The low-density lipoprotein receptor gene family codes for a class of structurally related cell surface receptors that fulfill diverse biological functions in different organs, tissues, and cell types.^[3] The role that is most commonly associated with this evolutionarily ancient family is cholesterol homeostasis (maintenance of appropriate concentration of cholesterol). In humans, excess cholesterol in the blood is captured by low-density lipoprotein (LDL) and removed by the liver via endocytosis of the LDL receptor.^[4] Recent evidence indicates that the members of the LDL receptor gene family are active in the cell signalling pathways between specialized cells in many, if not all, multicellular organisms.^[5][6]

Low-density lipoprotein receptor domain class A
Structure of a cysteine-rich repeat from the low-density lipoprotein receptor.[1]
Identifiers
Symbol	Ldl_recept_a
Pfam	PF00057
InterPro	IPR002172
SMART	SM00192
PROSITE	PS50068
SCOP2	1ldl / SCOPe / SUPFAM
Membranome	5
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Low-density lipoprotein receptor domain class B
Structure of the LDL receptor extracellular domain at endosomal pH.[2]
Identifiers
Symbol	Ldl_recept_b
Pfam	PF00058
Pfam clan	6CL0186
InterPro	IPR000033
SMART	SM00135
PROSITE	PS51120
SCOP2	1lrx / SCOPe / SUPFAM
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary PDB 1ijq​, 1n7d​, 1npe​

There are seven members of the LDLR family in mammals, namely:

• LDLR
• VLDL receptor (VLDLR)
• ApoER2, or LRP8
• Low density lipoprotein receptor-related protein 4
  • also known as multiple epidermal growth factor (EGF) repeat-containing protein (MEGF7)
• LDLR-related protein 1
• LDLR-related protein 1b
• Megalin.

Human proteins containing this domain

Listed below are human proteins containing low-density lipoprotein receptor domains:

Class A

C6; C7; 8A; 8B; C9; CD320; CFI; CORIN; DGCR2; HSPG2; LDLR; LDLRAD2; LDLRAD3; LRP1; LRP10; LRP11; LRP12; LRP1B; LRP2; LRP3; LRP4; LRP5; LRP6; LRP8; MAMDC4; MFRP; PRSS7; RXFP1; RXFP2; SORL1; SPINT1; SSPO; ST14; TMPRSS4; TMPRSS6; TMPRSS7; TMPRSS9 (serase-1B); VLDLR;

Class B

EGF; LDLR; LRP1; LRP10; LRP1B; LRP2; LRP4; LRP5; LRP5L; LRP6; LRP8; NID1; NID2; SORL1; VLDLR;

See also

• Soluble low-density lipoprotein receptor-related protein (sLRP) - impaired function is related to Alzheimer's disease.

Structure

Modular structure of LDL receptor family members. Domains depicted hatched are differentially spliced and occur in some receptor isoforms only

The members of the LDLR family are characterized by distinct functional domains present in characteristic numbers. These modules are:

• LDL receptor type A (LA) 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;
• LDL receptor type B repeats, also known as EGF precursor homology regions, containing EGF-like repeats and YWTD beta propeller domains;
• 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.

In addition to these domains which can be found in all receptors of the gene family, LDL receptor and certain isoforms of ApoER2 and VLDLR contain a short region which can undergo O-linked glycosylation, known as O-linked sugar domain. ApoER2 moreover, can harbour a cleavage site for the protease furin between type A and type B repeats which enables production of a soluble receptor fragment by furin-mediated processing.