The LU domain ( Ly-6 antigen/

uPAR The Urokinase receptor, also known as urokinase plasminogen activator surface receptor (uPAR) or CD87 (Cluster of Differentiation 87), is a protein encoded in humans by the PLAUR gene. It is a multidomain glycoprotein tethered to the cell membran ...

) is an evolutionarily conserved protein domain of the

three-finger protein Three-finger proteins or three-finger protein domains (3FP or TFPD) are a protein superfamily consisting of small, roughly 60-80 amino acid residue protein domains with a common tertiary structure: three beta strand loops extended from a hydrophob ...

superfamily. This domain is found in the

extracellular domain An ectodomain is the domain of a membrane protein that extends into the extracellular space (the space outside a cell). Ectodomains are usually the parts of proteins that initiate contact with surfaces, which leads to signal transduction.A notable ...

s of

cell-surface receptor Cell surface receptors (membrane receptors, transmembrane receptors) are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving (binding to) extracellular molecules. They are specialized integral me ...

s and in either GPI-anchored or

secreted 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical ...

globular protein In biochemistry, globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (formi ...

s, for example the Ly-6 family,

CD59 CD59 glycoprotein, also known as MAC-inhibitory protein (MAC-IP), membrane inhibitor of reactive lysis (MIRL), or protectin, is a protein that in humans is encoded by the ''CD59'' gene. It is an LU domain and belongs to the LY6/uPAR/ alpha-ne ...

, and Sgp-2. A variety of GPI-linked cell-surface glycoproteins are composed of one or more copies of a conserved LU domain of about 100 amino-acid residues. Among these proteins, most contain only a single LU domain, though small numbers of exceptions are known; well-studied family member

has three tandem LU domains.

Structure

This domain folds into five antiparallel

beta sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a g ...

s, a structure common to the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

. The domain typically contains ten well- conserved

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

residues involved in five

disulfide bond In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

s, though some examples such as two of the three

domains have fewer.

Examples

Besides uPAR, other receptors with LU domains include members of the transforming growth factor beta receptor (TGF-beta) superfamily, such as the

activin type 2 receptor The activin type 2 receptors belong to a larger TGF-beta receptor family and modulate signals for transforming growth factor beta ligands. These receptors are involved in a host of physiological processes including, growth, cell differentiation, ...

; and bone morphogenetic protein receptor, type IA. Other LU domain proteins are small globular proteins such as CD59 antigen,

LYNX1 Ly6/neurotoxin 1 is a protein in humans that is encoded by the ''LYNX1'' gene. Alternatively spliced variants encoding different isoforms have been identified. Function This gene encodes a member of the Ly-6/ neurotoxin gene family, a group o ...

SLURP1 Secreted Ly-6/uPAR-related protein 1 is a protein that in humans is encoded by the ''SLURP1'' gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''ge ...

, and

SLURP2 {{Short pages monitor