Immunoglobulin superfamily

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Immunoglobulin superfamily
SCOP2
1tlk / SCOPe / SUPFAM
OPM superfamily193
OPM protein3bib
CDDcd00096
Membranome2
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary
Immunoglobulin-like (ligands)
Identifiers
SymbolIg protein ligands
Membranome64
Immunoglobulin-like adhesion molecules
Identifiers
SymbolIgSF CAM
Membranome110

The immunoglobulin superfamily (IgSF) is a large

superfamily based on shared structural features with immunoglobulins (also known as antibodies); they all possess a domain known as an immunoglobulin domain or fold. Members of the IgSF include cell surface antigen receptors, co-receptors and co-stimulatory molecules of the immune system, molecules involved in antigen presentation to lymphocytes, cell adhesion molecules, certain cytokine receptors and intracellular muscle proteins. They are commonly associated with roles in the immune system. Otherwise, the sperm-specific protein IZUMO1
, a member of the immunoglobulin superfamily, has also been identified as the only sperm membrane protein essential for sperm-egg fusion.

Immunoglobulin domains

Proteins of the IgSF possess a

disulfide bonds formed between cysteine residues in the B and F strands, stabilize the Ig-fold.[citation needed
]

Classification

The Ig like domains can be classified as IgV, IgC1, IgC2, or IgI.[3]

Most Ig domains are either variable (IgV) or constant (IgC).

  • IgV: IgV domains with 9 beta strands are generally longer than IgC domains with 7 beta strands.
  • IgC1 and IgC2: Ig domains of some members of the IgSF resemble IgV domains in the amino acid sequence, yet are similar in size to IgC domains. These are called IgC2 domains, while standard IgC domains are called IgC1 domains.
  • IgI: Other Ig domains exist that are called intermediate (I) domains.[4]

Members

The Ig domain was reported to be the most populous family of proteins in the human genome with 765 members identified.[5] Members of the family can be found even in the bodies of animals with a simple physiological structure such as poriferan sponges. They have also been found in bacteria, where their presence is likely to be due to divergence from a shared ancestor of eukaryotic immunoglobulin superfamily domains.[6]

Members of the immunoglobulin superfamily
Molecule function/category Examples Description
Antigen receptors Antigen receptors found on the surface of T and B lymphocytes in all jawed vertebrates belong to the IgSF. Immunoglobulin molecules (the antigen receptors of B cells) are the founding members of the IgSF. In humans, there are five distinct types of immunoglobulin molecule all containing a heavy chain with four Ig domains and a light chain with two Ig domains. The antigen receptor of T cells is the T-cell receptor (TCR), which is composed of two chains, either the TCR-alpha and -beta chains, or the TCR-delta and gamma chains. All TCR chains contain two Ig domains in the extracellular portion; one IgV domain at the N-terminus and one IgC1 domain adjacent to the cell membrane.
Antigen presenting molecules The ligands for TCRs are major histocompatibility complex (MHC) proteins. These come in two forms; MHC class I forms a dimer with a molecule called beta-2 microglobulin (β2M) and interacts with the TCR on cytotoxic T cells and MHC class II has two chains (alpha and beta) that interact with the TCR on helper T cells. MHC class I, MHC class II and β2M molecules all possess Ig domains and are therefore also members of the IgSF.
Co-receptors Co-receptors and accessory molecules: Other molecules on the surfaces of T cells also interact with MHC molecules during TCR engagement. These are known as
helper T cells and the co-receptor CD8 is found on cytotoxic T cells. CD4 has four Ig domains in its extracellular portion and functions as a monomer. CD8, in contrast, functions as a dimer with either two identical alpha chains or, more typically, with an alpha and beta chain. CD8-alpha and CD8-beta each has one extracellular IgV domain in its extracellular portion. A co-receptor complex is also used by the BCR, including CD19
, an IgSF molecule with two IgC2-domains.
Antigen receptor accessory molecules A further molecule is found on the surface of T cells that is also involved in signaling from the TCR.
CD3
is a molecule that helps to transmit a signal from the TCR following its interaction with MHC molecules. Three different chains make up CD3 in humans, the gamma chain, delta chain and epsilon chain, all of which are IgSF molecules with a single Ig domain.

Similar to the situation with T cells, B cells also have cell surface co-receptors and accessory molecules that assist with cell activation by the B Cell Receptor (BCR)/immunoglobulin. Two chains are used or signaling, CD79a and CD79b that both possess a single Ig domain.

Co-stimulatory or inhibitory molecules Co-stimulatory or inhibitory molecules: Co-stimulatory and inhibitory signaling receptors and ligands control the activation, expansion and effector functions of cells. One major group of IgSF co-stimulatory receptors are molecules of the CD28 family;
B7-H4 (B7x/B7-S1).[8]
Receptors on Natural killer cells
Receptors on
Leukocytes
IgSF CAMs
  • NCAMs
  • ICAM-1
  • CD2 subset
  • Type IIa and Type IIb RPTPs, described in Receptor tyrosine kinases/phosphatases subsection below
Cytokine receptors
Growth factor receptors
Receptor tyrosine kinases/phosphatases
Ig binding receptors
Cytoskeleton
Others

References

  1. PMID 9609690
    .
  2. PMID 14690046
    .
  3. ISBN 978-0-12-369441-6
    . Retrieved 28 November 2010.
  4. PMID 8176743
    .
  5. PMID 11237011
    .
  6. PMID 8880921
    .
  7. PMID 16156851
    .
  8. PMID 15771580
    .
  9. S2CID 21801197
    .
  10. ^ Fraser CC, Howie D, Morra M, Qiu Y, Murphy C, Shen Q, Gutierrez-Ramos JC, Coyle A, Kingsbury GA, Terhorst C (February 2002). "Identification and characterization of SF2000 and SF2001, two new members of the immune receptor SLAM/CD2 family". Immunogenetics. 53 (10–11): 843–50.
    S2CID 10257502
    .
  11. ^ Tangye SG, Nichols KE, Hare NJ, van de Weerdt BC (September 2003). "Functional requirements for interactions between CD84 and Src homology 2 domain-containing proteins and their contribution to human T cell activation". Journal of Immunology. 171 (5): 2485–95.
    PMID 12928397
    .

External links
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