J chain

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Identifiers
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External IDsGeneCards: [1]
Orthologs
SpeciesHumanMouse
Entrez
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Immunoglobulin M (IgM) pentameric antibody molecule (consisting of five base units).
1: Base unit.
2: Heavy chains.
3: Light chains.
4: J chain.
5: Intermolecular disulfide bonds.
File:Dimeric IgA schematic 01.svg
Schematic of immunoglobulin A dimer showing H-chain (blue), L-chain (red), J-chain (magenta) and secretory component (yellow).

A J chain is a protein component of the antibodies IgM and IgA.[1] It is a 137 residue polypeptide,[2] encoded by the IGJ gene.[3][4][5]

Structure

The J Chain's molecular weight is approximately 15 kDa. It exhibits a standard immunoglobulin folding structure of two β-pleated sheets of four ribbons folded against one another. It has 8 cystine residues. Two of these residues link the α chains of IgA or the μ chains of IgM via disulfide bridges, effectively serving as the "glue" between two Fc regions of the antibody.[6]

The J-chain shows a large degree of homology between avian and human species, suggesting that it serves an important function.[6]

Function

The J Chain is required for IgM or IgA to be secreted into mucosa.[2] As part of a polymeric immunoglobulin (pIg), the J-chain is essential for binding of pIg to the pIgR, which forms the secretory component upon excretion of the secretory pIg by epithelial cells.[7] This binding facilitates transport of the J-chain positive pIg molecules from the basal to the apical sides of epithelial cells.

Because IgM and IgA are the only two types of antibody that polymerize, initial hypotheses stated that J chain was required for polymerization. However, it was subsequently found that IgM is able to polymerize in the absence of J chain as both a pentamer and a hexamer, however, both of these exist to lesser numbers in organisms lacking J chains. In such cases, there are also fewer IgA dimers.[2]

The J-chain also plays a role in the activation of complement. J-chain negative IgM hexamers are 15-20 times more effective at activating complement than J-chain positive IgM pentamers.[7] A consequence of this lack of complement activation is it allows J-chain positive pIgM to bind antigens without causing excessive damage to epithelial membranes through complement activation.

References

  1. Levinson. Medical Microbiology and Immunology (11 ed.). McGrawHill. pp. 405–6.
  2. 2.0 2.1 2.2 Schroeder, Harry; Wald, David; Greenspan, Neil (2008). "Chapter 4: Immunoglobulins: Structure and Function". In Paul, William. Fundamental Immunology (Book) (6th ed.). Philadelphia, PA: Lippincott Williams & Wilkins. pp. 125–151. ISBN 0-7817-6519-6.
  3. Max EE, McBride OW, Morton CC, Robinson MA (Sep 1986). "Human J chain gene: chromosomal localization and associated restriction fragment length polymorphisms". Proc Natl Acad Sci U S A. 83 (15): 5592–6. doi:10.1073/pnas.83.15.5592. PMC 386334. PMID 3016707.
  4. Max EE, Korsmeyer SJ (May 1985). "Human J chain gene. Structure and expression in B lymphoid cells". J Exp Med. 161 (4): 832–49. doi:10.1084/jem.161.4.832. PMC 2189063. PMID 2984306.
  5. "Entrez Gene: IGJ immunoglobulin J polypeptide, linker protein for immunoglobulin alpha and mu polypeptides".
  6. 6.0 6.1 Kiyono, Hiroshi; Kunisawa, Jaw; McGhee, Jerry; Mestecky, Jiri (2008). "Chapter 31: The Mucosal Immune System". In Paul, William. Fundamental Immunology (Book) (6th ed.). Philadelphia, PA: Lippincott Williams & Wilkins. pp. 983–1030. ISBN 0-7817-6519-6.
  7. 7.0 7.1 Johansen FE, Braathen R, Brandtzaeg P (September 2000). "Role of J Chain in Secretory Immunoglobulin Formation". Scandinavian Journal of Immunology. 52 (3): 240–248. doi:10.1046/j.1365-3083.2000.00790.x. PMID 10972899.

Further reading