J chain

A J chain is a protein component of the is_associated_with::antibodies is_associated_with::IgM and is_associated_with::IgA. It is a 137 residue polypeptide, encoded by the IGJ is_associated_with::gene.

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

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

Function
The J Chain is required for IgM or IgA to be secreted into mucosa.

Because IgM and IgA are the only two types of antibody that is_associated_with::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 is_associated_with::pentamer and a is_associated_with::hexamer, however, both of these exist to lesser numbers in organisms lacking J chains. In such case, there are also fewer IgA dimers.