Sequence and expression pattern of J chain in the amphibian, Xenopus laevis

We have determined the cDNA sequence encoding J chain, a polypeptide accessory molecule associated with polymeric Ig, from the anuran amphibian, Xenopus laevis (South African clawed frog). The translated polypeptide consists of 164 amino acid residues, including the signal sequence, and is somewhat longer than the corresponding sequence in mouse and cow, the two mammalian species in which the signal sequence of J chain has been determined. J chain in several mammalian species (human, mouse, cow and rabbit) has eight Cys residues. In the human chain, two of these Cys residues, the second and third in the sequence, have been shown to form disulfide bridges to heavy chains in IgM or IgA; the remaining Cys residues form intrachain disulfide bonds. The Xenopus J chain contains only seven of these Cys residues. Ser is found at the position corresponding to the third Cys in mammalian J chains. Northern blot analysis, performed on RNA isolated from various organs of 3-month old frogs, indicated that the highest level of expression was in the intestine. Transcripts corresponding to J chain were also detected in the spleen and at very low levels in the thymus.