THE JOINING (J)-CHAIN IS PRESENT IN INVERTEBRATES THAT DO NOT EXPRESSIMMUNOGLOBULINS

Joining (J) chain is a component of polymeric, but not monomeric, immu noglobulin (Ig) molecules and may play a role in their polymerization and transport across epithelial cells, To date, study of the J chain h as been confined to vertebrates that produce Ig and in which the J cha in displays a considerable degree of structural homology, The role of the J chain in Ig polymerization has been questioned and, since the J chain can be expressed in lymphoid cells that do not produce Ig, it is possible that the J chain may have other functions, To explore this p ossibility, we have surveyed J-chain gene, mRNA, and protein expressio n by using reverse transcriptase-coupled PCR, Northern blot analysis, and immunoblot analysis in invertebrate species that do not produce Ig , We report that the J-chain gene is expressed in invertebrates (Mollu sca, Annelida, Arthropoda, Echinodermata, and Holothuroidea), as well as in representative vertebrates (Mammalia, Teleostei, Amphibia), Furt hermore, J-chain cDNA from the earthworm has a high degree of homology (68-76%) to human, mouse, and bovine J chains, Immunohistochemical st udies reveal that the J chain is localized in the mucous cells of body surfaces, intestinal epithelial cells, and macrophage-like cells of t he earthworm and slug, This study suggests that the J chain is a primi tive polypeptide that arose before the evolution of Ig molecules and r emains highly conserved in extant invertebrates and vertebrates.