Evolutionary variation of immunoglobulin mu heavy chain RNA processing pathways: origins, effects, and implications

Immunoglobulins (Ig) can occur in two physical forms, soluble (secreted) and membrane bound. The soluble form is secreted from B cells, and is present in the blood and other fluids where it plays a role as an immune effector molecule. The membrane-bound form of the Ig molecule is inserted into the B-cell membrane, where it serves as a receptor for antigen. The function of the membrane-bound Ig as a receptor for antigen requires additional accessory molecules, the membrane Ig plus accessory molecules are referred to, collectively, as the B-cell receptor (BCR) complex. The secreted and membrane-bound forms of an Ig result from alternative patterns of RNA processing of the primary transcript from the heavy chain gene. IgM is the only class of Ig known to be conserved in all vertebrate species (perhaps exclusive of the agnathan fish). While the structure of the IgM heavy (mu) chain gene has been highly conserved in vertebrate evolution, the patterns of alternative RNA processing of the mu transcript show surprising diversity. In particular, the bony fish (teleosts) produce membrane mu-chain message by a splicing pathway that is quite different from that seen in other vertebrates; it results in the production of membrane IgM that lacks the C mu 4 domain. How this unusual RNA splicing pattern could have evolved and its implications for the function of the BCR in the bony fishes are considered here.