Evolution of variable and constant domains and joining segments of rearranging immunoglobulins.

The rearranging immunoglobulins (Igs) are a family of recognition and defense proteins found in all vertebrate classes. These proteins consist of two types of polypeptide chains; each of these contains a variable (V) domain, a joining (J) segment, and a constant (C) region, which can itself consist of one to four domains. The distinction between light and heavy chains is an ancient one phylogenetically that is reflected in the structures of V, J, and C regions. Despite the early emergence of these genetic elements, conservatism is apparent in the peptide structures encoded by V, J, and C exons. C regions of heavy chains did not evolve as single units; rather the individual domains show their own clustering patterns, which apparently are independent of heavy-chain designation or species. C-region domains of light chains and the T cell receptor beta chain are similar to one another and to the most carboxyl-terminal domain of heavy chains. Comparison of the light chains of sharks, bullfrogs, chickens, and mammals indicated that a phylogenetic distinction can be made between kappa and lambda light chains. V and J segments of the rearranging T cell receptors alpha, gamma, and delta are homologous to the corresponding segments of Igs, but their C regions form a group that is markedly distinct from those of conventional Igs and Tcr beta.