Immunoglobulin light chain isotypes in the teleost Trematomus bernacchii

Three immunoglobulin light chain (IgL) isotypes TrbeL1, TrbeL2, and TrbeL3were identified in the Antarctic teleost Trematomus bernacchii by immunoscreening a cDNA expression library, and using RT-PCR, and 5_ RACE. One of them was distinguished in two subisotypes TrbeL1A and TrbeL1B. Real-time PCR experiments showed that the different isotypes were expressed in similar ratios in the various tissues analyzed. Interestingly, the expression level of TrbeL1A isotypewas very high in all tissues. Molecular models of the CH1–CL domain pairings were built and minimized for the different isotypes. Several differences were identified in the superimposable structures mainly in the loops. In addition, the isotype-specific residues determined a different distribution of the charges on the external CL domain surface. Phylogenetic trees of 43 isotype representative sequences of CL domain from teleost species, built by different methods, indicated that all teleost light chain isotypes are distributed into three groups. Furthermore, the split of the group IgL1 into two subgroups, one of them carrying a micro-satellite DNA insertion, may have occurred in the Acanthopterygean ancestor.