Figure 1.

Structure and evolutionary relationships of the catfish IgD heavy chain. (A) Inferred exon structure encoding the secreted and membrane forms of the catfish δ chain. The exons are based on the deduced protein sequence of the Ig domain and on the identification of δ1 and δ2 exons in the germ line. (B) Complete amino acid sequence of the membrane form of catfish δ, inferred from the full-length cDNA M5 (accession no. U67437U67437). The sequence is shown by domain, and alignments with the homologous sequences of human and mouse δ C regions are boxed. The antigen receptor CART motif is marked with asterisks. The sequences of mouse δ are from ref. 11, and those of human δ are from ref. 12. The aligned regions (numbering of Kabat et al., ref. 28) are, for human δ1, 114–224; human δ2, 243–363; human δ3, 364–478; mouse δ1, 116–222; mouse δ3, 364–478. The sequence of the catfish δ secreted segment (δ sec) is taken from cDNA S1 (accession no. U67438U67438).