Phylogeny of immunoglobulin heavy chain isotypes: structure of the constant region of Ambystoma mexicanum upsilon chain deduced from cDNA sequence.

An RNA polymerase chain reaction strategy was used to amplify and clone a cDNA segment encoding for the complete constant part of the axolotl IgY heavy (C upsilon) chain. C upsilon is 433 amino acids long and organized into four domains (C upsilon 1-C upsilon 4); each has the typical internal disulfide bond and invariant tryptophane residues. Axolotl C upsilon is most closely related to Xenopus C upsilon (40% identical amino acid residues) and C upsilon 1 shares 46.4% amino acid residues among these species. The presence of additional cysteines in C upsilon 1 and C upsilon 2 domains is consistent with an additional intradomain S-S bond similar to that suggested for Xenopus C upsilon and C chi, and for the avian C upsilon and the human C epsilon. C upsilon 4 ends with the Gly-Lys dipeptide characteristic of secreted mammalian C gamma 3, human C epsilon 4, and avian and anuran C upsilon 4, and contains the consensus [G/GT(AA)] nucleotide splice signal sequence for joining C upsilon 4 to the transmembrane region. These results are consistent with the hypothesis of an ancestral structural relationship between amphibian, avian upsilon chains, and mammalian epsilon chains. However, these molecules have different biological properties: axolotl IgY is secretory Ig, anuran and avian IgY behave like mammalian IgG, and mammalian IgE is implicated in anaphylactic reactions.