Evolution of vertebrate IgM: complete amino acid sequence of the constant region of Ambystoma mexicanum μ chain deduced from cDNA sequence

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Abstract
CDNA clones coding for the constant region of the Mexican axolotl (Ambystoma mexicanum) μ heavy immunoglobulin chain were selected from total spleen RNA, using a cDNA polymerase chain reaction technique. The specific 5′-end primer was an oligonucleotide homologous to the JH segment of Xenopus laevis μ chain. One of the clones, JHA/3, corresponded to the complete constant region of the axolotl μ chain, consisting of a 1362-nucleotide sequence coding for a polypeptide of 454 amino acids followed in 3′ direction by a 179-nucleotide untranslated region and a polyA+ tail. The axolotl Cμ is divided into four typical domains (Cμ1-Cμ4) and can be aligned with the Xenopus Cμ with an overall identity of 56% at the nucleotide level. Percent identities were particularly high between Cμ1 (59%) and Cμ4 (71%). The C-terminal 20-amino acid segment which constitutes the secretory part of the μ chain is stronglyhomologous to the equivalent sequences of chondrichthyans and of other tetrapods, including a conserved N-linked oligosaccharide, the penultimate cysteine and the C-terminal lysine. The four Cμ domains of 13 vertebrate species ranging from chondrichthyans to mammals were aligned and compared at the amino acid level. The significant number of μ-specific residues which are conserved into each of the four Cμ domains argues for a continuous line of evolution of the vertebrate μ chain. This notion was confirmed by the ability to reconstitute a consistent vertebrate evolution tree based on the phylogenic parsimony analysis of the Cμ4 sequences.