Analysis of a cDNA sequence encoding the immunoglobulin heavy chain of the Antarctic teleost Trematomus bernacchii.

A spleen cDNA library was constructed from the Antarctic teleost Trematomus bernacchii and immunoscreened with rabbit IgG specific for T. bernacchii Ig heavy chain. Eleven cDNA clones, varying in size and encoding the entire heavy chain or parts of it, were isolated. Here the complete nucleotide and deduced amino acid sequences of clone 2C2 encoding the secretory IgH chain form are reported. Comparison of the amino acid sequence of the entire constant region of the T. bernacchii Ig heavy chain with those from other teleosts and two holostean fish showed percent identity ranging 53.6-60.6%, with the highest values found for Salmoniformes. The multiple sequence alignment revealed the presence of two remarkable insertions: one at the VH-CH1 boundary and a second one, not found in any other IgM heavy chain, localised at the CH2-CH3 boundary. The latter occurred in the region proposed to act as a 'hinge', and resulted in a CH2-CH3 hinge peptide longer than any other IgM hinge. Differences were also found in the number and position of putative N-glycosylation sites of the compared sequences. It is suggested that the unusual features found in the T. bernacchii Ig heavy chain might contribute to the flexibility of the Ig molecule and help understand more about the adaptation of Ig molecules to the polar sea environment.