Analysis of two IgM isotypes in Atlantic salmon and brown trout.

Atlantic salmon (Salmo salar) possesses two distinct subpopulations of polymeric IgM which are separable by anion exchange chromatography. Consistent with this finding there are two isotypic IgM heavy chain genes, CmuA and CmuB, in the genome of this species, presumably as a result of ancestral tetraploidy. In the present study it was shown that IgM of brown trout (Salmo trutta) is also separated into two subpopulations by anion exchange chromatography, while IgM of rainbow trout (Oncorhynchus mykiss) and Arctic char (Salvelinus alpinus) are eluted in one peak. Molecular cloning of IgM heavy chain cDNAs from brown trout revealed messages of two distinct constant region genes, named CmuA and CmuB. As deduced from the translated cDNA sequences (and in agreement with isoelectric focusing of the corresponding proteins) the mean pI values of the heavy chains in brown trout differ with only 0.14 units, in comparison to a 0.67 unit difference in salmon. Based on the present sequence analysis we suggest that an additional cysteine near the C-terminus of CmuB is critical in relation to the fractionation of IgM by anion exchange chromatography, for example by altering the overall structure of the IgM polymer and the exposure of charged residues. Most likely, the Cmu subvariant with the characteristic extra cysteine residue arose in the ancestor of Atlantic salmon and brown trout, i.e. after the three genera Salmo, Oncorhynchus and Salvelinus radiated.