Phylogeny of immunoglobulin structure and function; characterization of the cysteine-containing peptide involved in the pentamerization of shark IgM.

Nurse shark (Ginglymostoma cirratum) immunoglobulins were studied in an attempt to further define the relationship between the naturally occurring monomeric 7S and the pentameric 19S forms of extracellular IgM. A peptide containing the cysteine involved in the formation of intersubunit disulfide bonds linking 7S monomers into pentamers was isolated from the H chain of the 19S molecule and characterized. A similar peptide was also isolated from the H chain of the naturally occurring 7S molecule. These observations serve to substantiate previous claims that the two shark proteins belong to the same immunoglobulin class.