Sequence analysis of homogeneous peptides of shark immunoglobulin light chains by tandem mass spectrometry: correlation with gene sequence and homologies among variable and constant region peptides of sharks and mammals.

Morphologically, sharks are living fossils that are remarkably similar to their Devonian ancestors of ca. 400 million years ago. If a parallel conservation in biochemical properties characterizes shark evolution, knowledge of the properties of shark immunoglobulins should provide information on the structure of primordial immunoglobulins and their genes. The problem of polyclonality of shark immunoglobulins has precluded detailed analysis of shark immunoglobulin light polypeptide chains. Here, we approach the problem of obtaining direct sequence information on polyclonal light chains of shark immunoglobulins by isolating homogeneous peptides from tryptic digests of shark light chains and sequencing these by tandem mass spectrometry. To confirm the location of the peptides, we isolated a complementary DNA (cDNA) clone from a sandbar shark cDNA library in the expression vector lambda gt11, identifying the clone by its ability to produce a peptide serologically detectable using rabbit antibody to purified shark light chain. The correspondence between peptide sequence and that derived from gene sequence provided direct proof that the gene studied was that of a major expressed serum light chain. Using this combined approach, we isolated homogeneous peptides from both constant and variable regions. The variable region peptides showed homology to corresponding sequences of mammalian V lambda and V kappa sequences. The constant region gene sequence we obtained was homologous to mammalian C lambda sequence. The four constant region tryptic peptides we sequenced corresponded exactly to stretches of the C lambda sequence derived from the DNA sequence. The combined approach described here shows that shark light chains exhibit heterogeneity at both the protein and gene level, but that the constant regions of these chains can be identified as homologs of mammalian lambda chains and that evolutionary conservation has occurred in V region sequences ranging from elasmobranchs to man.