Amino Acid Sequence Studies of Immunoglobulins: Implications for the Storage, Processing, and Expression of Genetic Information

Antibody molecules form a highly complex set of proteins. A central problem in immunology has been how the information coding for these proteins is stored, processed, and expressed. The constant region sequences of two rat K light chain allotypes have been partially sequenced. These allotypes segregate in the Mendelian fashion. In the eighty-one constant region residues compared, 10 amino acid substitutions and one size difference were found. This large number of substitutions raise the possibility that the structural genes for both forms may exist in all rats and the inherited marker is a regulatory gene controlling the expression of one or the other forms. The diversity of immunoglobulins is reflected in the diversity of myeloma proteins and much of our knowledge of antibodies comes from studies of myeloma proteins. However, the window, created by the myeloma tumors, may be a biased one. By comparing the N-terminal amino acid sequences of myeloma light chains from two inbred strains of mice, BALB/c and NZB, we have found differences which suggest that different populations of lymphocytes are being transformed in the two strains. Thus the true diversity of immunoglobulins may be greater than that seen in myeloma proteins. By sequencing a set of closely related variable regions, one can ask the question -- what are the protein products co9ed by a single germ line gene? The nearly complete variable regions of twenty-two K chains have been sequenced using newly developed automated sequencing technology. This data shows that at least six genes code for this set, assuming that the somatic diversity generating mechanisms cannot produce multiple parallel mutations. Within each subset of these sequences, coded for by at least one gene, additional variations occur both inside and outside the hypervariable regions, although predominately inside. In addition, the sequence of the approximately twelve residues preceeding the constant region do not correlate with the rest of the variable region. We have termed this region the S or switch region and suggest that it is coded for by a separate segment of DNA that is reorganized during differentiation much in the same way as V and C regions are rearranged.