Structure and function of immunoglobulin genes and immunoglobulin precursors

To gain information on the origin of antibody diversity (somatic mutation or germ line hypothesis) it is necessary to determine the number of V region genes. For this purpose the capacity of a distinct V region probe to hybridize and quantify V genes of the same and different subgroups should be established. Relevant information on this issue was obtained from the extent of cross-hybridization of a distinct L chain cDNA with mRNAs coding for L chains of the same and different subgroups. The results indicated that: (1) V regions of similar amino acid sequence are coded by similar nucleotide sequence (this is not self-evident because of the degeneracy of the genetic code); (2) the nucleic acid probe to one V region may anneal and quantify V genes of members of the same subgroup. Molecular hybridizations of the cDNA probe with nuclear DNA showed that: (1) the number of kappa type C genes is small (about 2 per haploid genome); (2) the number of V genes presumably is also small; (3) there is no amplification of these genes in myeloma cells that produce large amounts of the Ig. These results support the somatic mutation model for the generation of antibody diversity. New information on the structure and controlled expression of Ig genes was obtained from the study of L chain precursors, which are the immediate product of L chain mRNA translation in vitro. In the precursors extra peptide segments (19-22 residues in length) precede the N-terminus of the mature L chain. Amino acid sequence analyses of the precursors provide evidence that: (1) the gene coding for the V region is larger than hitherto known; (2) duplication of a short DNA segment occurred in the structural gene coding for the MOPC-321 precursor; (3) translation of the L chain mRNA may be contingent on the nucleotide sequence coding for the extra piece; (4) cleavage of the extra piece may regulate secretion of mature L chain; (5) the extra piece is remarkably hydrophobic, suggesting that the role of the extra piece is to anchor the precursor in cell membranes, in a manner similar to the function of the "hydrophobic domain" of membrane bound proteins. We propose that most precursor molecules are directed to the endoplasmic reticulum where the extra piece is cleaved to yield mature Ig destined for secretion; a few precursor molecules escape cleavage and are anchored by means of the hydrophobic extra piece in the cell-surface membrane to serve as the antigen-recognizing receptor.