Mouse IgG2a Antibody [MOPC-173] (Alexa Fluor® 488) (A18725)

The complete amino acid sequence of the murine monoclonal immunoglobulin MOPC 173 (IgG2a, kappa) is reported. The heavy chain contains 447 amino-acid residues, and one carbohydrate prosthetic group attached to the ASX residue 299. The kappa light chain is composed of 214 residues. The H chains are covalently linked by 3 interchain disulfide bridges. The H-L bond-forming cysteine of the H chain is between the VH and the CH1 domain. Intrachain bridges are disposed linearly, according to the classical model. There is no simple relationship between the primary structure and any given function of a particular domain. This is presumably due to the fact that the selection pressure exerts itself on the three-dimensional structure which may retain a conserved general organization as a result of balanced multiple mutations. Selection seems to act in two ways: --horizontally, in a multigene system such as the immunoglobulin classes (C domains of the heavy chains), leading to interclass homologies which are particularly marked for all the COOH-terminal domains of H and L chains which have, in addition a fair degree of homology with human beta2 microglobulin (about 30% identities); --vertically, in which case strictly homologous domains appear extremely conserved between distinct animal species. Conservation of the VH domains seems just as high as conservation of the CH domains. The VH region contains 3 types of positions: invariant and subgroup characteristic ("framework") which may be accounted for by a rather small number of germ-line genes, and hypervariable for which the origin of diversity, somatic or germinal, cannot be decided from sequence data alone. Murine VK domains, although basically built according to the same pattern, show a much more marked polymorphism of the framework, which might necessitate a higher number of basic germ-line genes. Finally, a hypothetical model of the switch mechanism is proposed. Rotational symmetry regions can be deduced at the DNA level from the known amino acid sequences of the switch peptides for the three translocational systems: H, kappa and lambda. These would provide recognition signals for restriction-like enzymes such as those which operate in prokaryotes. An implication of this model is the definition of an exact limit between the V and the C regions of all immunoglobulin chains.

The IgG2a producing MOPC 173 tumor synthesizes heavy (H) and light (L) chains and assembles them into H2L2 utilizing heavy chain dimers (H2) and H2L as the major precursors. Although the tumor cells secrete only H2L2, they synthesize excess L chains which appear to be degraded and are not secreted. A L chain-producing variant arising spontaneously from the MOPC 173 tumor also failed to secrete any L chains. MOPC 173 tumor cells were cloned in the spleens of normal BALB/c mice and 1 of 23 spleen clones appeared to differ from the parent tumor in demonstrating a) a transient block in the assembly of H2L2 at the H2 level, leading to delayed formation and secretion of H2L2, b) appreciable amounts of non-covalently bonded H2L, c) an abnormal intracellular immunoglobulin species, H4, d) production of equimolar amounts of H and L chains. The patterns of immunoglobulin synthesis and assembly demonstrable in the above studies were also observed when tumor cells were studied in situ. Tryptic peptide mapping of the H and L chains produced by the MOPC 173 tumor and the variant spleen clone failed to demonstrate any differences in the H chains, but there were definite chemical differences in the L chains. These studies indicate that variant myeloma cells producing structurally altered immunoglobulins may continually be arising in myeloma tumors.