Light Chain Dimer Research Articles

Stabilization of Dissociable IgA2 Proteins by Secretory Component

Abstract The Am2(+) genetic variant of human IgA2 immunoglobulins is potentially unstable because it lacks disulfide bridges between the heavy (H) and light (L) chains. It dissociates spontaneously under denaturing conditions into heavy and light chain dimer subunits (H2, L2), without the need for prior reduction. Isolated secretory component (SC) binds to human Am2(+), IgA2 myeloma proteins in vitro, and stabilizes them so that they will no longer dissociate into subunits in the presence of denaturants. Both human and bovine SC will bind to and stabilize the polymer forms of these myeloma proteins, but only human SC will bind to and stabilize monomeric proteins. The reaction involves the whole IgA2 molecule rather than just its subunits. Moreover, SC will stabilize a pepsin F(ab′)2 fragment. Since stabilization is prevented by iodoacetic acid, covalent disulfide bonding appears to be involved. The ability of SC to stabilize the potentially dissociable Am2(+) genetic variant of IgA2 may enhance its capacity to function in external secretions.

Absence of Disulfide Bonds Linking the Heavy and Light Chains: A Property of a Genetic Variant of γA2 Globulins

The gammaA2 globulins have attracted considerable interest because of the absence of the disulfide bonds linking the heavy and light chains characteristic of all other human immune globulins. Recently a genetic marker (Am(2)) has been found which delineates two genetic variants of gammaA2 globulins that are controlled by allelic genes. These differ markedly in incidence in different populations with a marked preponderance in Caucasians of the type possessing the Am(2) marker. A study of 22 gammaA2 myeloma proteins primarily from Caucasians showed that 20 were Am(2)(+), and 2 were Am(2)(-). All of the positive type dissociated in the presence of acid or urea into heavy and light chain dimers without reduction of disulfide bonds, as expected from the earlier studies. However, the two Am(2)(-) proteins failed to dissociate in the presence of acid, urea, guanidine, or detergent. It was only after partial reduction and alkylation that these molecules split into heavy and light chains. Thus the unique absence of disulfide bonds linking the heavy and light chains is a property of only one genetic variant and not of all gammaA2 proteins.

Synthesis, assembly, and secretion of gamma globulin by mouse myeloma cells. I. Adaptation of the Merwin plasma cell tumor-11 to culture, cloning, and characterization of gamma globulin subunits.

MPC-11 myeloma tumor cells were adapted to growth in continuous culture. The cultured cells resembled the parent tumor in that they produced the fully assembled gamma globulin molecules as well as six unassembled molecules. Although cultured and tumor cells synthesized excess light chains, the molar ratio of light (L) to heavy (H) chains was approximately 1.7:1 in the culture, and 3.5:1 in the tumor. The cultured cells also produced fewer half molecules and free light chains than the parent tumor. Peptide column analysis did not reveal differences in the primary structure of the H chains derived from the parent tumor and the culture. The L chains may have differed by a minor peptide. As much as 20% of the newly labeled cytoplasmic proteins and almost 100% of the proteins secreted by the cultured myeloma cells could be precipitated by specific antiserum. The immune precipitates contained seven different gamma globulin molecules, six of which were characterized according to their molecular size and H and L chain content as fully assembled molecules (H(2)L(2)), heavy chain dimers (H(2)), half molecules (HL), H, light chain dimers (L(2)), and L chains. All gamma globulin subunits as well as the complete H(2)L(2) molecule were produced and secreted by splenic clones of the parent MPC-11 tumor, and agar clones of the cultured cells. This indicates that the various gamma globulin subunits were produced by the same cell and did not reflect cellular heterogeneity with respect to gamma globulin synthesis.

IgG myeloma with closed tetrameric Bence Jones proteinemia

A forty-one year old woman with multiple myeloma of sixteen months' duration died with the clinical picture of plasma cell leukemia. During the course of her illness her serum contained λ type IgG myeloma protein and λ, type Bence Jones protein, but concomitant Bence Jones proteinuria was absent. The serum Bence Jones protein was composed of two disulfide linked light chain dimers which were noncovalently bound to form a closed tetramer. The protein had a corrected sedimentation coefficient of 5.4 S and a molecular weight of about 88,000. The absence of Bence Jones proteinuria is explained by the high molecular weight of the tetramer.