A monoclonal lambda 1-bearing anti-dextran antibody from a lambda-defective mouse strain

A point mutation in the constant region of Ig lambda1 prevents normal B cell development due to defective BCR signaling

Surface expression of B cell antigen receptors (BCRs) containing Ig and Igalpha/Igbeta generates signals required to transit discrete developmental checkpoints. The mechanism by which BCR components collaborate to initiate signals is still unclear. The expression of Iglambda1 in SJL mice is 50-fold lower than in other strains. Here, we demonstrate by gene targeting that a point mutation, which changes a glycine to a valine in the lambda1 constant region, is responsible for this defect. In vitro experiments show that Ig receptors bearing this mutation, while expressed normally, are deficient in signaling. These findings reveal a direct involvement of the Ig light chain (IgL) in B cell signaling and development beyond the requirement of light chains for BCR assembly.

Assembly of immunoglobulin light chains as a prerequisite for secretion. A model for oligomerization-dependent subunit folding

Oligomeric proteins usually have to assemble into their final quartenary structure to be secreted. However, most immunoglobulin (Ig) light (L) chains can be exported as free chains, whereas only a few Ig L chains, here referred to as export-incompetent, have to assemble with Ig heavy (H) chains into antibody molecules to be secreted. In the absence of Ig H chain expression, these export-incompetent Ig L chains remain bound to BiP as partially folded monomers with only one of the two internal disulfide bonds being formed. To understand the apparent discrepancy in Ig L chain export, we performed assembly studies with chimeric Ig chains and found that the variable (V) domain of the export-incompetent NS1 kappa chain cannot mediate homodimer formation. Conversely, the V domain of the export-competent J558L lambda1 chain supports homodimer formation and, concordantly, these Ig L chains are secreted as noncovalently or covalently linked homodimers. We show that the export-incompetent mutant lambda1 FS62 chain forms disulfide bonds in both domains only upon pairing with Ig H chain and is secreted as part of an antibody. Therefore, Ig L chain assembly seems to be a prerequisite for complete folding, indicating that Ig L chain secretion generally depends on either homo- or heterodimer formation. We discuss a mechanism that controls oligomerization by monitoring the conformation of individual subunits that cannot proceed in folding prior to successful assembly.