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 lambda(1) 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 lambda(1) 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.