The variable domain of nonassembled Ig light chains determines both their half-life and binding to the chaperone BiP.

Not much is known about the features that determine the biological stability of a molecule retained in the endoplasmic reticulum (ER). Ig light (L) chains that are not secreted in the absence of Ig heavy (H) chain expression bind to the ER chaperone BiP as partially folded molecules until they are degraded. Although all Ig L chains have the same three-dimensional structure when part of an antibody molecule, the degradation rate of unassembled Ig L chains is not identical. For instance, the two nonsecreted murine Ig L chains, kappaNS1 and lambdaFS62, are degraded with half-lives of approximately 1 and 4 hr, respectively, in the same NS1 myeloma cells. Furthermore, the BiP/lambdaFS62 Ig L chain complex appears to be more stable than the BiP/kappaNS1 complex. Here, we used the ability of single Ig domains to form an internal disulfide bond after folding as a measure of the folding state of kappaNS1 and lambdaFS62 Ig L chains. Both of these nonsecreted L chains lack the internal disulfide bond in the variable (V) domain, whereas the constant (C) domain was folded in that respect. In both cases the unfolded V domain provided the BiP binding site. The stability of BiP binding to these two nonsecreted proteins was quite different, and both the stability of the BiP:Ig L chain complex and the half-life of the Ig L chain could be transferred from one Ig L chain isotype to the other by swapping the V domains. Our data suggest that the physical stability of BiP association with an unfolded region of a given light chain determines the half-life of that light chain, indicating a direct link between chaperone interaction and delivery of partially folded substrates to the mammalian degradation machinery.