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 sta bility of a molecule retained in the endoplasmic reticulum (ER), Ig li ght (L) chains that are not secreted in the absence of Ig heavy (H) ch ain expression bind to the ER chaperone BiP as partially folded molecu les until they are degraded. Although all Ig L chains have the same th ree-dimensional structure when part of an antibody molecule, the degra dation rate of unassembled Ig L chains is not identical. For instance, the two nonsecreted murine Ig L chains, kappa(NS1) and lambda(FS62), are degraded with half-lives of approximately 1 and 4 hr, respectively , in the same NS1 myeloma cells, Furthermore, the BiP/lambda(FS62) Ig L chain complex appears to be more stable than the BiP/kappa(NS1) comp lex, Here, we used the ability of single Ig domains to form an interna l disulfide bond after folding as a measure of the folding state of ka ppa(NS1) and lambda(FS62) Ig L chains, Both of these nonsecreted L cha ins lack the internal disulfide bond in the variable (V) domain, where as the constant (C) domain was folded in that respect. In both cases t he unfolded V domain provided the BiP binding site. The stability of B iP 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 s tability of BiP association with an unfolded region of a given light c hain determines the half-life of that light chain, indicating a direct link between chaperone interaction and delivery of partially folded s ubstrates to the mammalian degradation machinery.