Novel mechanisms control the folding and assembly of lambda 5/14.1 and VpreB to produce an intact surrogate light chain

Surrogate light chain, which escorts the mu heavy chain to the cell surface , is a critical component of the pre-B cell receptor complex. The two prote ins that comprise the surrogate light chain, VpreB and lambda 5/14.1, conta in both unique regions and ig-like domains. The unique regions have been po stulated to function in the assembly of the surrogate light chain. However, by using transient transfection of COS7 cells, we show that deletion of th e unique regions of both proteins did not inhibit the assembly of surrogate light chain, Instead, in who folding studies showed that the unique region of lambda 5/14.1 acts as an intramolecular chaperone by preventing the fol ding of this protein when it is expressed in the absence of its partner, Vp reB. The Ig domains of both lambda 5/14.1 and VpreB are atypical. The one i n VpreB lacks one of the canonical beta strands whereas tbe one in lambda 5 /14.1 has an extra beta strand, Deletion of the extra beta strand in lambda 5/14.1 completely abrogated the formation of the surrogate light chain, de monstrating that complementation of the incomplete Ig domain in VpreB by th e extra beta strand in lambda 5/14.1 was necessary and sufficient for the f olding and assembly of these proteins, Our studies reveal two novel mechani sms for regulating surrogate light chain formation: (i) the presence of an intramolecular chaperone that prevents folding of the unassembled subunit b ut that remains part of the mature assembled protein, and (ii) splitting an Ig domain between two proteins to control their folding and assembly.