Novel mechanisms control the folding and assembly of λ5/14.1 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 proteins that comprise the surrogate light chain, VpreB and λ5/14.1, contain both unique regions and Ig-like domains. The unique regions have been postulated to function in the assembly of the surrogate light chain. However, by using transient transfection of COS7 cells, we show that deletion of the unique regions of both proteins did not inhibit the assembly of surrogate light chain. Instead, in vivo folding studies showed that the unique region of λ5/14.1 acts as an intramolecular chaperone by preventing the folding of this protein when it is expressed in the absence of its partner, VpreB. The Ig domains of both λ5/14.1 and VpreB are atypical. The one in VpreB lacks one of the canonical β strands whereas the one in λ5/14.1 has an extra β strand. Deletion of the extra β strand in λ5/14.1 completely abrogated the formation of the surrogate light chain, demonstrating that complementation of the incomplete Ig domain in VpreB by the extra β strand in λ5/14.1 was necessary and sufficient for the folding and assembly of these proteins. Our studies reveal two novel mechanisms for regulating surrogate light chain formation: (i) the presence of an intramolecular chaperone that prevents folding of the unassembled subunit but that remains part of the mature assembled protein, and (ii) splitting an Ig domain between two proteins to control their folding and assembly.