Minegishi Y, Hendershot L M, Conley M E
Department of Immunology, St. Jude Children's Research Hospital, Memphis, TN 38105, USA.
Proc Natl Acad Sci U S A. 1999 Mar 16;96(6):3041-6. doi: 10.1073/pnas.96.6.3041.
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 lambda5/14.1, 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 lambda5/14.1 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 lambda5/14.1 and VpreB are atypical. The one in VpreB lacks one of the canonical beta strands whereas the one in lambda5/14.1 has an extra beta strand. Deletion of the extra beta strand in lambda5/14.1 completely abrogated the formation of the surrogate light chain, demonstrating that complementation of the incomplete Ig domain in VpreB by the extra beta strand in lambda5/14.1 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.
替代轻链可将μ重链转运至细胞表面，是前B细胞受体复合物的关键组成部分。构成替代轻链的两种蛋白质，即VpreB和λ5/14.1，既包含独特区域，也含有免疫球蛋白（Ig）样结构域。据推测，独特区域在替代轻链的组装过程中发挥作用。然而，通过瞬时转染COS7细胞，我们发现删除这两种蛋白质的独特区域并不会抑制替代轻链的组装。相反，体内折叠研究表明，当λ5/14.1在没有其伙伴VpreB的情况下表达时，其独特区域可作为分子内伴侣蛋白，阻止该蛋白质折叠。λ5/14.1和VpreB的Ig结构域都不典型。VpreB中的Ig结构域缺少一条典型的β链，而λ5/14.1中的Ig结构域则多一条β链。删除λ5/14.1中多余的β链会完全消除替代轻链的形成，这表明λ5/14.1中多余的β链对VpreB中不完整Ig结构域的互补作用对于这些蛋白质的折叠和组装是必要且充分的。我们的研究揭示了两种调节替代轻链形成的新机制：（i）存在一种分子内伴侣蛋白，可阻止未组装亚基的折叠，但仍作为成熟组装蛋白的一部分；（ii）将一个Ig结构域拆分在两种蛋白质之间以控制它们的折叠和组装。
