Molecular components and assembly of mu.surrogate light chain complexes in pre-B cell lines

We describe the molecular components, subunit assembly, and cell surface expression of mu-chain complexes in mu+kappa- pre-B cell lines as revealed by two-dimensional gel electrophoresis. The mu-chain complexes of these cell lines contain several previously unreported components, p42(6.4), p39(6.7), p18(8.6), and p14(7.0), in addition to lambda 5, VpreB1, VpreB3 (formerly named 8HS20), MB-1(Ig-alpha), and B29(Ig-beta). These new components are not detected in mu+kappa+ immature B cell lines. The mu-chain associates with lambda 5, VpreB3, and p56(5.0) at an early phase of assembly, preceding the association of other molecules. mu-Associated VpreB3 decreased during assembly as the association of VpreB1 became dominant, suggesting that the change in the ratio of these two VL-like surrogate light chains is involved in the mechanism of assembly. Lambda 5, VpreB1, p56(5.0), p32(5.0), p36(5.5), and p14(7.0) were shown to be expressed on the cell surface in association with mu-chain. The association of the other molecules with mu-chain is most likely restricted to the intracellular compartment. An interaction between VpreB1 and VpreB3 was also suggested. These findings might be important for understanding the function of mu-chain complexes in pre-B cells. A possible signaling mechanism of mu/surrogate light chain complexes is discussed.