Structure, biosynthesis, and transduction properties of the human mu-psi L complex: similar behavior of preB and intermediate preB-B cells in transducing ability.

In human preB cells, the mu chain is associated with a surrogate light chain composed of the VpreB and lambda-like gene products. Using anti-peptide antibodies directed against VpreB and lambda-like epitopes, we identified the discrete components of the mu-psi L (pseudo-light) chain complex in various preB cell lines, and in intermediate preB-B cells that co-expressed the psi L and the kappa chain. The lambda-like gene product was identified as a single band at 20 kDa, disulfide linked to the mu chain. VpreB was detected at 16 kDa and, depending upon the cell lines, an isoform of this polypeptide was also present at 15 kDa. In addition, lambda-like--VpreB chain complexes not associated with mu were identified both in cell lysates and culture supernatants. Pulse-chase experiments indicated that VpreB was transiently associated with two new polypeptides of molecular weights 17.5 and 36 kDa. Expression of mu-psi L and co-expression of mu-psi L and microL at the surface of preB and intermediate preB-B cells respectively was detected by cytofluorimetry. The signal transduction ability of the complex in both types of cells was shown by measuring the calcium mobilization and the phosphorylation of tyrosyl residues upon stimulation by anti-mu. Signal events were similar in both cases, but differed from those induced in a mature B cell line. This points to a definite function of the preB cell receptor and suggests that the intermediate preB-B cell line still lacks some molecular components that condition initiation of a mature B cell transduction signal.