2 TYPES OF MU-CHAIN COMPLEXES ARE EXPRESSED DURING DIFFERENTIATION FROM PRE-B TO MATURE B-CELLS

Immunoglobulin μ chains synthesized in murine pre-B cells are known to be associated with surrogate light chains designated as omega (ω), iota (ι) and B34. In addition to these molecules, we identified the complexes of polypeptides (50, 40, 27 and 15.5 kd) associated with surface or intracellular μ chains of pre-B cell lines. Most of these polypeptides were continuously synthesized and associated with μ chains in virgin B cells lines, although some of them scarcely bound to the μκ dimer or μ2κ2 tetramer concomitantly present in the same clone or population. However, in ure B cells they were no longer detectable except B34. Cross-linking of μm chains on the surface of pre-B cells resulted in an increase in intracellular free Ca2+, indicating that the μm chain complex on the surface of pre-lB cell lines acted as a signal transduction molecule. However, the receptor cross-linkage of pre-B cell lines did not induce the increased inositol phospholipid metabolism usually observed in virgin and mature B cell lines. These results suggest that, during the differentiation from pre-B to mature B cells, the cells express two types of μ chain complexes which exhibit different structures as a whole and possess different signal transducing capacities.