COOH terminus of membrane IgM is essential for an antigen-specific induction of some but not all early activation events in mature B cells

Transfectants of mature B cell lines that bind phosphorylcholine were made in order to understand the role of the COOH terminus of the μ chain of membrane IgM (mIgM) in generation of antigen-specific signals. A chimeric receptor (I-Aα tail) was constructed by replacing 40 amino acids from the μ COOH terminus with that of major histocompatibility complex class II I-Aα chain. The effect of wild-type and chimeric tails were studied on representative immediate-early antigen-specific signals. The I-Aα tail hybrid, but not the wild-type receptor, was defective in antigen-driven Ca²⁺ mobilization, although it could effectively endocytose ligand-receptor complexes. Signal(s) transduced through the wild-type receptor led to transient induction of selected immediate-early gene messages (Egr-1, c-fos, Jun) above basal levels. However, the signal(s) generated after crosslinking of the I-Aα tail receptor either showed no effect (c-fos) or actually repressed basal level expression of Egr-1 and Jun. Thus, we have established that receptor-mediated endocytosis can be distinguished from other early events associated with B cell activation, based on their differential dependence upon the structural fidelity of the COOH-terminal sequence of mIgM.