By generating phosphorylcholine (PC)-specific, wild-type (mu), and chimeric (mu-I-A alpha) antigen receptor transfectants of mature B cells, we have shown that the COOH terminus of the mu heavy chain is essential for three major functions: immediate signal transduction (measured as changes in intracellular Ca2+), antigen presentation, and induction of immunoglobulin M secretion. A more detailed analysis of structural requirements of the COOH-terminal domains contributing to these functions was achieved by systematically replacing the spacer, cytoplasmic, and transmembranal domains of the mu-I-A alpha chimeric chain with those of mu. Using this rescue approach, we show that the carboxyl two-thirds of the transmembranal domain (proximal to the cytoplasmic domain) is required for induction of intracellular Ca2+, whereas the complete transmembranal domain is required for the function of antigen presentation but is dispensable for induction of antibody secretion.

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