Differential structure-function requirements of the transmembranal domain of the B cell antigen receptor

By generating phosphorylcholine (PC)-specific, wild-type (μ), and chimeric (μ-I-Aα) antigen receptor transfectants of mature B cells, we have shown that the COOH terminus of the μ heavy chain is essential for three major functions: immediate signal transduction (measured as changes in intracdlular 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 μ-I-Aα chimeric chain with those of μ. 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.