The pre-B cell receptor (BCR) complex, consisting of µ heavy chain, a pseudo-light chain, and the Mb-1/B29 heterodimer, directs the transition to the mature B cell stage. Plasma membrane expression of the pre-BCR is extremely low, despite its presumed signaling function. We have compared assembly and intracellular transport of the pre-BCR complex with that of the BCR complex in mature B cells. Synthesis and assembly rate of pre-BCR and BCR components are comparable. However, the pre-BCR is subject to a highly efficient retention mechanism, which only allows exit of a few percent of the complexes from the endoplasmic reticulum (ER). This small transported pool of pre-BCR complexes is significantly enriched for protein-tyrosine kinase activity, as compared with the ER-localized receptor pool. Accordingly, the Src-related tyrosine kinase Lyn was found in the transported glycoprotein fraction but not in association with ER-localized glycoproteins. Upon introduction of a conventional light chain into pre-B cells, plasma membrane receptor levels increased, but the efficiency of intracellular transport of the receptor complex was not restored to that in mature B cells. This indicates that the ER retention mechanism is not selective for the pseudo-light chain and may be inherent to pre-B cells. We propose that this retention mechanism contributes to the regulation of pre-BCR-mediated signal transduction.
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