Internalization of B cell and pre‐B cell receptors is regulated by tyrosine kinase and phosphatase activities

Prior to the expression of the B cell antigen receptor, the μ heavy chain associates with two non‐polymorphic polypeptides, λ like and VpreB, which form a pseudo‐light chain complex in pre‐B cells and pre‐B cell lines. Surface expression of the so‐called pre‐B cell receptor (pre‐BCR) occurs only in the presence of Igα and Igβ, known to be involved both in B cell antigen receptor (BCR) signaling and trafficking. Although the pre‐BCR organization is consistent with an efficient transport to the cell surface, most of the newly synthesized receptor remains within the cells, and so far, no data are available concerning the rate of exit from the endoplasmic reticulum. Using the human pre‐B cell line Nalm‐6, we found that only a small fraction (2%) of newly synthesized pre‐BCR is transported to the cell surface within 4‐6 h after synthesis, where it is constitutively re‐internalized. Membrane Ig‐heavy chain cross‐linking induced internalization of surface pre‐BCR within a few minutes, and the mechanisms underlying endocytosis were analyzed by immunofluorescence and confocal microscopy. Prein‐cubation of the cells with either genistein or orthovanadate, which inhibit, respectively, tyrosine kinases and tyrosine phosphatases, blocked pre‐BCR internalization in a dose‐dependent manner, indicating that both activities are required for endocytosis. BCR internalization was also inhibited in a reversible manner by the drugs. In contrast, neither drug affected the size of the steadystate pool of internalized transferrin receptors. Thus, our data show that tyrosine phosphorylation and dephosphorylation are both required for cross‐linkinginduced pre‐BCR and BCR internalization.