B cell antigen receptor (BCR) cross-linking activates both Src family and Syk tyrosine kinases, resulting in increased cellular protein-tyrosine phosphorylation and activation of several downstream signaling enzymes. To define the role of Syk in these events, we expressed the BCR in the AtT20 mouse pituitary cell line. These nonlymphoid cells endogenously expressed the Src family kinase Fyn but not Syk. Anti-IgM stimulation of these cells failed to induce most of the signaling events that occur in B cells. BCR-expressing AtT20 transfectants were generated that also expressed Syk. Syk expression reconstituted several signaling events upon anti-IgM stimulation, including Syk phosphorylation and association with the BCR, tyrosine phosphorylation of numerous proteins including Shc, and activation of mitogen-activated protein kinase. In contrast, Syk expression did not reconstitute anti-IgM-induced inositol phosphate production. A catalytically inactive Syk mutant could associate with the BCR and become tyrosine phosphorylated but could not reconstitute downstream signaling events. Expression of the Src family kinase Lck instead of Syk also did not reconstitute signaling. Thus, wild type Syk was required to reconstitute several BCR-induced signaling events but was not sufficient to couple the BCR to the phosphoinositide signaling pathway.

The antigen receptor of B lymphocytes (BCR) plays important roles in recognition of foreign antigens and self-components to allow the immune system to make appropriate antibody responses. The BCR is a complex between membrane immunoglobulin and the Ig-alpha and Ig-beta heterodimer. Site-directed mutagenesis experiments have shown that the mu heavy chain transmembrane domain plays a key role in the association of mIgM with Ig-alpha/Ig-beta. In the absence of complex formation, mIgM is retained in the endoplasmic reticulum, and this function is also specified by the mu chain transmembrane domain. The ability of various mutant mIgM molecules to associate with Ig-alpha/Ig-beta correlates well with their ability to induce signal transduction reactions such as protein tyrosine phosphorylation and phosphoinositide breakdown. Thus, the signaling ability of the BCR appears to reside in the Ig-alpha/Ig-beta heterodimer. The cytoplasmic domains of Ig-alpha and Ig-beta each contain an ITAM sequence, which is defined by its limited homology with subunits of the T-cell antigen receptor and of Fc receptors. Moreover, chimeric proteins containing these ITAMs and surrounding sequences from the cytoplasmic domains of Ig-alpha or Ig-beta exhibit signaling function characteristics of the intact BCR. The Ig-alpha and Ig-beta chimeras are each capable of inducing all of the BCR signaling events tested and thus represent redundant functions. Cross-linking these chimeras leads to their phosphorylation and to binding of the intracellular tyrosine kinases Lyn and Syk. The BCR expressed in the nonlymphoid AtT20 cells, which express the Src-family tyrosine kinase Fyn but not Syk, was not able to trigger vigorous signaling reactions. Introduction of the active form of Syk into these cells restored some signaling events. These results are consistent with a model in which the ITAMs act to initiate the BCR signaling reactions by binding and activating tyrosine kinases.

The B cell Ag receptor (BCR) has two important functions: first, it binds and takes up Ag for presentation to T lymphocytes; and second, it transmits signals that regulate B cell development. Normal expression of the BCR requires the association of the Ag binding subunit, membrane IgM (mIgM), with the signaling component, the Ig-αβ heterodimer. After assembly in the endoplasmic reticulum, the intact BCR travels through the secretory pathway to the cell surface. In this paper, we report two variants of the B lymphoma cell lines, WEHI 279 and WEHI 231, that have both lost the ability to express μ heavy chain and consequently do not express mIgM. However, these variants do express the Ig-αβ heterodimer. In one variant, WEHI 279*, the Ig-αβ remained trapped intracellularly in the absence of mIgM. The other variant, 303.1.5.LM, expressed an aberrantly glycosylated Ig-αβ on the cell surface that was capable of signaling after cross-linking with anti-Ig-β Abs. Further characterization uncovered a point mutation in the 303.1.5.LM mb1 gene that would change a proline for a leucine in the extracellular domain of Ig-α. The 303.1.5.LM Ig-αβ could not associate with a wild-type mIgM after μ heavy chain was reconstituted by DNA transfection. Thus, this mutation could define a region of the Ig-α polypeptide that is important for recognition by the endoplasmic reticulum quality control system, for association with glycosylating enzymes, and for the association of Ig-αβ subunits with mIgM subunits to create a complete BCR complex.