Activation of the Syk tyrosine kinase is insufficient for downstream signal transduction in B lymphocytes

A mutant form of Syk carrying a combination of a K395A substitution in the kinase domain and substitutions of three phenylalanines (3F) for the three C-terminal tyrosines was expressed in a murine B cell lymphoma cell line, BCL1.3B3 to interfere with normal Syk regulation as a means to examine the Syk activation step in BCR signaling. Introduction of this kinase-inactive mutant led to the constitutive activation of the endogenous wildtype Syk enzyme in the absence of receptor engagement through a 'dominant-positive' effect. Under these conditions, Syk kinase activation occurred in the absence of phosphorylation on Syk tyrosine residues. Although Syk appears to be required for BCR-induced apoptosis in several systems, no increase in spontaneous cell death was observed in these cells. Surprisingly, although the endogenous Syk kinase was enzymatically active, no enhancement in the phosphorylation of cytoplasmic proteins, including phospholipase Cγ2 (PLCγ2), a direct Syk target, was observed.These data indicate that activation of Syk kinase enzymatic activity is insufficient for Syk-dependent signal transduction. This observation suggests that other events are required for efficient signaling. We speculate that localization of the active enzyme to a receptor complex specifically assembled for signal transduction may be the missing event.The B cell antigen receptor (BCR) is a multi-subunit complex that acts as a key sensor regulating the response of lymphocytes to their environment (reviewed in [1-7]). In mature B cells, activation through the BCR stimulates cellular proliferation and differentiation. In immature B cells, activation through the BCR induces either a state of unresponsiveness, termed anergy, or death by apoptosis, depending on the physical nature and concentration of the antigen [8-25]. In some B cell lymphomas, activation through the BCR can induce cell cycle arrest and apoptosis in vitro and tumor dormancy in vivo [19,26-28]. The core of the multi-subuni