Regulation of signaling in B cells through the phosphorylation of Syk on linker region tyrosines - A mechanism for negative signaling by the Lyn tyrosine kinase

The B cell antigen receptor (BCR) is coupled to the mobilization of Ca2+ by the protein-tyrosine kinase, Syk. Syk, recruited to the clustered BCR, becomes phosphorylated on three tyrosines (Tyr-317, Tyr-342, and Tyr-346) located within the linker region that separates the C-terminal catalytic domain from the N-terminal tandem Src homology 2 domains. Phosphorylation within the linker region can be either activating or inhibitory to Ca2+ mobilization depending on the sites that are modified. Syk that is not phosphorylated on linker region tyrosines couples the BCR to Ca2+ mobilization through a phosphoinositide 3-kinase-dependent pathway. The phosphorylation of Tyr-342 and -346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of Ca2+ mobilization via a phosphoinositide 3-kinase-independent pathway. The phosphorylation of Tyr-317 strongly dampens the Ca2+ signal. In cells that lack the Src family kinase, Lyn, the phosphorylation of the inhibitory Tyr-317 is suppressed leading to elevated production of inositol 1,4,5-trisphosphate and an amplified Ca2+ signal. This provides a novel mechanism by which Lyn functions as an inhibitor of BCR-stimulated signaling. Thus, Syk and Lyn combine to determine the pathway through which the BCR is coupled to Ca2+ mobilization as well as the magnitude and duration of the Ca2+ flux.