The catalytic activity of Src‐family tyrosine kinase is required for B cell antigen receptor signaling

Abstract

The Src family protein-tyrosine kinases (PTKs) are known to be important for B cell antigen receptor (BCR) signaling. To study the mechanism of action of Src-PTK in BCR signaling, kinase deficient-and Src homology 2 (SH2)-mutants of Src-PTK were transfected into Lyn-deficient B cells and analyzed. Kinase activity of Src-PTK was essential for tyrosine phosphorylation of Syk and calcium mobilization upon receptor ligation, whereas these events were not affected by the mutation of SH2 domain. Receptor-mediated tyrosine phosphorylation of Lyn was still observed in Syk-deficient B cells. These results demonstrate that the BCR-induced phosphorylation of Src-PTK is independent of Syk and that the kinase activity of Src-PTK is critical for BCR signaling.Key words: Kinase activity; BCR signaling; Src-PTK; SH2 domain; Syk showed that Fyn or Lyn interacts with resting BCR primarily through the non-phosphorylated Iga chain Immunoreceptor Tyrosine based Activation Motif (ITAM) (low-affinity binding) and that tyrosine-phosphorylated Igc~ ITAM induces the binding to Fyn SH2 domain (high-affinity binding), leading to the increased kinase activity of Fyn [8]. However, the in vivo role of the Src-PTK SH2 domain in BCR signaling process has not been established. Moreover, it has not been clearly demonstrated that the kinase activity of Src-PTK plays a role in receptor signaling.To address the in vivo functions of Src-PTK domains in regulating BCR signal transduction, we mutated residues required for SH2 or kinase function of Src-PTK and analyzed the ability of these constructs to restore BCR signaling. In this report, we demonstrate that the catalytic activity of Src-PTK is essential for BCR signaling and suggest that the SH2 domain of Src-PTK is not crutical for early signaling events.

Regulation of the pp72syk protein tyrosine kinase by platelet integrin alpha IIbbeta 3

“…Similarly, increased tyrosine phosphorylation of Syk in adherent A5 cells was associated with increased Syk activity in vitro (not shown). In the case of immune response receptors, Syk activation appears to require the concerted action of a Src family kinase and autophosphorylation (Takata and Kurosaki, 1995; El‐Hillal et al ., 1997). To determine whether Syk activation through α IIb β 3 involves autophosphorylation, A5 cells were transfected with wild‐type or a kinase‐inactive form of Syk(K402R) and assayed for Syk phosphorylation after adhesion to mAb D57.…”

Phosphorylation of spleen tyrosine kinase at Y346 negatively regulates ITAM-mediated signaling and function in platelets

“…These results are consistent with the idea that Y346 is phosphorylated mostly by SFKs, thus representing a major target of SFK-mediated regulation of Syk activity following engagement of ITAM-bearing receptors. The key role of SFKs in the initial events of ITAM-bearing receptor signaling had been shown in multiple systems ( 8 , 10 , 55 , 56 , 57 ), but the targets of SFK-dependent regulation on Syk remained largely unknown. Our results together with the data on mast cells ( 54 ) firmly establish Syk Y346 as a phosphorylation site for SFKs.…”