Nucleocytoplasmic Trafficking of the Syk Protein Tyrosine Kinase

Abstract

The protein tyrosine kinase Syk couples the B-cell receptor (BCR) for antigen to multiple intracellular signaling pathways and also modulates cellular responses to inducers of oxidative stress in a receptorindependent fashion. In B cells, Syk is found in both the nuclear and cytoplasmic compartments but contains no recognizable nuclear localization or export signals. Through the analysis of a series of deletion mutants, we identified the presence of an unconventional shuttling sequence near the junction of the catalytic domain and the linker B region that accounts for Syk's subcellular localization. This localization is altered following prolonged engagement of the BCR, which causes Syk to be excluded from the nucleus. Nuclear exclusion requires the receptor-mediated activation of protein kinase C and new protein synthesis. Both of these processes also potentiate the activation of caspase 3 in cells in response to oxidative stress in a manner that is dependent on the localization of Syk outside of the nucleus. In contrast, restriction of Syk to the nucleus greatly diminishes the stress-induced activation of caspase 3.Syk is a 72-kDa protein tyrosine kinase with an N-terminal, tandem pair of SH2 domains separated by a long linker (linker B) from a C-terminal catalytic domain (40). Syk plays a critical role in B cells in coupling the B-cell receptor (BCR) for antigen to multiple downstream signaling pathways, including the activation of phospholipase C␥, mobilization of calcium from intracellular stores, and activation of the Ras/extracellular signal-regulated kinase (ERK), phosphatidylinositol 3-kinase, and NF-B pathways (1,4,7,18). Upon receptor cross-linking, the tandem SH2 domains of Syk bind two phosphorylated tyrosines in the conserved immunoreceptor tyrosine-based activation motifs (ITAMs) located within the cytoplasmic domains of the immunoglobulin-␣ (CD79a) and immunoglobulin-␤ (CD79b) components of the BCR complex (3). Subsequent phosphorylation of Syk induces its activation, leading to the binding and/or phosphorylation of adaptor proteins and downstream effectors (6,7,9,13,16,20,32,33). Consequently, Syk-deficient B cells are largely nonresponsive to BCR aggregation (36) and B-cell development in Syk-deficient mice is largely blocked at the transitions from pro-B cells to pre-B cells and from immature to mature B cells (5, 39).Increasing evidence indicates that Syk also has fundamental cellular functions that are receptor and ITAM independent. Paradoxically, in B lymphocytes, the expression of Syk is required for BCR-induced apoptosis, but its presence also protects cells from apoptosis induced by ceramide, osmotic stress, or oxidative stress (9,25,27,28,35,37). While the activation of Syk by BCR engagement requires both intact SH2 domains, its participation in oxidative stress signaling does not, suggesting an ITAM-independent mechanism (8, 26).In lymphoid and epithelial cells, Syk has been reported to reside in both the nucleus and cytoplasm (21,41,43), as has its close family member . In breast canc...

Syk Tyrosine Kinase Is Linked to Cell Motility and Progression in Squamous Cell Carcinomas of the Head and Neck

“…However, Syk(L) and Syk(S) were present in both nuclei and cytoplasm in our transfected 011A cells (not shown). This is consistent with observations by Zhou et al (36) in B lymphocytes and breast cancer cells. Syk(S) expression was detected by RT-PCR or Western blot in all six Syk-positive cell lines in this study and its expression was usually lower than Syk(L).…”

In-depth Analyses of Kinase-dependent Tyrosine Phosphoproteomes Based on Metal Ion-functionalized Soluble Nanopolymers

“…The identification of substrates important in the regulation of apoptosis also is consistent with the reported role for Syk in the enhancement of cell survival (37). In addition, 27% of the phosphotyrosine-containing proteins were annotated to the nucleus, consistent with previous studies suggesting important roles for the kinase in the nucleus (23,38,39). The most abundant category for the molecular function of substrates was protein binding (44%), which is consistent with the known role of Syk and tyrosine phosphorylation in regulating protein-protein interactions.…”

Spleen Tyrosine Kinase Functions as a Tumor Suppressor in Melanoma Cells by Inducing Senescence-like Growth Arrest

“…Also, loss of nuclear Syk expression was shown to be closely associated with the metastatic property of gastric cancer (38). Consistently, Zhou and colleagues (39) identified a shuttling sequence in Syk that accounts for its nucleocytoplasmic trafficking. It remains to be assessed whether such nuclear activity and localization of Syk occurs in melanoma cells and what structural element(s) of Syk are involved in tumor suppression.…”

“…However, because Syk modulates the p53 pathway in melanoma cells, we speculate that it may integrate p53-mediated stress responses in melanocytes. Accordingly, a link between Syk and oxidative stress signaling has been observed in B lymphocytes (39,45). Like apoptosis, senescence is a stress response that prevents neoplastic transformation.…”