The Syk protein tyrosine kinase can function independently of CD45 or Lck in T cell antigen receptor signaling.

Abstract

The protein tyrosine phosphatase CD45 is a critical component of the T cell antigen receptor (TCR) signaling pathway, acting as a positive regulator of Src family protein tyrosine kinases (PTKs) such as Lck. Most CD45‐deficient human and murine T cell lines are unable to signal through their TCRs. However, there is a CD45‐deficient cell line that can signal through its TCR. We have studied this cell line to identify a TCR signaling pathway that is independent of CD45 regulation. In the course of these experiments, we found that the Syk PTK, but not the ZAP‐70 PTK, is able to mediate TCR signaling independently of CD45 and of Lck. For this function, Syk requires functional kinase and SH2 domains, as well as intact phosphorylation sites in the regulatory loop of its kinase domain. Thus, differential expression of Syk is likely to explain the paradoxical phenotypes of different CD45‐deficient T cells. Finally, these results suggest differences in activation requirements between two closely related PTK family members, Syk and ZAP‐70. The differential activities of these two kinases suggest that they may play distinct, rather than completely redundant, roles in lymphocyte signaling.

Nonredundant Roles of Src-Family Kinases and Syk in the Initiation of B-Cell Antigen Receptor Signaling

“…The NP 25 -BSA-induced phosphorylation appeared to be more transient for Erk, Iga, and Syk, but not Akt and SFKs upon stimulation for 5 min. LAT-Csk delayed the phosphorylation of Syk, Iga, Erk, and Akt in anti-BCR-activated cells, but almost completely abolished the phosphorylation of these proteins in NP 25 -BSA-and NP 40 -Ficoll-treated cells (Fig. 3D).…”

“…3B). PP2 did not block NP 40 -Ficoll-triggered calcium flux completely, but again, it had a significantly stronger impact on NP 40 -Ficoll than on anti-BCR-mediated signaling (Fig. 3C).…”

“…The following reagents were used for cell stimulation: anti-CD3 Ab (hamster IgG1 145-2C11; Exbio Praha), goat anti-Armenian hamster IgG, goat anti-mouse IgM F(ab) 2 , goat anti-chicken IgY F(ab) 2 Abs (all from Jackson ImmunoResearch), 4-hydroxy-3-nitrophenylacetyl (NP) conjugated to BSA at molecular ratio 25:1 (NP 25 -BSA) and 5:1 (NP 5 -BSA), 4-hydroxy-3-iodo-5-nitrophenylacetyl (NIP) conjugated to BSA at molecular ratio 25:1 (NIP 25 -BSA), and NP conjugated to Ficoll at molecular ratio 40:1 (NP 40 -Ficoll) (all from Biosearch Technologies, Novato, CA). The following cell inhibitors were used: PP2, Syk inhibitor IV-BAY 61-3606 (both from Calbiochem, Merck, Darmstadt, Germany), and piceatannol (Sigma-Aldrich).…”

“…Indeed, Syk was reported to phosphorylate both ITAM tyrosines in Iga when coexpressed in the insect cell line S2 (15), and Syk, but not its T cell paralogue ZAP70, was reported to phosphorylate ITAMs in vitro and in COS cells (14,39). Moreover, ectopic expression of Syk was able to rescue TCR signaling in the J.CaM1.6 T cell line, which is deficient in SFK Lck (40).…”

Redundant role for Zap70 in B cell development and activation

“…In the standard model of TCR signaling, phosphorylation of ITAM on the CD3 subunits by Src family kinases is a prerequisite for Syk family kinases to bind and become activated. While this appears to be true for Zap70, Syk can be activated by TCR signaling even in the absence of Src family kinase activation, probably because, unlike Zap70, it can phosphorylate the ITAM directly [35][36][37]. Despite this, Zap70 has been reported to be much better than Syk at promoting phosphorylation of the TCRf chain by recruiting Lck to the signaling complex [38].…”