Cutting Edge: Fyn Is Essential for Tyrosine Phosphorylation of Csk-Binding Protein/Phosphoprotein Associated with Glycolipid-Enriched Microdomains in Lipid Rafts in Resting T Cells

Abstract

In resting T cells, Csk is constitutively localized in lipid rafts by virtue of interaction with a phosphorylated adaptor protein, Csk-binding protein (Cbp)/phosphoprotein associated with glycolipid-enriched microdomains, and sets an activation threshold in TCR signaling. In this study, we examined a kinase responsible for Cbp phosphorylation in T cell membrane rafts. By analyzing T cells from Fyn−/− mice, we clearly demonstrated that Fyn, but not Lck, has its kinase activity in membrane rafts, and plays a critical role in Cbp phosphorylation, Cbp-Csk interaction, and Csk kinase activity. Naive CD44lowCD62 ligandhigh T cells were substantially reduced in Fyn−/− mice, presumably due to the inhibition of Cbp phosphorylation. Thus, Fyn mediates Cbp-Csk interaction and recruits Csk to rafts by phosphorylating Cbp. Csk recruited to rafts would then be activated and inhibit the kinase activity of Lck to keep resting T cells in a quiescent state. Our results elucidate a negative regulatory role for Fyn in proximal TCR signaling in lipid rafts.

Csk-binding Protein Mediates Sequential Enzymatic Down-regulation and Degradation of Lyn in Erythropoietin-stimulated Cells

“…The involvement of Cbp in reducing Lyn kinase activity after Epo stimulation of immature erythroid cells, complements studies showing inactivation of Src and Fyn in other cell types (47)(48)(49). The crucial role of recruiting Csk/Ctk by Cbp Tyr 314 was also demonstrated in erythroid cells.…”

“…The data presented here detail how Lyn, like Src and Fyn, is negatively regulated by Cbp (30,31,47,48). The involvement of Cbp in reducing Lyn kinase activity after Epo stimulation of immature erythroid cells, complements studies showing inactivation of Src and Fyn in other cell types (47)(48)(49).…”

SRC Homology 2 Domain-containing Leukocyte Phosphoprotein of 76 kDa (SLP-76) N-terminal Tyrosine Residues Regulate a Dynamic Signaling Equilibrium Involving Feedback of Proximal T-cell Receptor (TCR) Signaling

“…However, the regulation of phosphorylation of PAG is not well understood. Previous studies indicated that Fyn, but not Lck, is predominantly responsible for the phosphorylation of PAG in resting peripheral T cells (44). Interestingly, although one study suggested that CD45 phosphatase could act on Tyr 371 of PAG (45), another failed to confirm this finding (15).…”

Transmembrane phosphoprotein Cbp senses cell adhesion signaling mediated by Src family kinase in lipid rafts

“…It was already shown that Cbp can be a target of SFK (8-10), but it remained unclear whether there are specificities among the SFK members. Previous analysis showed that phosphorylation of Cbp is substantially down-regulated in the immune system of Fyn-deficient mice (37), suggesting the potential role of Fyn in Cbp phosphorylation. To further verify this finding, we analyzed the raft fraction in neonatal brains of Fyn-deficient mice.…”