Interaction between Sam68 and Src Family Tyrosine Kinases, Fyn and Lck, in T Cell Receptor Signaling

Noemi Fusaki,Akihiro Iwamatsu,Makio Iwashima,Jun-Ichi Fujisawa

doi:10.1074/jbc.272.10.6214

Abstract

The Src family protein-tyrosine kinase, Fyn, is associated with the T cell receptor (TCR) and plays an important role in TCR-mediated signaling. We found that a human T cell leukemia virus type 1-infected T cell line, Hayai, overexpressed Fyn. To identify the molecules downstream of Fyn, we analyzed the tyrosine phosphorylation of cellular proteins in the cells. In Hayai, a 68-kDa protein was constitutively tyrosine-phosphorylated. The 68-kDa protein was coimmunoprecipitated with various signaling proteins such as phospholipase C gamma1, the phosphatidylinositol 3-kinase p85 subunit, Grb2, SHP-1, Cbl, and Jak3, implying that the protein might function as an adapter. Purification and microsequencing of this protein revealed that it was the RNA-binding protein, Sam68 (Src associated in mitosis, 68 kDa). Sam68 was associated with the Src homology 2 and 3 domains of Fyn and also those of another Src family kinase, Lck. CD3 cross-linking induced tyrosine phosphorylation of Sam68 in uninfected T cells. These data suggest that Sam68 participates in the signal transduction pathway downstream of TCR-coupled Src family kinases Fyn and Lck in lymphocytes, that is not only in the mitotic pathway downstream of c-Src in fibroblasts.

Highlights

The Src family protein-tyrosine kinase, Fyn, is associated with the T cell receptor (TCR) and plays an important role in TCR-mediated signaling
Altered Expression of Tyrosine Kinases in Human T cell leukemia virus type 1 (HTLV-1)-infected Cells—Since several reports suggest that HTLV-1-infected T cells exhibit altered expression of tyrosine kinases [22, 23], we analyzed the expression of tyrosine kinases in HTLV-1-infected and -uninfected T cell lines by immunoblotting (Fig. 1)
We demonstrated in this study that Sam68, a mitotic target of c-Src, was constitutively tyrosine-phosphorylated in a HTLV-1-infected T cell line, Hayai, which overexpressed the Src family kinase, Fyn

Summary

Introduction

The Src family protein-tyrosine kinase, Fyn, is associated with the T cell receptor (TCR) and plays an important role in TCR-mediated signaling. The Src family protein-tyrosine kinases, Fyn (associated with TCR [3]) and Lck (associated with coreceptors CD4 and CD8), have been shown to be responsible for T cell activation [1].

Results

Conclusion