Janus kinase 2 is associated with a box 1-like motif and phosphorylates a critical tyrosine residue in the cytoplasmic region of cytotoxic T lymphocyte associated molecule-4

It is a consensus that a cytotoxic T lymphocyte associated molecule-di (CTL A-4) transduces inhibitory signal for T cell activation under physiological condition, indicating that this molecule is an important regulator of T ce ll homeostasis in vivo. It has been reported that phosphorylation and depho sphorylation of tyrosine residue Y-165 in the cytoplasmic region of CTLA-4 play an important role in its negative signaling and cell surface expressio n. Some signaling molecules such as Src homology 2 protein tyrosine phospha tase 2 (SHP-2) and the p85 subunit of phosphatidylinositol 3 kinase (PI3 ki nase) associate with phosphorylated tyrosine residue Y-165, through Src hom ology 2 (SH2) domains. On the other hand, the adapter complex proteins, AP- 2 and AP-50 interact with the same tyrosine residue when unphosphorylated, resulting in clathrin-mediated endocytosis of CTLA-4 molecules. The objecti ve of this study is to identify a tyrosine kinase that can directly bind an d phosphorylate the critical tyrosine residue, Y-165 in the cytoplasmic dom ain of CTLA-4. Here, we demonstrated that 1) Janus Kinase 2 (Jak2) was dire ctly associated with a box 1-like motif in the cytoplasmic tail of CTLA-4 m olecule, 2) Jak2 phosphorylated Y-165 residue in the cytoplasmic region of CTLA-4 molecule, and 3) Jak2 was associated with CTLA-4 in HUT 78 T cell li nes. (C) 2000 Wiley-Liss, Inc.