Evidence of Autophosphorylation in Txk: Y91 Is an Autophosphorylation Site.

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  • Kashiwakura Jun-ichi
    Department of Biochemistry, Hoshi University
  • Suzuki Noboru
    Departments of Immunology and Medicine, St. Marianna University School of Medicine
  • Takeno Mitsuhiro
    Departments of Immunology and Medicine, St. Marianna University School of Medicine
  • Itoh Saotomo
    Research Institute of Tuberculosis, Japan Anti-Tuberculosis Association
  • Oku Teruaki
    Department of Biochemistry, Hoshi University
  • Sakane Tsuyoshi
    Departments of Immunology and Medicine, St. Marianna University School of Medicine
  • Nakajin Shizuo
    Department of Biochemistry, Hoshi University
  • Toyoshima Satoshi
    Pharmaceuticals and Medical Devices Evaluation Center, National Institute of Health Sciences

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We have previously shown that Txk, a member of Tec family tyrosine kinase, is expressed in Th1 and Th0 cells and directly contributes to gene transcription of Th1-related proteins, including interferon (IFN)-γ, through nuclear translocation in response to mitogenic stimuli. Btk, another member of Tec family tyrosine kinase, has been shown to have a Src family tyrosine kinase-dependent transphosphorylation site and an autophosphorylation site. However, little is known about the phosphorylation mechanism of Txk, except that 420 tyrosine residue was identified as the transphosphorylation site. In this study, we found that Txk autophosphorylated itself by using an in vitro kinase assay. To elucidate the role of phosphorylation in Txk function, we studied IFN-γ secretion by Jurkat T cells expressing mutant Txk proteins. While transfection with the wild-type Txk resulted in increased IFN-γ production, the function was abrogated by disruption of the ATP biding site, which is presumably involved in the autophosphorylation mechanism. The results suggest that phosphorylated Txk is an active form to promote IFN-γ synthesis. The 91 tyrosine residue of Txk is deduced to be an autophosphorylation site by comparing its structure with Btk. In Jurkat cells transfected with Txk Y91A, IFN-γ production was decreased in comparison with the wild-type Txk transfected Jurkat cells. These data suggest that phosphorylation of the 91 tyrosine residue in Txk plays a positive regulatory role in Txk function.

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