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From the * Department of Oncology, Institute of Medical Science, University of Tokyo, Tokyo 108, Japan; The 75-kD HS1 protein is highly tyrosine-phosphorylated during B cell antigen receptor
(BCR)-mediated signaling. Owing to low expression of HS1, WEHI-231-derived M1 cells,
unlike the parental cells, are insensitive to BCR-mediated apoptosis. Here, we show that
BCR-associated tyrosine kinases Lyn and Syk synergistically phosphorylate HS1, and that Tyr378 and Tyr-397 of HS1 are the critical residues for its BCR-induced phosphorylation. In addition, unlike wild-type HS1, a mutant HS1 carrying the mutations Phe-378 and Phe-397 was
unable to render M1 cells sensitive to apoptosis. Wild-type HS1, but not the mutant, localized
to the nucleus under the synergy of Lyn and Syk. Thus, tyrosine phosphorylation of HS1 is required for BCR-induced apoptosis and nuclear translocation of HS1 may be a prerequisite for
B cell apoptosis.
Department of Molecular Immunology, Medical Institute of Bioregulation, Kyushu University,
Fukuoka 812-62, Japan; § Department of Biochemistry, Fukui Medical School, Fukui 910-11, Japan;
Research Institute for Bioscience, Science University of Tokyo, Noda 278, Japan; and the ¶ Department of Biochemistry, Kobe University, School of Medicine, Kobe 650, Japan
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