TI  - Human spleen tyrosine kinase  <prot>p72Syk</prot>  associates with the Src-family kinase
       <prot>p53</prot>/<prot>56Lyn</prot>  and a <prot>120-kDa phosphoprotein</prot>.
PG  - 359-63
AB  - The 72-kDa spleen tyrosine kinase (<prot>Syk</prot>) and Src-family kinase <prot>p53</prot>/<prot>56Lyn</prot>
      (<prot>Lyn</prot>) contribute to signaling via the B-cell antigen receptor complex.
      Here we show that  <prot>Syk</prot>  and  <prot>Lyn</prot>  from human B lymphocytes can interact
      directly.  <prot>Syk</prot>  and  <prot>Lyn</prot>  coimmunoprecipitated from mature and activated
      B-cell lines, and gel-purified  <prot>Syk</prot>  and  <prot>Lyn</prot>  reassociated in vitro,
      demonstrating their direct interaction. This  <prot>Syk</prot> - <prot>Lyn</prot>  interaction may be
      dependent on the stage of B-cell differentiation, since  <prot>Syk</prot> - <prot>Lyn</prot> 
      associations were not detected in pre-B and myeloma cell lines and <prot>Syk</prot>
      from an immature B-cell line did not reassociate with <prot>Lyn</prot> in vitro.
      Serine/threonine kinase activity was also associated with <prot>Syk</prot>.
      Crosslinking of cell surface IgM led to rapid activation of both tyrosine
      and serine/threonine protein kinase activities that resulted in
      phosphorylation in vitro of proteins coprecipitating with <prot>Syk</prot>--in
      particular, a serine/threonine phosphorylated protein 120 kDa in size
      (<prot>pp120</prot>). Several phosphoproteins, including one of 72 kDa and one of 120
      kDa, coprecipitated with <prot>phospholipase C-gamma 1</prot> (<prot>PLC gamma 1</prot>). Sequential
      immunoprecipitation identified the 72-kDa protein associated with  <prot>PLC
      gamma 1</prot>  as  <prot>Syk</prot> . The 120-kDa serine/threonine phosphorylated protein that
      coprecipitated with <prot>PLC gamma 1</prot> resembled the <prot>Syk</prot>-associated <prot>pp120</prot> by
      several criteria. Thus,  <prot>pp120</prot>  may serve as a link between  <prot>Syk</prot>  and  <prot>PLC
      gamma 1</prot> , coupling the B-cell antigen receptor to the phosphatidylinositol
      pathway.
AD  - Department of Microbiology, University of Washington Medical Center,
