RECONSTITUTION OF THE B-CELL ANTIGEN RECEPTOR SIGNALING COMPONENTS INCOS CELLS

To elucidate interactions occurring between B cell protein tyrosine ki nases and the signaling components of the B cell antigen receptor, we have co transfected into COS cells individual tyrosine kinases togethe r with chimeric cell surface receptors containing the cytoplasmic doma ins of Iga or Ig beta, Of the tyrosine kinases transfected (Lyn, Blk, Hck, Syk, Fyn), only Blk was able to phosphorylate and subsequently as sociate with cotransfected Ig alpha and Ig beta chimeras in vivo, Asso ciation between Blk and the Ig alpha and Ig beta cytoplasmic domains w as shown by mutational analyses to be the result of an SH2-phosphotyro sine interaction, We identified the tyrosine residues of the Ig alpha and Ig beta cytoplasmic domains phosphorylated by Blk. The enzymatic a ctivity and membrane association of Blk were required for the observed phosphorylation of the Ig alpha and Ig beta chimeras, Sequences withi n the amino-terminal unique domain of Blk are responsible for recognit ion and subsequent phosphorylation of the Ig alpha chimera since trans fer of the unique region of Blk to Fyn results in the chimeric kinase' s ability to phosphorylate the cytoplasmic domain of Ig alpha. These f indings indicate that the unique domain of Src family kinases may dire ct recognition of certain substrates leading to their phosphorylation.