Identification of a 52-kDa molecule (p52) coprecipitated with the Ig receptor-related MB-1 protein that is inducibly phosphorylated by the stimulation with phorbol myristate acetate.

Triggering of the Ig receptor (IgR) induces the activation in multiple intracellular signal transduction reactions including protein tyrosine phosphorylation, activation of phospholipase C, increased inositoltriphosphate, increased diacylglycerol, intracellular Ca2+ mobilization, and activation of protein kinase C. The IgR-complex, composed of mu-chain, L chain, Ig-alpha (MB-1), and Ig-beta (B29) proteins, is a functional unit both for expression of IgR and for signal transduction into cells, possibly by physical association with the down-stream functional molecules. An important functional motif ((D or E)-X7-(D or E)-Y-X3-L-X7-Y-X2-(L or I)) in the cytoplasmic domain of MB-1 molecule was shown to bind with several phosphoprotein components including src-type tyrosine kinases and phosphatidylinositol-3 kinase. To further study the functional components, we analyzed the phosphoprotein molecules coprecipitated with MB-1 protein. We found that a 52-kDa protein is coprecipitated with MB-1 protein and is inducibly phosphorylated by the stimulation with PMA. A rat mAb, prepared by immunizing the 52-kDa protein purified from SDS-PAGE, could detect the similar 52-kDa phosphoprotein (p52) expressed on the cell surface. In comparison with the 52-kDa protein in the immunoprecipitate of MB-1, the p52 migrated to the same position on 2-D gel electrophoresis (nonequilibrium pH gradient gel electrophoresis/SDS-PAGE). An in vitro kinase reaction analysis demonstrated that the p52 is tightly associated with the tyrosine kinase molecule(s), one of which is an 80-kDa protein containing an apparent autophosphorylation activity. These molecules would provide the informations of the down-stream molecules in the cascade reactions of the IgR-mediated signal transduction.