The cDNA encoding the rat equivalent of the human hematopoietic tyrosine phosphatase, also known as leukocyte phosphatase, was isolated from a rat basophilic leukemia mast cell cDNA library. By two-dimensional electrophoresis, the protein expressed in the mast cells was of a size (40 kDa) and pI (6.9) predicted from the deduced amino acid sequence. Thus, although previously shown to be preferentially expressed in T cells and B cells, the phosphatase is also found in mast cells. By

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... ence microscopy, rat hematopoietic tyrosine phosphatase localized to discrete, globular compartments within the cytoplasm and was not found either in the nucleus or associated with the cell surface membrane. Aggregation of high affinity IgE receptors in the mast cells induced tyrosine phosphorylation of the phosphatase. The tyrosine phosphorylation was mimicked by stimulation with calcium ionophore A23187 but not by direct activation of protein kinase C. Since phosphorylation of the phosphatase was dramatically reduced when the cells were activated in Ca 2؉ -free media, it is dependent on a rise in intracellular Ca 2؉ . These data strongly suggest that hematopoietic tyrosine phosphatase may be involved in the IgE receptor-mediated signaling cascade. Mast cells and basophils play a central role in allergic and inflammatory reactions. They express high affinity IgE receptors (Fc⑀RI) 1 on their cell surfaces that, when aggregated, initiate biochemical events that lead to the release of inflammatory mediators. In the rat basophilic leukemia (RBL-2H3) mast cell line, aggregation of Fc⑀RI induces activation of phospholipases A 2 , C, and D, an increase in intracellular Ca 2ϩ concentration, and activation and translocation of protein kinase C from the cytosol to the plasma membrane (1-5). In addition, numerous proteins become tyrosine phosphorylated following receptor aggregation. These include the ␤ and ␥ subunits of the receptor, phospholipase C-␥1, Vav, Nck, and paxillin; protein-tyrosine kinases such as Lyn, Syk, Fak, and Btk; and other unidentified proteins (6 -21).