AGGREGATION OF IGE RECEPTORS IN RAT BASOPHILIC LEUKEMIA 2H3 CELLS INDUCES TYROSINE PHOSPHORYLATION OF THE CYTOSOLIC PROTEIN-TYROSINE-PHOSPHATASE HEPTP

The cDNA encoding the rat equivalent of the human hematopoietic tyrosi ne 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 c ells and B cells, the phosphatase is also Pound in mast cells. By immu nofluorescence microscopy, rat hematopoietic tyrosine phosphatase loca lized to discrete, globular compartments within the cytoplasm and was not found either in the nucleus or associated with the cell surface me mbrane. Aggregation of high affinity IgE receptors in the mast cells i nduced tyrosine phosphorylation of the phosphatase. The tyrosine phosp horylation was mimicked by stimulation with calcium ionophore A23187 b ut not by direct activation of protein kinase C. Since phosphorylation of the phosphatase was dramatically reduced when the cells were activ ated in Ca2+ free media, it is dependent on a rise in intracellular Ca 2+. These data strongly suggest that hematopoietic tyrosine phosphatas e may be involved in the IgE receptor-mediated signaling cascade.