CD45 is essential for Fc epsilon RI signaling by ZAP70, but not Syk, in Syk-negative mast cells

The ZAP70/Syk family of protein tyrosine kinases plays an important role in Ag receptor signaling. Structural similarity of Syk and ZAP70 suggests the ir functional overlap. Previously, it was observed that expression of eithe r ZAP70 or Syk reconstitutes Ag receptor signaling in Syk-negative B cells. However, in CD45-deficient T cells, Syk, but not ZAP70, restores cell rece ptor-signaling pathway. To study the function of Syk, ZAP70, and CD45 in ma st cells, a Syk/CD45 double-deficient variant of RBL-2H3 cells was characte rized. After transfection, stable cell lines were isolated that expressed Z AP70, Syk, CD45, ZAP70 plus CD45, and Syk plus CD45. IgE stimulation did no t induce degranulation in parental double-deficient cells, nor in the cells expressing only CD45, ZAP70 expression did not restore Fc epsilon RI signa ling unless CD45 was coexpressed in the cells. However, Syk alone restored the IgE signal transduction pathway. The coexpression of CD45 with Syk had no significant effects on the responses to Fc epsilon RI-aggregation. There was much better binding of Syk than ZAP70 to the phosphorylated Fc epsilon RI gamma-ITAM. Furthermore, unlike Syk, ZAP70 required CD45 to display rec eptor-induced increase in kinase activity. Therefore, in mast cells, ZAP70, but not Syk, requires CD45 for Ag receptor-induced signaling.