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RI Signaling by ZAP70, But Not Syk, in Syk-Negative Mast Cells
Receptors and Signal Transduction Section, Oral Infection and Immunity Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892
The ZAP70/Syk family of protein tyrosine kinases plays an important
role in Ag receptor signaling. Structural similarity of Syk and ZAP70
suggests their functional overlap. Previously, it was observed that
expression of either ZAP70 or Syk reconstitutes Ag receptor signaling
in Syk-negative B cells. However, in CD45-deficient T cells, Syk, but
not ZAP70, restores T cell receptor-signaling pathway. To study the
function of Syk, ZAP70, and CD45 in mast cells, a Syk/CD45
double-deficient variant of RBL-2H3 cells was characterized. After
transfection, stable cell lines were isolated that expressed ZAP70,
Syk, CD45, ZAP70 plus CD45, and Syk plus CD45. IgE stimulation did not
induce degranulation in parental double-deficient cells, nor in the
cells expressing only CD45. ZAP70 expression did not restore Fc
RI
signaling 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
FcRI-aggregation. There was much better binding of Syk than ZAP70 to
the phosphorylated FcRI
-ITAM. Furthermore, unlike Syk, ZAP70
required CD45 to display receptor-induced increase in kinase activity.
Therefore, in mast cells, ZAP70, but not Syk, requires CD45 for Ag
receptor-induced signaling.
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