The Fas receptor is physically associated with a series of tyrosine-phosphorylated proteins in human eosinophils. (A) Cells (5 × 10⁶) were stimulated with anti-Fas MoAb (IgM) for the indicated times. The cell lysates were immunoprecipitated with anti-Fas MoAb (IgG3). The immunoprecipitates were analyzed by immunoblotting with an MoAb to ptyr, RC20. (Upper panel) A prominent 56-kD protein and two additional tyrosine-phosphorylated proteins associated with the receptor in unstimulated cells. In addition, 88- and 110-kD tyrosine-phosphorylated proteins inducibly coimmunoprecipitated with the Fas receptor. (Lower panel) Stimulation of 3 × 10⁶ eosinophils with control MoAb did not result in an increase in either the quantity or the degree of tyrosine phosphorylation of proteins associated with the Fas receptor. (B) Cells (3 × 10⁶) were stimulated with anti-Fas MoAb (IgM) for the indicated times. The cell lysates were immunoprecipitated with 4G10 MoAb (upper panel) or control IgG2b MoAb (lower panel), and the immunoprecipitates were examined for the presence of Fas receptor protein. The Fas receptor inducibly coimmunoprecipitated with ptyr proteins (upper panel). The positions of molecular size standards for (A) and (B) are on the left. All data are representative of at least three independent experiments.