Characterization of tyrosine-phosphorylated proteins. (A and B) Western blots of Lyn and Syk immunoprecipitated with anti-Lyn (A) or anti-Syk (B) from Triton X-100 lysates of KG1a cells stimulated for 30 minutes with MOPC21 (control, lane 1), 4A1 (lane 2), or MEM59 (lane 3) as described in the legend to Fig 1, probed with anti-pTyr (top panels), and reprobed with anti-Lyn (A, bottom panel) or anti-Syk (B, bottom panel). IgG (H), heavy chain of immunoglobulin G. (C) (Top two panels) Western blots of modified RIPA-washed Syk immunoprecipitates from Triton X-100 lysates prepared as described above, probed with anti-pTyr, and reprobed with anti-Syk. (Bottom two panels) Autoradiograms of in vitro phosphorylation of Syk and MBP by aliquots of the modified RIPA-washed Syk immunoprecipitates. Syk kinase activities were determined by measuring the incorporation of³²PO₄ into Syk per se and MBP and expressed as values relative to the specific activity of Syk in the control sample (lane 1).