Grb2-Sos complex recruits tyrosine-phosphorylated Shc and CrkL after FcRI aggregation. We studied Grb2-bound phosphoproteins in U937IF cells after FcR stimulation. (A) Antiphosphotyrosine blot performed on anti-Grb2 immunoprecipitates. Lane 1 is a control immunoprecipitation with rabbit IgG. Anti-Grb2 immunoprecipitates from resting U937IF cells (lane 2) and from U937IF cells stimulated with anti-FcR and RM (both Fab′₂ fragments) for 1 minute (lane 3), for 5 minutes (lane 4), for 10 minutes (lane 5), or for 30 minutes (lane 6), respectively. Lane 7 represents a whole cell lysate of U937IF cells stimulated with anti-FcR after RM antibodies for 1 minute. (B) The same membrane was stripped and reprobed with anti-Sos (first panel), anti-Shc (second panel), anti-CrkL (third panel), and anti-Grb2 (fourth panel), respectively. Lanes are identical to those in (A).