CRP stimulates tyrosine phosphorylation of syk and PLCγ2. syk and PLCγ2 were immunoprecipitated from lysates of platelets stimulated by CRP or collagen and immunoblotted for phosphotyrosine as described in the Materials and Methods. (A) Tyrosine phosphorylation of syk (i and ii) and PLCγ2 (iii and iv) in CRP-stimulated platelets. The time course of tyrosine phosphorylation stimulated by CRP (3 μg/mL) is shown in (i) and (iii). The concentration response relationship (90 seconds of incubation) for tyrosine phosphorylation by CRP is shown in (ii) and (iv). The location of syk, PLCγ2, and the heavy chain of the immunoprecipitating antibody (IgG) are shown. An unknown band of 75 kD (indicated by the arrow in [i]) was present in syk immunoprecipitates. Three prominent uncharacterized proteins of approximately 47, 78, and 120 kD were present in PLCγ2 immunoprecipitates and are indicated by arrows in (iii) and (iv). Reprobing for syk and PLCγ2 showed that similar levels of each protein were precipitated under all conditions (not shown). (B) Tyrosine phosphorylation of syk and PLCγ2 in collagen-stimulated platelets. Platelets were stimulated with collagen for 90 seconds. The antiphosphotyrosine blots of syk and PLCγ2 immunoprecipitates shown in the upper part of each figure were stripped and reprobed for syk and PLCγ2 as described in the Materials and Methods and are shown in the lower part of each figure. Results are representative of three experiments.