Figure 3.

Tyrosine phosphorylation of (A) Vav and (B) CD22 after BCR crosslinking in B cells from CD19-deficient (CD19^−/−) mice and wild-type littermates. Purified splenic B cells (10⁷/lane) were incubated with anti-IgM antibodies (40 μg/ml) and processed as in Fig. 1. Proteins of interest were immunoprecipitated with specific antibodies plus anti-MAPK antibodies, normal rabbit serum (CTL, A), or normal rat serum (CTL, B), as in Fig. 2. Immunoprecipitated proteins were fractionated by SDS/PAGE and transferred onto a membrane for subsequent anti-phosphotyrosine immunoblotting (Upper). Subsequently, all blots were stripped of anti-phosphotyrosine antibody and reprobed with anti-MAPK antibodies to quantitate the amount of proteins within the immunoprecipitates (Lower). These results are representative of those obtained with three littermate pairs of mice.