Ligation of CD19 in immature B cells induces tyrosine phosphorylation and activation of Tec. (A) Immature B-cell lines, RS4;11, 380, and NALM6 were incubated with anti-CD19 monoclonal antibody or isotype-matched nonreactive Ig for 15 minutes at 4°C followed by goat antimouse Ig antiserum for 1 minute at 37°C. Proteins immunoprecipitated with anti-Tec rabbit antibody were separated by SDS-PAGE and transferred to a nitrocellulose membrane. The membrane was probed with antiphosphotyrosine antibody (pTyr; upper panel), then stripped and reprobed with anti-Tec rabbit antibody (lower panel). The position of Tec is indicated. (B) CD19 cross-linking in immature B cells increases Tec kinase activity. RS4;11 cells were stimulated as above and Tec kinase activity was assessed by in vitro kinase assay. Molecular mass markers (in kilodaltons) are indicated. The position of Tec is indicated by the arrow.