Figure 7.

BCR-mediated activation of Btk and TFII-I depends on the levels of Bright in lipid rafts. (A) Dominant-negative Bright mutants alter Btk phosphorylation in B-cell lines. Whole cell lysates prepared from retrovirally transduced cell lines established in Figure 4A were stimulated for 5 min using either 500 ng α-μ or 500 ng α-μ+500 ng α-CD19 for 5 × 10⁸ cells as indicated. After anti-Btk immunoprecipitation, westerns were performed with anti-Btk (loading control) and anti-pY anti-sera. (B) Transgenic Bright over-expression inhibits Btk and TFII-I phosphorylation in B-cell subpopulations. Lipid raft extracts prepared using RIPA buffer (described in Figure 6D; equivalent of ∼10⁶ cells per lane) were subjected to α-Btk IP, followed by western blotting using the antibodies indicated.