Thr-133 in RasGRP3 is crucial for BCR-mediated Ras activation. (A) Schematic diagram of RasGRP3 mutants. By homology modeling based on structures of Sos, an N-terminal ≈400-residue catalytic module of chicken RasGRP3 is thought to be sufficient for the Ras-specific nucleotide exchange activity. This segment of RasGRP3 includes a core region known as the cdc25 domain (residues 130–399) and an additional domain, known as the Ras exchange motif domain, located N-terminal to the cdc25 domain (residues 1–129). RasGRP3 cDNAs encoding these mutations were transfected into RasGRP3-deficient DT40 cells. A single clone expressing each RasGRP3 mutant was extensively analyzed and represented, although some critical experiments were carried out, at least, using two different clones. EFh, EF hand; C1, protein kinase C conserved region 1 domain. (B) Expression of surface BCR on various mutant DT40 cells. Filled and opened histograms represent unstained cells and cells stained with FITC-anti-chicken IgM, respectively. (C) Expression of RasGRP3 in various mutant DT40 cells. Whole-cell lysates (2 × 106 cells per lane) were subjected to Western blot analysis using anti-chicken RasGRP3 Ab (Upper) or anti-extracellular signal-regulated kinase Ab (Lower). (D) BCR-mediated Ras activation in various mutant DT40 cells. DT40 B cells (1 × 107 cells per lane) were stimulated with M4 (5 μg/ml) for the indicated period. Lysates were incubated with RBD-bound beads for measuring GTP-bound Ras. Proteins eluted from beads were resolved in SDS/PAGE followed by Western blot analysis using anti-Ras mAb (Upper). For measuring total Ras, parts of the lysates (1 × 106 cells per lane) were subjected to Western blot analysis without incubation with beads (Lower). (E) Comparison of protein sequences of the RasGRP family surrounding Thr-133 (denoted by an asterisk).