The presence of the atypical SH3 domain-binding motif reduces the ability of SLP-65 to activate p38 and JNK. The slp-65^–/– B cells (lanes 1-4) and transfectants expressing equal amounts of either full-length SLP-65 (lanes 5-8) or s-SLP-65 (lanes 9-12) were analyzed for the ability to activate distinct MAPK families on BCR engagement. The activation of Erk (A), p38 (B), and JNK (C) was monitored by immunoblotting of cleared cellular lysates with antibodies to phospho-Erk, phospho-p38, and phospho-JNK, respectively (top panels). Equal loading was controlled by probing the membranes with anti-Erk, anti-p38, or antiactin antibodies, respectively (bottom panels). The time of BCR activation (in minutes) is denoted above each lane. Relative molecular masses of marker proteins are indicated on the left.