| BCR ligation induced rapid tyrosine phosphorylation of HPK 1 mainly by Syk and Lyn , resulting in its association with BASH and catalytic activation . ^^^ BCR mediated activation of HPK 1 was impaired in Syk or BASH deficient B cells . ^^^ The functional SH 2 domain of BASH and Tyr 379 within HPK 1 which we identified as a Syk phosphorylation site were both necessary for interaction of both proteins and efficient HPK 1 activation after BCR stimulation . ^^^ |