passages.id: split_4_test_888_passage
passages.type: progene_text
passages.text: BCR - induced tyrosine phosphorylation of the phosphatidylinositol 3-kinase catalytic subunit was abolished in Syk - deficient cells consistent with a receptor - proximal role for Syk in BCR - mediated phosphatidylinositol 3-kinase activation ; in contrast , it was maintained in Btk - deficient cells , suggesting Btk functions downstream of phosphatidylinositol 3-kinase .