PHYSICAL AND FUNCTIONAL ASSOCIATION OF CORTACTIN WITH SYK IN HUMAN LEUKEMIC-CELL LINE K562

Human leukemic cell line K562 is induced to differentiate into the meg akaryocytic lineage by stimulation with 12-O-tetradecanoylphorbol-18-a cetate (TPA). We demonstrate here that TPA stimulation increases tyro sine phosphorylation of an 80-kDa protein at an early stage of megakar yocytic differentiation and that this 80-kDa protein is identical with cortactin. Since tyrosine kinase Syk, was activated by TPA stimulatio n, we examined the possibility that cortactin is a potential substrate of Syk in K562 cells. TPA-induced tyrosine phosphorylation of cortact in was decreased profoundly by overexpression of dominant-negative Syk . Furthermore, cortactin was associated with Syk even before TPA stimu lation. Since cortactin was previously referred as an 80/85-kilodalton pp60(src) substrate, we examined the association between Src and cort actin, whereas its association could not be detected. These data sugge st that Syk phosphorylates cortactin in K562 cells upon TPA treatment.