The phosphatidylinositol polyphosphate 5-phosphatase SHIP1 associates withthe Dok1 phosphoprotein in Bcr-Abl transformed cells

The initial phase of chronic myelogenous leukemia (CML) is triggered by con stitutive protein tyrosine kinase activity bf the chimeric kinase p210(bcr- abl) (Bcr-Abl). A major substrate of Bcr-Abl was recently identified as the RasGAP-associated 62 kDa docking protein Dok1. Here, we report complex for mation between endogenous Dok1 and the SH2 domain-containing phosphatidylin ositol polyphosphate 5-phosphatase SHIP1 in hematopoietic cells expressing Bcr-Abl. Expression of Bcr-Abl induced tyrosine phosphorylation of both Dok 1 and SHIP1 and the formation of a Dok1/SHIP1 complex. Tyr(P) SHIP1 was als o bound to Shc in Bcr-Abl expressing cells. A small amount of Shc/SHIP1/Dok 1 trimolecular complex was detected and this was due to binding of Dok1 to SHIP1 that was bound to Shc. In contrast, association of Dok1 with SHIP1 or RasGAP was mutually exclusive. Both the SH2 domain of SHIP1 and the PTB do main of Dokl were required for complex formation between the two proteins. Neither the specific activity of SHIP1 as an inositol phosphate 5-phosphata se nor the subcellular localization of SHIP1 appeared to be altered by tyro sine phosphorylation. However, the Dok1/SHIP1 complex was only detected in the cytosolic fraction of Bcr-Abl transformed hematopoietic cells. We propo se that interaction between Dokl and SHIP1 modulates the ability of these t wo proteins to interact with other cytosolic binding partners. (C) 2000 Els evier Science Inc. All rights reserved.