FMS-Like tyrosine kinase 3 (FLT3) is expressed in hematopoietic stem cells in the marrow. Activation of FLT3 initiates downstream signaling cascades that are important for the survival, proliferation and differentiation of stem cells. Whie expression of this tyrosine kinase is normally lost as hematopoietic cells differentiate, many acute myeloid (AML) leukemias aberrantly express FLT3. Additionally, up to one-third of AML cases harbor activating mutations in FLT3 including internal tandem duplications in the juxtamembrane domain (FLT3/ITD) and, less frequently, point mutations in the kinase domain such as FLT3/D835Y. The presence of a FLT3/ITD mutation in AML portends a poor prognosis. FLT3 is a promising therapeutic target, and several tyrosine kinase inhibitors (TKIs) directed against FLT3 are currently in clinical trials. These agents have dramatic in vitro activity; however, the results of clinical trials have been largely disappointing. While many hypotheses to explain these results have been suggested, we believe that a more complete understanding of normal and mutant FLT3 biology will be required before we are able to effectively target this pathway. Although the clinical implications of FLT/ITD mutations have been described in detail and at least some of the relevant downstream signaling pathways have been characterized, relatively little is known about the cohort of proteins that interacts with FLT3. In order to idenify a panel of proteins that interact with FLT3/ITD, we performed a screen using co- immunoprecipitation assays coupled with mass spectroscopy. These experiments generated a panel of potential interactors, including dedicator of cytokinesis 2 (DOCK2). DOCK2 activates small G-proteins including the Rho-subfamily small GTPases Rac1 and Rac2, and affects various aspects of cellular function including stem cell homing and retention in the marrow, cell motility, cytoskeletal organization and cell proliferation. In this proposal our goal is to determne the functional consequences of the interaction between DOCK2 and wild type FLT3 as well as the interaction between DOCK2 and FLT3 with activating mutations. Next, we will assess how the interaction between DOCK2 and FLT3, FLT3/ITD and FLT3/D835Y affects the response of the tyrosine kinase receptors to clinically relevant inhibitors. We hypothesize that characterizin the interaction of FLT3 and FLT3/ITD with interactors such as DOCK2 will provide a deeper understanding of this tyrosine kinase's function, suggest mechanisms that could enhance the efficacy of TKI therapy, and identify new drug targets in order to develop more efficacious treatments for patients with FLT3/ITD+ AML.