The transcription factor Runx-1 is required for the ontogeny of all definitive hematopoiesis, and plays a specific role in megakaryopoiesis during later stages of development. Germline mutations in Runx-1 cause Familial Platelet Disorder with Propensity to Develop AML (FPD/AML), and acquired mutations occur in a subset of patients with myelodysplastic syndrome (MDS) and acute myelogenous leukemia. Although many of the reported Runx-1 mutations affect DNA binding and/or its interaction with the cofactor CBF-beta, other mutations occur outside of these binding regions and have unknown mechanistic effects. In this study, we purified Runx-1 containing multiprotein complexes from murine megakaryocytic cells in order to identify potential novel Runx-1 associated factors whose interaction may be altered by Runx-1 mutations. Here we report the identification of the key megakaryocyte ets transcription factor Fli- 1 as a direct Runx-1 binding partner. This interaction involves the negative regulatory DNA binding and activation domains of Runx-1 (amino acids 179–370), and a region around the Ets DNA binding region of Fli-1 (amino acids 281–361). The interaction is lost in the MDS- associated Y254X Runx-1 mutation. We also show that Runx-1 and Fli-1 co-occupy the c-mpl promoter in primary megakaryocytes and act synergistically in transcriptional reporter assays. Interestingly, the interaction between Runx-1 and Fli- 1 occurs in murine L8057 megakaryoblastic cells only after they have been induced to differentiate, even though both proteins are expressed abundantly in uninduced cells. The interaction correlates with assembly of a large multiprotein complex that also includes the key megakaryocyte transcription factor GATA-1 and its cofactor Friend of GATA-1 (FOG- 1) based on gel filtration chromatography experiments. Furthermore, we show that Fli-1 from this large complex lacks phosphorylation of a specific residue that is phosphorylated on non-complexed Fli-1. Mutation of this site to aspartic acid, which mimics constitutive phosphorylation, disrupts the interaction between Fli-1 and Runx-1 and abrogates their transcriptional synergy. We propose that dephosphorylation of Fli-1 is a key event in the transcriptional activation of megakaryocyte terminal maturation by facilitating the assembly of a RUNX-1/FLI-1/GATA-1/FOG-1 enhancesome complex. These findings have implications for the differentiation of other cell types where interactions between Runx and ets family proteins occur.

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