A Possible Mechanism for Synergy in Transcriptional Activation between AML1 and PU.1: Exclusion of the Co-Repressor CBFA2T3 (ETO-2, MTG16).

AML1 and PU.1, important regulators of hematopoietic differentiation, interact with each other and are known to synergize in transcriptional activation (Zhang et al, 1996). PU.1 and AML1 also interact with mSin3, a component of a co-repressor complex that can include N-CoR, HDAC and CBFA2T1 (ETO, MTG8) or CBFA2T3 (ETO2, MTG16). CBFA2T3 is highly expressed in hematopoietic cells and is a target of a chromosomal translocation found in acute myeloid leukemia (t(16;21)). In transfected 293T cells, we demonstrate that both AML1 and PU.1 co-immunoprecipitate with the conserved N-terminal TAFH domain of CBFA2T3 but not the C-terminal MYND domain. Although AML1 and PU.1 independently co-immunoprecipitate with CBFA2T3, when all three proteins are over-expressed in 293T cells, AML1 and PU.1 co-immunoprecipitate with each other while excluding CBFA2T3. CBFA2T3 interacts with the non-runt portion of AML1 (AML1 C-terminus) while PU.1 can interact with both the runt domain of AML1 and AML1 C-terminus. Presumably, the interaction between AML1 and PU.1 shields the CBFA2T3 binding sites on both proteins. Since this region includes the binding site for mSin3, other co-repressors may also be excluded from an AML1/PU.1 complex. This may be one basis for the co-operation between AML1 and PU.1 in transcriptional activation.