HETERODIMERIZATION OF HOX PROTEINS WITH PBX1 AND ONCOPROTEIN E2A-PBX1GENERATES UNIQUE DNA-BINDING SPECIFICITIES AT NUCLEOTIDES PREDICTED TO CONTACT THE N-TERMINAL ARM OF THE HOX HOMEODOMAIN-DEMONSTRATION OF HOX-DEPENDENT TARGETING OF E2A-PBX1 IN-VIVO

Hox proteins control genetic programs that orchestrate development, an d a large subset of Hox proteins can bind DNA elements as heterodimers with the Pbx family of homeodomain proteins. A transcriptionally acti vated version of Phx1, E2a-Pbx1, is an oncoprotein in human pre-B cell leukemia that strongly suppresses differentiation and retains its abi lity to heterodimerize with Hox proteins. Because monomeric Hox protei ns bind very similar DNA motifs, it is unclear how they activate diver se developmental programs. Here we demonstrate that heterodimers conta ining different Hox proteins and a common Pbx1 or E2a-Pbx1 partner bin d different DNA motifs, Structural models suggest that the specificity of the Hox protein is altered by a conformation change involving resi dues in the N-terminal arm of the Hox homeodomain. Mutational analysis also supported the hypothesis that unique sequences in the N-terminal arm of the Hox homeodomain are at least partially responsible for med iating this specificity. lit vivo, Hox proteins directed E2a-Pbx1-medi ated transactivation with moderate specificity to cognate Hox-Pbx moti fs. Thus, the development specificity of individual Hox proteins may b e mediated, in part, by differential targeting of cellular genes by Pb x1-Hox complexes. Likewise, through its function as a common heterodim er partner, oncoprotein E2a-Pbx1 may be able to interfere with multipl e programs of development that are induced by the sequential or simult aneous expression of Hox proteins during hematopoiesis.