MEIS1 AND PKNOX1 BIND DNA COOPERATIVELY WITH PBX1 UTILIZING AN INTERACTION SURFACE DISRUPTED IN ONCOPROTEIN E2A-PBX1

E2a-Pbx1 is a chimeric transcription factor oncoprotein produced by th e t(1;19) translocation in human pre-B cell leukemia. Class I Hox prot eins bind DNA cooperatively with both Pbx proteins and oncoprotein E2a -Pbx1, suggesting that leukemogenesis by E2a-Pbx1 and Hox proteins may alter transcription of cellular genes regulated by Pbx-Hox motifs. Li kewise, in murine myeloid leukemia, transcriptional coactivation of Me is1 with HoxA7/A9 suggests that Meis1-HoxA7/9 heterodimers may evoke a berrant gene transcription. Here, we demonstrate that both Meis1 and i ts relative, pKnox1, dimerize with Pbx1 on the same TGATT-GAC motif se lected by dimers of Pbx proteins and unidentified partner(s) in nuclea r extracts, including those from t(1;19) pre-B cells. Outside their ho meodomains, Meis1 and pKnox1 were highly conserved only in two motifs required for cooperativity with Pbx1. Like the unidentified endogenous partner(s), both Meis1 and pKnox1 failed to dimerize significantly wi th E2a-Pbx1. The Meis1/pKnox1-interaction domain in Pbx1 resided predo minantly in a conserved N-terminal Pbx domain deleted in E2a-Pbx1. Thu s, the leukemic potential of E2a-Pbx1 may require abrogation of its in teraction with members of the Meis and pKnox families of transcription factors, permitting selective targeting of genes regulated by Pbx-Hox complexes. In addition, because most motifs bound by Pbx-Meis1/pKnox1 were not bound by Pbx1-Hox complexes, the leukemic potential of Meis1 in myeloid leukemias may involve shifting Pbx proteins from promoters containing Pbx-Hox motifs to those containing Pbx-Meis motifs.