THE HOX COOPERATIVITY MOTIF OF THE CHIMERIC ONCOPROTEIN E2A-PBX1 IS NECESSARY AND SUFFICIENT FOR ONCOGENESIS

E2a-Pbx1 chimeric oncoproteins result from fusion of the E2A and PBX1 genes at the sites of t(1;19) chromosomal translocations in a subset a cute lymphoblastic leukemias. Experimentally, E2a-Pbx1 transforms a va riety of cell types, including fibroblasts, myeloid progenitors, and l ymphoblasts. Structure-function studies have shown that contributions from both E2a and Pbx1 are necessary for oncogenesis, but the Pbx1 hom eodomain is dispensable and the required portion of Pbx1 has not been delineated. In this study, we used deletional and site-directed mutage nesis to identify portions of Pbx1 necessary for oncogenic and transcr iptional activities of E2a-Pbx1. These studies defined a motif (named the Hox cooperativity motif [HCM]) carboxy terminal to the Pbx homeodo main that is required for cooperative DNA binding, cellular transcript ional activity, and the oncogenic potential of E2a-Pbx1. The HCM is hi ghly conserved throughout the Pbx/exd subfamily of divergent homeodoma in proteins and functions in DNA-binding assays as a potential contact site for Hox dimerization. E2a-Pbx1 proteins with interstitial deleti on or single-point mutations in the HCM could neither activate transcr iption in cellular assays nor transform NIH 3T3 cells. An E2a-Pbx1 mut ant containing 50 amino acids of Pbs1b spanning the HCM but lacking th e homeodomain was capable of inducing fibroblast transformation. Thus, the HCM is a necessary and sufficient contribution of Pbx1 for oncoge nesis induced by E2a-Pbx1 and accounts for its homeodomain-independent transforming properties. Since subtle alterations of the Pbx HCM resu lt in complete abrogation of transforming activity whereas the homeodo main is entirely dispensable, we conclude that interactions mediated b y the HCM are more important for transformation by E2a-Pbx1 than inter actions with cognate Pbx DNA sites.