Pbx-hox heterodimers recruit coactivator-corepressor complexes in an isoform-specific manner

Homeobox (hox) proteins have been shown to regulate cell fate and segment i dentity by promoting the expression of specific genetic programs. In contra st to their restricted biological action in vivo, however, most homeodomain factors exhibit promiscuous DNA binding properties in vitro, suggesting a requirement for additional cofactors that enhance target site selectivity. In this regard, the pbx family of homeobox genes has been found to heterodi merize with and thereby augment the DNA binding activity of certain hox pro teins on a subset of potential target sites, Here we examine the transcript ional properties of a forced hox-pbx heterodimer containing the pancreas-sp ecific orphan homeobox factor pdx fused to pbx-1a. Compared to the pdx mono mer, the forced pdx-pbx1a dimer, displayed 10- to 20-fold-higher affinity f or a consensus hox-pbx binding site but was completely unable to bind a hox monomer recognition site. The pdx-pbx dimer stimulated target gene express ion via an N-terminal trans-activation domain in pdr that interacts with th e coactivator CREB binding protein. The pdx-pbx dimer was also found to rep ress transcription via a C-terminal domain in pbx-1a that associates with t he corepressors SMRT and NCoR The transcriptional properties of the pdx-pbx 1 complex appear to be regulated at the level of alternative splicing; a pd x-pbx polypeptide containing the pbx1b isoform, which lacks the C-terminal extension in pbx1a, was unable to repress target gene expression via NCoR-S MRT, Since pbx1a and pbx1b are differentially expressed in endocrine versus exocrine compartments of the adult pancreas, our results illustrate a nove l mechanism by which pbn proteins may modulate the expression of specific g enetic programs, either positively or negatively, during development.