The homeodomain of PDX-1 mediates multiple protein-protein interactions inthe formation of a transcriptional activation complex on the insulin promoter

Activation of insulin gene transcription specifically in the pancreatic bet a cells depends on multiple nuclear proteins that interact with each other and with sequences on the insulin gene promoter to build a transcriptional activation complex. The homeodomain protein PDX-1 exemplifies such interact ions by binding to the A3/4 region of the rat insulin I promoter and activa ting insulin gene transcription by cooperating with the basic-helix-loop-he lix (bHLH) protein E47/Pan1, which binds to the adjacent E2 site. The prese nt study provides evidence that the homeodomain of PDX-1 acts as a protein- protein interaction domain to recruit multiple proteins, including E47/Pan1 , BETA2/NeuroD1, and high-mobility group protein I(Y), to an activation com plex on the E2A3/4 minienhancer, The transcriptional activity of this compl ex results from the clustering of multiple activation domains capable of in teracting with coactivators and the basal transcriptional machinery. These interactions are not common to all homeodomain proteins: the LIM homeodomai n protein Lmx1.1 can also activate the E2A3/4 minienhancer in cooperation w ith E47/Pan1 but does so through different interactions. Cooperation betwee n Lmx1.1 and E47/Pan1 results not only in the aggregation of multiple activ ation domains but also in the unmasking of a potent activation domain on E4 7/Pan1 that is normally silent in non-beta cells. While more than one activ ation complex may be capable of activating insulin gene transcription throu gh the E2A3/4 minienhancer, each is dependent on multiple specific interact ions among a unique set of nuclear proteins.