THE HMG-BOX TRANSCRIPTION FACTOR HBP1 IS TARGETED BY THE POCKET PROTEINS AND E1A

A yeast two-hybrid screen has identified HBP1 as a transcription facto r capable of interacting with the pocket protein family. We show that HBP1 can interact with one of these, RE, both in vitro and in mammalia n cells. Two distinct RE binding sites are present within HBP1 - a hig h affinity binding site, mediated by an LXCXE motif and a separate low affinity binding site present within an activation domain. GAL4-fusio n experiments indicate that HBP1 contains a masked activation domain. Deletion of two independent N- and C-terminal inhibitor domains unmask s an activation domain which is 100-fold more active than the full len gth protein. The released activation capacity is repressed by RE, p130 and p107. In addition, E1A can repress the activity of HBP1 via conse rved region 1 sequences in a manner independent of the CBP coactivator . We show by stable expression in NIH3T3 cells that HBP1 has the capac ity to induce morphological transformation of cells in culture.