SPECIFICITY OF HOX PROTEIN FUNCTION DEPENDS ON DNA-PROTEIN AND PROTEIN-PROTEIN INTERACTIONS, BOTH MEDIATED BY THE HOMEO DOMAIN

Transcription of human HOX gene promoters in cultured cells is positiv ely and negatively regulated by HOX proteins interacting with specific target sequences. The human HOXD9 protein activates transcription of the HOXD9 promoter by interacting with the HCR sequence and is antagon ized by the HOXD8 protein. HOXD8 is not intrinsically a repressor, sin ce it can activate transcription on different targets. Complete or par tial HOXD8/HOXD9 homeo domain swapping indicates that the ability to r ecognize, and activate transcription from, the HCR target in vivo depe nds on the amino terminus and helix 1 of the homeo domain. The inhibit ory activity of HOXD8 is not affected by deletion of the homeo domain helix 2/3 region, whereas it requires the amino terminus/helix 1 regio n and an additional, effector domain located at the protein amino-term inal end. This activity is therefore DNA-binding independent, and poss ibly mediated by protein-protein interactions. Affinity chromatography experiments show that the homeo domain amino terminus/helix 1 region is able to mediate direct interactions between HOX proteins in solutio n. These data indicate that specificity of HOX protein function in viv o depends on both DNA-protein and protein-protein interactions, mediat ed by the same sub region of the homeo domain.