Hox proteins functionally cooperate with the GC box-binding protein system through distinct domains.

Hox genes encode a transcriptional factor that plays a key role in regulating position-specific cartilage pattern formation. We found that Hoxa-13 and Hoxd-13, which are members of the Abd-B subfamily of Hox genes and are crucial for the autopod development of the limb, stimulate transcription from the Bmp-4 promoter. This stimulation was dependent on the GC box within the promoter and independent of the putative Hox protein binding site. The stimulation by HoxA-13 was remarkably enhanced by cotransfection with members of a family of zinc finger GC box binding transcriptional factors including Sp1. The stimulation was suppressed by another Abd-B Hox protein, HoxA-11, indicating that each Abd-B Hox protein has a different effect on the target genes through the Sp1 system. We have identified multiple functional domains involved in transcriptional regulation, including three independent transcriptional activation domains (ADs) in HoxA-13. AD1 and AD3 in helices 1 and 2 of the homeodomain individually cooperate with Sp1-dependent stimulation. The homeodomain is also required for cooperation of the AD with Sp1. By contrast, AD2 strongly activates transcription in an Sp1-independent manner only when the homeodomain has been removed. These observations indicate that HoxA-13 regulates transcription through multiple pathways. In addition, we found that a helix 3 mutation of the HoxA-13 homeodomain behaves as a dominant negative form.