TRANSCRIPTIONAL REPRESSION BY MSX-1 DOES NOT REQUIRE HOMEODOMAIN DNA-BINDING SITES

This study investigates the transcriptional properties of Msx-1, a mur ine homeodomain protein which has been proposed to play a key role in regulating the differentiation and/or proliferation state of specific cell populations during embryogenesis. We show, using basal and activa ted transcription templates, that Msx-1 is a potent repressor of trans cription and can function through both TATA-containing and TATA-less p romoters. Moreover, repression in vivo and in vitro occurs in the abse nce of DNA-binding sites for the Msx-1 homeodomain. Utilizing a series of truncated Msx-1 polypeptides, we show that multiple regions of Msx -1 contribute to repression, and these are rich in alanine, glycine, a nd proline residues. When fused to a heterologous DNA-binding domain, both N- and C-terminal regions of Msx-1 retain repressor function, whi ch is dependent upon the presence of the heterologous DNA-binding site . Moreover, a polypeptide consisting of the full-length Msx-1 fused to a heterologous DNA-binding domain is a more potent repressor than eit her the N- or C-terminal regions alone, and this fusion retains the ab ility to repress transcription in the absence of the heterologous DNA site. We further show that Msx-1 represses transcription in vitro in a purified reconstituted assay system and interacts with protein comple xes composed of TBP and TFIIA (DA) and TBP, TFIIA, and TFIIB (DAB) in gel retardation assays, suggesting that the mechanism of repression is mediated through interaction(s) with a component(s) of the core trans cription complex. We speculate that the repressor function of Msx-1 is critical for its proposed role in embryogenesis as a regulator of cel lular differentiation.