PREP1 (aka PKNOX1, PBX-knotted homeobox 1) was identified by Chen et al. (1997) in their effort to characterize genes mapping on the human chromosome 21. At the same time, PREP1 was identified as one of the components of the human transcription factor complex UEF3, urokinase enhancer factor 3. PREP1 was found to be a homeodomain-containing DNA-binding protein belonging to the TALE (three aminoacid loop extension) superclass characterized by the MEINOX domain, most closely related to TGIF (TGF β–induced factor homeobox) and MEIS1 (Meis homeobox 1). ChIP-seq analysis shows that in vivo PREP1 recognizes the decameric TGAXTGACAG consensus, both alone and as dimer with PBX1 (Penkov et al. 2013). PREP1 heterodimerizes with PBX through the amino-terminal MEIS-A and MEIS-B motifs present in the MEINOX domain (Berthelsen et al. 1998); PREP1 binds PBX1 in the cytoplasm and is transported to the nucleus where it prevents PBX nuclear export (Berthelsen et al. 1999...
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