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Mol. Cell. Biol., Apr 1996, 1632-1640, Vol 16, No. 4
Q Lu and MP Kamps
Genetic studies have identified a family of divergent homeodomain proteins,
including the human protooncoprotein Pbx1 and its drosophila homolog
extradenticle (Exd), which function as cofactors with a subset of Hox and
HOM-C proteins, and are essential for specific target gene expression.
Pbx1/Exd binds DNA elements cooperatively with a large subset of Hox/HOM-C
proteins containing a conserved pentapeptide motif, usually YPWMR, located
just N terminally to their homeodomains. The pentapeptide is essential for
cooperative DNA binding with Pbx1. In this study, we identify structural
determinants of Pbx1 that are required for cooperative DNA binding with the
pentapeptide-containing Hox protein HoxA5. We demonstrate that the
homeodomain of Pbx1 contains a surface that binds the pentapeptide motif
and that the Pbx1 homeodomain is sufficient for cooperative DNA binding
with a Hox protein. A sequence immediately C terminal to the Pbx1
homeodomain, which is highly conserved in Pbx2 and Pbx3 and predicted to
form an alpha-helix, enhances monomeric DNA binding by Pbx1 and also
contributes to maximal cooperativity with Hox proteins. Binding studies
with chimeric HoxA5-Pbx1 fusion proteins suggest that the homeodomains of
Pbx1 and HoxA5 are docked on the representative element, TTGATTGAT, in
tandem, with Pbx1 recognizing the 5' TTGAT core motif and the Hox protein
recognizing the 3' TGAT core. The proposed binding orientation permits Hox
proteins to exhibit further binding specificity on the basis of the
identity of the four residues 3' to their core binding motif.
Copyright © 1996, American Society for Microbiology
Structural determinants within Pbx1 that mediate cooperative DNA binding with pentapeptide-containing Hox proteins: proposal for a model of a Pbx1-Hox-DNA complex
Department of Pathology, School of Medicine, University of California, San Diego, La Jolla 92093, USA.
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