AN EXTRADENTICLE-INDUCED CONFORMATIONAL CHANGE IN A HOX PROTEIN OVERCOMES AN INHIBITORY FUNCTION OF THE CONSERVED HEXAPEPTIDE MOTIF

HOX homeoproteins control cell identities during animal development by differentially regulating target genes, The homeoprotein encoded by t he extradenticle (exd) gene can selectively modify HOX DNA binding, su ggesting that it contributes to HOX specificity in vivo, HOX-EXD inter actions are in part mediated by a conserved stretch of amino acids ter med the hexapeptide found in many HOX proteins, Here, we demonstrate t hat a 20 bp oligonucleotide from the 5' region of the mouse Hoxb-1 gen e, a homolog of Drosophila labial (lab), is sufficient to direct an ex pression pattern in Drosophila that is very similar to endogenous lab, In vivo, this expression requires lab and exd and, in vitro, LAB requ ires EXD to bind this oligonucleotide, In contrast, LAB proteins with mutations in the hexapeptide bind DNA even in the absence of EXD, More over, a hexapeptide mutant of LAB has an increased ability to activate transcription in vivo. Partial proteolysis experiments suggest that E XD can induce a conformational change in LAB, These data are consisten t with a mechanism whereby the LAB hexapeptide inhibits LAB function b y inhibiting DNA binding and that an EXD-induced conformational change in LAB relieves this inhibition, promoting highly specific interactio ns with biologically relevant binding sites.