The HoxB1 hexapeptide is a prefolded domain: Implications for the Pbx1/Hoxinteraction

Hox proteins are transcriptional regulators that bind consensus DNA sequenc es. The DNA-binding specificity of many of these Hox proteins is modulated by the heterodimerization with partners, such as the Pbx proteins. This coo perative heterodimerization is accomplished through a conserved hexapeptide motif found N-terminal to the Hox DNA-binding homeodomain. Several human l eukemias have been associated with a chromosomal translocation involving ei ther the Hox gene (i.e., NUP98/HOXA9) or the gene encoding Pbx 1 (E2A/PBX1) . The transforming ability of these fusion oncoproteins relies at least par tially on the ability to interact with one another through this hexapeptide motif. Herein we describe NMR structural calculations of the hexapeptide o f HoxB 1 (N alpha -acetyl-Thr-Phe-Asp-Trp-Met-Lys-amide) that has been show n to mediate binding between HoxB1 and Pbx1 and a hexapeptide consensus seq uence (N alpha -acetyl-Leu-Phe- Pro-Trp-Met-Arg-amide). The consensus pepti de exists in two conformations caused by cia-trans isomerization of the Phe -Pro peptide bond. The structures of the HoxB 1 peptide and the trans form of the consensus peptide reveal a turn very similar to that found as part o f the HoxB1/Pbx1/DNA complex in the X-ray crystal structure. This observati on implies that this region is at least partially 'preformed' and thus read y to interact with Pbx1 and stabilize binding of Pbx1 and HoxB 1 to DNA. Th e structural results presented here provide a starting point for synthesizi ng potential nonpeptide or cyclical peptide antagonists that mimic the inte raction of these transcriptional cofactors resulting in a potential chemoth erapeutic for certain types of leukemias.