Homology modeling based solution structure of Hoxc8-DNA complex: role of context bases outside TAAT stretch.

The 3D structure of neither Hoxc8 nor Hoxc8-DNA complex is known. The repressor protein Hoxc8 binds to the TAAT stretch of the promoter of the osteopontin gene and modulates its expression. Over expression of the osteopontin gene is related to diseases like osteoporosis, multiple sclerosis, cancer et cetera. In this paper we have proposed a 3D structure of Hoxc8-DNA complex obtained by Homology modeling and molecular dynamics (MD) simulation in explicit water. The crystal structure (9ant.pdb) of Antennapedia homeodomain in complex with its DNA sequence was chosen as the template based on (i) high sequence identity (85% for the protein and 60% for the DNA) and (ii) the presence of the TAAT stretch in interaction with the protein. The resulting model was refined by MD simulation for 2.0ns in explicit water. This refined model was then characterized in terms of the structural and the interactional features to improve our understanding of the mechanism of Hoxc8-DNA recognition. The interaction pattern shows that the residues Ile(195), Gln(198), and Asn(199), and the bases S2-(4)TAATG(8) are most important for recognition suggesting the stretch TAATG as the "true recognition element" in the present case. A strong and long-lived water bridge connecting Gln(198) and the base of S1-C(7) complementary to S2-G(8) was observed. Our predicted model of Hoxc8-DNA complex provides us with features that are consistent with the available experimental data on Hoxc8 and the general features of other homeodomain-DNA complexes. The predictions based on the model are also amenable to experimental verification.