Role of conserved salt bridges in homeodomain stability and DNA binding

The sequence information available for homeodomains reveals that salt bridges connecting pairs 19/30, 31/42, and 17/52 are frequent, whereas aliphatic residues at these sites are rare and mainly restricted to proteins from homeotherms. We have analyzed the influence of salt and hydrophobic bridges at these sites on the stability and DNA binding properties of human Hesx-1 homeodomain. Regarding the protein stability, our analysis shows that hydrophobic side chains are clearly preferred at positions 19/30 and 31/42. This stabilizing influence results from the more favorable packing of the aliphatic side chains with the protein core, as illustrated by the three-dimensional solution structure of a thermostable variant, herein reported. In contrast only polar side chains seem to be tolerated at positions 17/52. Interestingly, despite the significant influence of pairs 19/30 and 31/42 on the stability of the homeodomain, their effect on DNA binding ranges from modest to negligible. The observed lack of correlation between binding strength and conformational stability in the analyzed variants suggests that salt/hydrophobic bridges at these specific positions might have been employed by evolution to independently modulate both properties.