DNA-induced secondary structure of the carboxyl-terminal domain of histone H1.

Abstract We have studied the secondary structure of the carboxyl-terminal domains of linker histone H1 subtypes H10 (C-H10) and H1t (C-H1t), free in solution and bound to DNA, by IR spectroscopy. The carboxyl-terminal domain has little structure in aqueous solution but becomes extensively folded upon interaction with DNA. The secondary structure elements present in the bound carboxyl-terminal domain include the α-helix, β-structure, turns, and open loops. The structure of the bound domain shows a significant dependence on salt concentration. In low salt (10 mm NaCl), there is a residual amount of random coil, 7% in C-H10 and 12% in C-H1t. In physiological salt concentrations (140 mm NaCl), the carboxyl termini become fully structured. Under these conditions, C-H10 contained 24% α-helix, 25% β-structure, 17% open loops, and 33% turns. The latter component could include a substantial proportion of the 310 helix. Despite their low sequence identity (∼30%), the representation of the different structural motifs in C-H1t was similar to that in C-H10. Examination of the changes in the amide I components in the 20–80 °C temperature interval showed that the secondary structure of the DNA-bound C-H1t is for the most part extremely stable. The H1 carboxyl-terminal domain appears to belong to the so-called disordered proteins, undergoing coupled binding and folding.