Macromolecular crowding induces a molten globule state in the C-terminal domain of histone H1

We studied the secondary structure of the C-terminal domains of the histone H1 subtypes H1 degrees ( C- H1 degrees) and H1t ( C- H1t) in the presence of macromolecular crowding agents ( Ficoll 70 and PEG 6000) by IR spectroscopy. The carboxyl-terminal domain has little structure in aqueous solution but became extensively folded in the presence of crowding agents. In 30% PEG, C- H1 degrees contained 19% alpha- helix, 28% beta- sheet, 16% turns, and 31% open loops. Similar proportions were observed in 30% Ficoll 70 and for C- H1t in both crowding agents. The proportions of secondary structure motifs were comparable to those of the DNA- bound domain. Kratky plots of the small-angle x-ray scattering showed that in crowding agents the C- terminus had the compaction of a globular state. Progressive dissipation of the secondary structure and a linear increase in partial heat capacity with temperature together with increased binding of ANS indicated that the C- terminus is not cooperatively folded in crowded conditions. Native-like secondary structure and compactness in absence of folding cooperativity indicate that the C- terminus in crowding agents is in a molten globule state. Folding of the C- terminus in crowded conditions may increase the rate of the transition toward the DNA- bound state and facilitate H1 diffusion inside cell nuclei.