Conformational changes in subfractions of calf thymus histone H1

Abstract

Calf thymus histone H1 was fractionated by the method of Kincade and Cole using a very shallow guanidine hydrochloride gradient. A possible new H1 subfraction, about 5-8% of the H1, was found and characterized by amino acid analysis and electrophoresis. The effects of salt concentration and pH on the conformation of each of the 4 major subfractions were studied by measuring the fluorescence anisotropy of the tyrosine emission and the circular dichroism (CD) of the peptide bond. Upon the addition of salt to aqueous solutions of neutral pH, all 4 subfractions show an instantaneous change in fluorescence anisotropy, fluorescence intensity, tyrosine absorbance and CD. The folding associated with this instantaneous change is highly cooperative and involves the region of the molecule containing the lone tyrosine, which becomes buried in the folded form. The folding of subfraction 3a is more sensitive to salt than the other major subfractions. Upon folding approximately 13% of the residues of subfractions 1b and 2 form .alpha. and .beta. structure; 3a and 3b have approximately 16% of the residues in .alpha. and .beta. structures. There is no evidence for interactions between the subfractions. In salt-free solutions each of the 4 major subfractions shows very little change in conformation in going from low to neutral pH, but each shows a very sharp transition near pH 9. This transition gives rise to a marked increase in fluorescence anisotropy and fluorescence intensity, and involves the formation of both .alpha. and .beta. structure in a manner similar to that of the salt-induced state.