Neutron Scattering Studies and Modeling of the HMG 14 Core Nucleosome Complex

Abstract

There is considerable evidence relating the nonhistone proteins HMG 14 and HMG 17 with the structure of active or protentially active chromatin. In this study, bulk nucleosome core particles prepared from chicken erythrocytes and the complex formed by binding two HMG 14 molecules per nucleosome core were studied by small-angle neutron scattering techniques. By varying the H₂O/D₂O ratio, hence the contrast between the solvent and the particles, it was possible to determine the radius of gyration of the protein and of the DNA independently and as a function of HMG 14 binding. The results show an increase in the radius of gyration of 0.9 ± 0.6 Å for protein and of 2.7 ± 0.6 Å for DNA, upon binding HMG 14 to the nucleosome. These changes are considered in light of several postulated modes for the unfolding or perturbation of the nucleosome structure. Modeling calculations demonstrate that the observed changes in radius of gyration for the DNA and for the protein are too small to be consistent with an overall unfolding or opening of the core particle upon HMG 14 binding. However, the observed changes are consistent with several models which involve only minor changes in the structure. It is postulated that the differences observed may be an indication of the type of conformational change occurring in active nucleosomes. (Research sponsored by the U.S. Department of Energy, under contract W-7405-ENG-26 with the Union Carbide Corporation; and NIH Research Grants GM 10334 to D.E. Olins and GM 29818 to G.J. Bunick.)