Neutron scattering studies and modeling of high mobility group 14 core nucleosome complex

Considerable evidence relates the nonhistone proteins high mobility group (HMG) 14 and HMG 17 with the structure of active or potentially 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 use of small-angle neutron scattering techniques. By varying the H2O/2H2O ratio, and 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 of 0.9 +/- 0.6 A (mean +/- SEM) in the protein radius of gyration and of 2.7 +/- 0.6 A in the DNA radius of gyration upon binding of HMG 14 to the nucleosome. These changes are considered in the 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 that 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.