Structural investigations of chromatin core protein by nuclear magnetic resonance.

A complex derived from chromatin containing one molecule of each of histones H2A, H2B, H3, and H4, termed core protein, was studied by 13C and 1H nuclear magnetic resonance. 13C line widths, when analyzed and compared with those of native and thermally unfolded representative globular proteins, showed that regions of the core protein possess considerable mobility. Studies of Calpha and Cbeta line widths, and Calpha spin-spin relaxation times, show that this mobility arises from sections of random-coil polypeptide. It is argued that these regions are N-terminal "tails", attached to C-terminal globular polypeptides. The 270-MHz 1H nuclear magnetic resonance spectrum shows numerous ring current shifted resonances, indicating that the C-terminal globular domain has a precise tertiary structure. The globular domain most likely forms the histone "core" of the chromatin monomer particle, whilst the basic tails probably wind around the grooves of the double helix, enabling the basic side chains to interact with the DNA phosphate groups. Some biological implications of this model are considered.