Low resolution models of self-assembled histone fibers from X-ray diffraction studies

Abstract

X-ray diffraction data from self-assembled histone fibers are presented for three systems: H4, H3-H4, and the four core histones H2A, H2B, H3 and H4. These data have been obtained under conditions of high ionic strength and high protein concentration which are thought to promote histone conformation similar to that found in intact chromatin. The low angle equatorial scattering (R<,05 A ) is analysed, and, with additional constraints imposed by electron microscopy data, four low resolution fibrillar models are derived. Two features common so all the possible models are a maximum outer iameter of approximately 60 A and a subfibril diameter of approximately 25 A. It is the interference of the Rrotein subfibrils across a central region of low electron-density -a 10 R "hole" -which gives rise to the characteristic diffraction peak at 36 A. Possible relationships of the models of the histone fibers to the structure of the histone component of chromatin are suggested. I NTRODUCT I ONRecently, a low resolution model of the chromatin "core" particle, consisting of 140 base pairs of DNA and eight histones, has been derived from a combination of single crystal X-ray diffraction and electron microscopy data1. The particle is described as a flat cylinder 110 A in diameter and 0 57 A in height. A similar shape and dimensions were found to be consistent 2,3with the low angle neutron scattering from core particles in solutionThe neutron experiments furthermore determined that the bulk of the histone proteins occupies the central region of the particle to a radius of approximately 32 A and that the DNA is supercoiled on the outside. Some conclusions may be drawn concerning the conformation of the DNA. Presumably the strong 28 A periodicity apparent in the crystal data corresponds to the pitch of the DNA superhelix wound about the histone core,

The impact of solubility and electrostatics on fibril formation by the H3 and H4 histones

“…11 While the fibril diameter is comparable, the morphology of the histone fibrils is more worm-like. Similar EM images have been reported for ''immature fibrils'' formed by human H2A at elevated temperatures, 51 fibrils formed from individual and combinations of calf thymus histones, 52 and the small, indistinct fibrils formed at pH 4 by the repeat domain of Pmel17 involved in melanosome structure.…”

“…The pH dependence of the thermal unfolding of H2A-H2B and H3-H4 has been reported, although at lower ionic strengths. 41,42 Between pH 4.5 and 5.5, the mid-point of the thermal unfolding transition, T M , for the H3-H4 heterodimer was [50][51][52][53][54][55][56][57][58][59][60] C, depending on the protein concentration, while the T M of H2A-H2B was 10-15 C lower. Thus, H3-H4 is more stable, but forms fibrils much more readily than H2A-H2B or the individual H2A and H2B monomers which are only partially folded even under stabilizing conditions.…”

Neutron‐scattering studies of the structure of chromatin core particles in solution

“…A more detailed description will be found elsewhere. 33 The first family of models includes a variety of spherical cores with the full range of DNA superhelices. The results of these combinations are not illustrated.…”

Conformational Prerequisites for Formation of Amyloid Fibrils from Histones

“…This observation is in good agreement with the results of earlier studies that showed that coincubation of the core histones H2A, H2B, H3 and H4 at high protein concentration and high ionic strength led to the formation of histone fibers, which contained H2A:H2B:H3:H4 in an approximately 1 : 1 : 1 : 1 molar ratio. 95 Interestingly, when the fibrillation of individual histones and their different mixtures was analyzed, the arginine-rich histones H3 and H4 and their mixtures were shown to assemble most readily at relatively low ionic strength (between 0.02 M and 0.15 M), whereas the lysine-rich histones H2A and H2B polymerize only at high ionic strength (O0.5 M). Furthermore, in solutions of pairs of histones or of all four histones, there was an equilibrium between low molecular mass complexes and fibrils, with dimer being the basic unit in the assembly.…”