Stoichiometry of Rtt109 complexes with Vps75 and histones H3-H4

Histone acetylation is one of many posttranslational modifications that affect nucleosome accessibility. Vps75 is a histone chaperone that stimulates Rtt109 acetyltransferase activity toward histones H3-H4 in yeast. In this study, we use sedimentation velocity and light scattering to characterize various Vps75-Rtt109 complexes, both with and without H3-H4. These complexes were previously ill-defined because of protein multivalency and oligomerization. We determine both relative and absolute stoichiometry and define the most pertinent and homogeneous complexes. We show that the Vps75 dimer contains two unequal binding sites for Rtt109, with the weaker binding site being dispensable for H3-H4 acetylation. We further show that the Vps75-Rtt109-(H3-H4) complex is in equilibrium between a 2:1:1 species and a 4:2:2 species. Using a dimerization mutant of H3, we show that this equilibrium is mediated by the four-helix bundle between the two copies of H3. We optimize the purity, yield, and homogeneity of Vps75-Rtt109 complexes and determine optimal conditions for solubility when H3-H4 is added. Our comprehensive biochemical and biophysical approach ultimately defines the large-scale preparation of Vps75-Rtt109-(H3-H4) complexes with precise stoichiometry. This is an essential prerequisite for ongoing high-resolution structural and functional analysis of this important multi-subunit complex.