Abstract
Vps75 is a histone chaperone that has been historically characterized as homodimer by X-ray crystallography. In this study, we present a crystal structure containing two related tetrameric forms of Vps75 within the crystal lattice. We show Vps75 associates with histones in multiple oligomers. In the presence of equimolar H3–H4 and Vps75, the major species is a reconfigured Vps75 tetramer bound to a histone H3–H4 tetramer. However, in the presence of excess histones, a Vps75 dimer bound to a histone H3–H4 tetramer predominates. We show the Vps75–H3–H4 interaction is compatible with the histone chaperone Asf1 and deduce a structural model of the Vps75–Asf1-H3–H4 (VAH) co-chaperone complex using the Pulsed Electron-electron Double Resonance (PELDOR) technique and cross-linking MS/MS distance restraints. The model provides a molecular basis for the involvement of both Vps75 and Asf1 in Rtt109 catalysed histone H3 K9 acetylation. In the absence of Asf1 this model can be used to generate a complex consisting of a reconfigured Vps75 tetramer bound to a H3–H4 tetramer. This provides a structural explanation for many of the complexes detected biochemically and illustrates the ability of Vps75 to interact with dimeric or tetrameric H3–H4 using the same interaction surface.
Highlights
Vacuolar Protein Sorting 75 (Vps75) is a histone chaperone belonging to the Nucleosome Assembly Protein (NAP-1) family [1]
We show the Vps75–H3–H4 interaction is compatible with the histone chaperone AntiSilencing Function 1 (Asf1) and deduce a structural model of the Vps75– Asf1-H3–H4 (VAH) co-chaperone complex using the Pulsed Electron-electron Double Resonance (PELDOR) technique and cross-linking MS/MS distance restraints
The model shows that whilst crosslinks between Vps75 and Asf1 were observed, these distance restraints can be satisfied without substantial contacts between Vps75 and Asf1 (Figure 7A)
Summary
Vacuolar Protein Sorting 75 (Vps75) is a histone chaperone belonging to the Nucleosome Assembly Protein (NAP-1) family [1]. The founding member of this family, Nap was originally identified from cell extracts as a factor capable of assembling nucleosomes in vitro [2]. Both Nap and Vps are capable of binding all core histones H2A, H2B, H3 and H4 [1,3]. All crystal structures published to date show Nap1 [5,6] and Vps75 [7,8,9] to be homo-dimeric proteins adopting characteristic ‘headphone’ folds, in which a long N-terminal helix self-associates in an antiparallel manner to form the dimerization interface which is capped at either end by a globular domain. In addition to the headphone fold, both Nap and Vps contain C-terminal acidic domains dispensable for histone binding [9,13]
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