Structural basis for histone H3 acetylation by Rtt109 in complex with histone chaperones Asf1 and Vps75

In eukaryotes, genetic material is stored as chromatin, a DNA-protein complex whose structure is tightly regulated. Histones are the main protein components of chromatin and their post-translational modifications (PTMs) influence several cellular processes, from gene expression to epigenetic regulation and inheritance. Histone acetyltransferases carry out the acetylation of histones, an abundant modification, the function of which is determined by the position of the modified residue. Histone chaperones can influence the specificity of the histone acetyltransferases, however, the underlying mechanism of this process is not fully understood. Rtt109 is a fungal histone acetyltransferase which is essential for the acetylation of newly synthesized histone H3. Two distinct histone chaperones, Asf1 and Vps75, have been reported to alter its activity and specificity: while Asf1 is necessary for H3 K56 acetylation, Vps75 promotes acetylation of H3 K9, K23 and K27, which are located in a long disordered N-terminal tail of H3. Despite the availability of structures of Rtt109 in complex with Vps75, the mechanism of regulation of Rtt109 activity by Asf1 and Vps75 remains elusive. In order to understand how Asf1 and Vps75 stimulate the Rtt109 activity towards specific substrates, I reconstituted in vitro the complex containing Rtt109, histones H3:H4 and both chaperones. With multi angle light scattering and nuclear magnetic resonance (NMR), I could show that the Vps75 dimer assembles a non-symmetric complex with one copy of Rtt109 and Asf1-bound histones. Using an integrative structural biology approach combining distance restraint information from NMR and low-resolution shape information from small-angle neutron scattering (SANS) data, I could obtain a structural model of this complex. The structure revealed that the chaperones form a bagel-shaped complex with Rtt109 and the histones, bringing the enzyme and the substrate together and positioning H3 K56 next to the Rtt109 active center. A combination of NMR data with biochemical experiments and computational studies, revealed that the flexible H3 tail is chaperoned by Asf1 and is guided towards the catalytic pocket of Rtt109 by both folded and unfolded structural elements of Vps75. These results, taken together with existing literature and further mutational studies, allowed me to propose a mechanism by which the histone chaperones promote acetylation of the disordered H3 N-terminal tail

Structural characterization of the Asf1-Rtt109 interaction and its role in histone acetylation.

Acetylation of histone H3 at lysine-56 by the histone acetyltransferase Rtt109 in lower eukaryotes is important for maintaining genomic integrity and is required for C. albicans pathogenicity. Rtt109 is activated by association with two different histone chaperones, Vps75 and Asf1, through an unknown mechanism. Here, we reveal that the Rtt109 C-terminus interacts directly with Asf1 and elucidate the structural basis of this interaction. In addition, we find that the H3 N-terminus can interact via the same interface on Asf1, leading to a competition between the two interaction partners. This, together with the recruitment and position of the substrate, provides an explanation of the role of the Rtt109 C-terminus in Asf1-dependent Rtt109 activation

Histone chaperone exploits intrinsic disorder to switch acetylation specificity

Histones, the principal protein components of chromatin, contain long disordered sequences, which are extensively post-translationally modified. Although histone chaperones are known to control both the activity and specificity of histone-modifying enzymes, the mechanisms promoting modification of highly disordered substrates, such as lysine-acetylation within the N-terminal tail of histone H3, are not understood. Here, to understand how histone chaperones Asf1 and Vps75 together promote H3 K9-acetylation, we establish the solution structural model of the acetyltransferase Rtt109 in complex with Asf1 and Vps75 and the histone dimer H3:H4. We show that Vps75 promotes K9-acetylation by engaging the H3 N-terminal tail in fuzzy electrostatic interactions with its disordered C-terminal domain, thereby confining the H3 tail to a wide central cavity faced by the Rtt109 active site. These fuzzy interactions between disordered domains achieve localization of lysine residues in the H3 tail to the catalytic site with minimal loss of entropy, and may represent a common mechanism of enzymatic reactions involving highly disordered substrate

Transfer of chemical elements in vapor-gas streams at the dehydration of secondary sulfates

The elemental composition of vapor-gas streams obtained during heating of secondary hydrous sulfates are presented. Samples of abundant sulfate intergrowth were collected at the Belovo waste heaps and heated at 60ºC in experiments to collect condensates of the releasing vapor-gas streams. A wide spectrum of major and trace elements was determined in the condensate. Chemical elements can be absorbed by the water vapor and migrate with this phase during the dehydration of hydrous sulfates. To determine the mechanisms of migration and the sources of elements in vapor-gas streams, a study of the features of certain hydrous sulphates (antlerite, goslarite, starkeyite, gunningite, siderotile, sideronatrite) by stepwise heating up to 60ºC was conducted. Alteration in the phase composition is controlled by powder X-ray diffractometry. It was determined, that antlerite and starkeite remain stable throughout the temperature range. The beginning of the separation of structural water in goslarite and siderotile occurs at 40°C. Goslarite and sideronatrite at 40°C lost water molecules and transformed to gunningite and Na-jarosite, correspondingly. Structure of siderotile was loosened. The modes of occurrence of the chemical elements in sulfates and pore solution determine the concentrations of elements in the condensates