Chaperone-mediated acetylation of histones by Rtt109 identified by quantitative proteomics.

Rtt109 is a fungal-specific histone acetyltransferase (HAT) that associates with either Vps75 or Asf1 to acetylate histone H3. Recent biochemical and structural studies suggest that site-specific acetylation of H3 by Rtt109 is dictated by the binding chaperone where Rtt109-Asf1 acetylates K56, while Rtt109-Vps75 acetylates K9 and K27. To gain further insights into the roles of Vps75 and Asf1 in directing site-specific acetylation of H3, we used quantitative proteomics to profile the global and site-specific changes in H3 and H4 during in vitro acetylation assays with Rtt109 and its chaperones. Our analyses showed that Rtt109-Vps75 preferentially acetylates H3 K9 and K23, the former residue being the major acetylation site. At high enzyme-to-substrate ratio, Rtt109 also acetylated K14, K18, K27 and to a lower extent K56 of histone H3. Importantly, this study revealed that in contrast to Rtt109-Vps75, Rtt109-Asf1 displayed a far greater site-specificity, with K56 being the primary site of acetylation. For the first time, we also report the acetylation of histone H4 K12 by Rtt109-Vps75, whereas Rtt109-Asf1 showed no detectable activity toward H4. This article is part of a Special Issue entitled: From protein structures to clinical applications.