Most histone acetyltransferases (HATs) function as multisubunit complexes in which accessory proteins regulate substrate specificity and catalytic efficiency. Rtt109 is a particularly interesting example of a HAT whose specificity and catalytic activity require association with either of two histone chaperones, Vps75 or Asf1. Here, we utilize biochemical, structural, and genetic analyses to provide the detailed molecular mechanism for activation of a HAT (Rtt109) by its activating subunit Vps75. The rate-determining step of the activated complex is the transfer of the acetyl group from acetyl CoA to the acceptor lysine residue. Vps75 stimulates catalysis (> 250-fold), not by contributing a catalytic base, but by stabilizing the catalytically active conformation of Rtt109. To provide structural insight into the functional complex, we produced a molecular model of Rtt109-Vps75 based on X-ray diffraction of crystals of the complex. This model reveals distinct negative electrostatic surfaces on an Rtt109 molecule that interface with complementary electropositive ends of a symmetrical Vps75 dimer. Rtt109 variants with interface point substitutions lack the ability to be fully activated by Vps75, and one such variant displayed impaired Vps75-dependent histone acetylation functions in yeast, yet these variants showed no adverse effect on Asf1-dependent Rtt109 activities in vitro and in vivo. Finally, we provide evidence for a molecular model in which a 12 complex of Rtt109-Vps75 acetylates a heterodimer of H3-H4. The activation mechanism of Rtt109-Vps75 provides a valuable framework for understanding the molecular regulation of HATs within multisubunit complexes.
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