Understanding histone acetyltransferase Rtt109 structure and function: how many chaperones does it take?

[Display omitted] ► Histone chaperones Vps75 and Asf1 stimulate Rtt109 acetyltransferase toward histone H3 approximately 100-fold. ► Rtt109-Asf1 acetylates H3 K56, while Rtt109-Vps75 acetylates H3 K9 and K27. ► Auto-acetylation of Rtt109 at K290 is essential and is not influenced by Vps75. ► Based on structural work, the stoichiometry of Vps75-Rtt109 is uncertain; it may be 2:1 or 2:2. ► Vps75 imports Rtt109 into the nucleus, stabilizes Rtt109, and positions H3 for acetylation by Rtt109. Rtt109 (Regulator of Ty1 Transposition 109) is a fungal-specific histone acetyltransferase required for modification of histone H3 K9, K27 and K56. These acetylations are associated with nascent histone H3 and play an integral role in replication-coupled and repair-coupled nucleosome assembly. Rtt109 is unique among acetyltransferases as it is activated by a histone chaperone; either Vps75 (Vacuolar Protein Sorting 75) or Asf1 (Anti-silencing Function 1). Recent biochemical, structural and genetic studies have shed light on the intricacies of this activation. It is now clear that Rtt109-Asf1 acetylates K56, while Rtt109-Vps75 acetylates K9 and K27. This reinforces that Asf1 and Vps75 activate Rtt109 via distinct molecular mechanisms. Structures of Rtt109-Vps75 further imply that Vps75 positions histone H3 in the Rtt109 active site. These structures however raise questions regarding the stoichiometry of the Rtt109-Vps75 complex. This has ramifications for determining the physiological Rtt109 substrate.