Characterization of the Pneumocystis carinii histone acetyltransferase chaperone proteins PcAsf1 and PcVps75

Rtt109 is a lysine acetyltransferase that acetylates histone H3 at lysine 56 (H3K56) in fungi. This acetylation event is important for proper DNA replication and repair to occur. Efficient Rtt109 acetyltransferase activity also requires a histone chaperone, vacuolar protein sorting 75 (Vps75), as well as the major chaperone of the H3-H4 dimer, anti-silencing factor 1 (Asf1). Little is known about the role of these proteins in the opportunistic fungal pathogen Pneumocystis carinii. To investigate the functions of Asf1 and Vps75 in Pneumocystis carinii, we cloned and characterized both of these genes. Here, we demonstrate that both genes, P. carinii asf1 (Pcasf1) and Pcvps75, function in a fashion analogous to their Saccharomyces cerevisiae counterparts. We demonstrate that both P. carinii Asf1 (PcAsf1) and PcVps75 can bind histones. Furthermore, when Pcasf1 is expressed heterologously in S. cerevisiae asf1δ cells, PcAsf1 can restore full H3 lysine acetylation. We further demonstrated that the Pcasf1 cDNA expressed in asf1δ S. cerevisiae cells can restore growth to wild-type levels in the presence of genotoxic agents that block DNA replication. Lastly, we observed that purified PcAsf1 and PcVps75 proteins enhance the ability of PcRtt109 to acetylate histone H3-H4 tetramers. Together, our results indicate that the functions of the Rtt109-Asf1-Vps75 complex in the acetylation of histone H3 lysine 56 and in DNA damage response are present in P. carinii DNA and cell cycle progression.